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What you’ll learn to do: Recognize that mutations are the basis of microevolution; and that adaptations enhance the survival and reproduction of individuals in a population
We’ve already learned about DNA and mutations, now we’ll learn about how these mutations can drive evolution. This type of evolution falls under the category of microevolution.
Microevolution is the change in allele frequencies that occurs over time within a population. This change is due to five different processes: mutation, selection (natural and artificial), gene flow, gene migration and genetic drift. This change happens over a relatively short (in evolutionary terms) amount of time compared to the changes termed ‘macroevolution’ which is where greater differences in the population occur.
Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution. Ecological genetics concerns itself with observing microevolution in the wild. Typically, observable instances of evolution are examples of microevolution; for example, bacterial strains that have antibiotic resistance.
Microevolution over time leads to speciation or the appearance of novel structure, sometimes classified as macroevolution. Macro and microevolution describe fundamentally identical processes on different scales.
- Understand the connection between genetics and evolution
- Understand how environmental changes and selective pressures impact the spread of mutations, contributing to the process of evolution
- Describe the different types of variation in a population
Darwin Meets Mendel—Not Literally
When Darwin came up with his theories of evolution and natural selection, he knew that the processes he was describing depended on heritable variation in populations. That is, they relied on differences in the features of the organisms in a population and on the ability of these different features to be passed on to offspring.
Darwin did not, however, know how traits were inherited. Like other scientists of his time, he thought that traits were passed on via blending inheritance. In this model, parents’ traits are supposed to permanently blend in their offspring. The blending model was disproven by Austrian monk Gregor Mendel, who found that traits are specified by non-blending heritable units called genes.
Although Mendel published his work on genetics just a few years after Darwin published his ideas on evolution, Darwin probably never read Mendel’s work. Today, we can combine Darwin’s and Mendel’s ideas to arrive at a clearer understanding of what evolution is and how it takes place.
Microevolution and Population Genetics
Microevolution, or evolution on a small scale, is defined as a change in the frequency of gene variants, alleles, in a population over generations. The field of biology that studies allele frequencies in populations and how they change over time is called population genetics.
Microevolution is sometimes contrasted with macroevolution, evolution that involves large changes, such as formation of new groups or species, and happens over long time periods. However, most biologists view microevolution and macroevolution as the same process happening on different timescales. Microevolution adds up gradually, over long periods of time to produce macroevolutionary changes. It is important to remember that both these processes are based on changes in DNA sequences, or mutations. Not all mutations are beneficial, just as not all are harmful. Furthermore, the impact of a particular mutation (benefit or harm) may change if the environment changes. This is natural selection in action.
Let’s look at three concepts that are core to the definition of microevolution: populations, alleles, and allele frequency.
A population is a group of organisms of the same species that are found in the same area and can interbreed. A population is the smallest unit that can evolve—in other words, an individual can’t evolve.
An allele is a version of a gene, a heritable unit that controls a particular feature of an organism.
For instance, Mendel studied a gene that controls flower color in pea plants. This gene comes in a white allele, w, and a purple allele, W. Each pea plant has two gene copies, which may be the same or different alleles. When the alleles are different, one—the dominant allele, W—may hide the other—the recessive allele, w. A plant’s set of alleles, called its genotype, determines its phenotype, or observable features, in this case flower color.
Allele frequency refers to how frequently a particular allele appears in a population. For instance, if all the alleles in a population of pea plants were purple alleles, W, the allele frequency of W would be 100%, or 1.0. However, if half the alleles were W and half were w, each allele would have an allele frequency of 50%, or 0.5.
In general, we can define allele frequency as
Sometimes there are more than two alleles in a population (e.g., there might be A, a, and Ai alleles of a gene). In that case, you would want to add up all of the different alleles to get your denominator.
It’s also possible to calculate genotype frequencies—the fraction of individuals with a given genotype—and phenotype frequencies—the fraction of individuals with a given phenotype. Keep in mind, though, that these are different concepts from allele frequency. We’ll see an example of this difference next.
This video talks about population genetics, which helps to explain the evolution of populations over time.
Selective and Environmental Pressures
Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency—a process known as adaptive evolution. Natural selection does not act on individual alleles, however, but on entire organisms. An individual may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce (fecundity), but if that same individual also carries an allele that results in a fatal childhood disease, that fecundity phenotype will not be passed on to the next generation because the individual will not live to reach reproductive age. Natural selection acts at the level of the individual; it selects for individuals with greater contributions to the gene pool of the next generation, known as an organism’s evolutionary (Darwinian) fitness.
Fitness is often quantifiable and is measured by scientists in the field. However, it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population. This concept, called relative fitness, allows researchers to determine which individuals are contributing additional offspring to the next generation, and thus, how the population might evolve.
There are several ways selection can affect population variation: stabilizing selection, directional selection, diversifying selection, frequency-dependent selection, and sexual selection. As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes displayed can become more similar or more disparate.
If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo stabilizing selection (Figure 1a). In a population of mice that live in the woods, for example, natural selection is likely to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most closely matched to that color will be most likely to survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them a bit lighter or a bit darker will stand out against the ground and be more likely to fall victim to predation. As a result of this selection, the population’s genetic variance will decrease.
When the environment changes, populations will often undergo directional selection (Figure 1b), which selects for phenotypes at one end of the spectrum of existing variation. A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. But as soot began spewing from factories, the trees became darkened, and the light-colored moths became easier for predatory birds to spot. Over time, the frequency of the melanic form of the moth increased because they had a higher survival rate in habitats affected by air pollution because their darker coloration blended with the sooty trees. Similarly, the hypothetical mouse population may evolve to take on a different coloration if something were to cause the forest floor where they live to change color. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.
In science, sometimes things are believed to be true, and then new information comes to light that changes our understanding. The story of the peppered moth is an example: the facts behind the selection toward darker moths have recently been called into question. Read this article to learn more.
Sometimes two or more distinct phenotypes can each have their advantages and be selected for by natural selection, while the intermediate phenotypes are, on average, less fit. Known as diversifying selection (Figure 1c), this is seen in many populations of animals that have multiple male forms. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which can’t overtake the alpha males and are too big to sneak copulations, are selected against. Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand, and would thus be more likely to be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.
In recent years, factories have become cleaner, and less soot is released into the environment. What impact do you think this has had on the distribution of moth color in the population?
[reveal-answer q=”173318″]Show Answer[/reveal-answer]
[hidden-answer a=”173318″]Moths have shifted to a lighter color.[/hidden-answer]
Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection). An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females; blue males are medium-sized and form strong pair bonds with their mates; and yellow males (Figure 2) are the smallest, and look a bit like females, which allows them to sneak copulations. Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. That is, the big, strong orange males can fight off the blue males to mate with the blue’s pair-bonded females, the blue males are successful at guarding their mates against yellow sneaker males, and the yellow males can sneak copulations from the potential mates of the large, polygynous orange males.
In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes—in one generation, orange might be predominant, and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored.
Negative frequency-dependent selection serves to increase the population’s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.
Males and females of certain species are often quite different from one another in ways beyond the reproductive organs. Males are often larger, for example, and display many elaborate colors and adornments, like the peacock’s tail, while females tend to be smaller and duller in decoration. Such differences are known as sexual dimorphisms (Figure 3), which arise from the fact that in many populations, particularly animal populations, there is more variance in the reproductive success of the males than there is of the females. That is, some males—often the bigger, stronger, or more decorated males—get the vast majority of the total matings, while others receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to get those matings, resulting in the evolution of bigger body size and elaborate ornaments to get the females’ attention. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males.
Sexual dimorphism varies widely among species, of course, and some species are even sex-role reversed. In such cases, females tend to have a greater variance in their reproductive success than males and are correspondingly selected for the bigger body size and elaborate traits usually characteristic of males.
The selection pressures on males and females to obtain matings is known as sexual selection; it can result in the development of secondary sexual characteristics that do not benefit the individual’s likelihood of survival but help to maximize its reproductive success. Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual’s survival. Think, once again, about the peacock’s tail. While it is beautiful and the male with the largest, most colorful tail is more likely to win the female, it is not the most practical appendage. In addition to being more visible to predators, it makes the males slower in their attempted escapes. There is some evidence that this risk, in fact, is why females like the big tails in the first place. The speculation is that large tails carry risk, and only the best males survive that risk: the bigger the tail, the more fit the male. This idea is known as the handicap principle.
The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be picky because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring.
In both the handicap principle and the good genes hypothesis, the trait is said to be an honest signal of the males’ quality, thus giving females a way to find the fittest mates— males that will pass the best genes to their offspring.
No Perfect Organism
Natural selection is a driving force in evolution and can generate populations that are better adapted to survive and successfully reproduce in their environments. But natural selection cannot produce the perfect organism. Natural selection can only select on existing variation in the population; it does not create anything from scratch. Thus, it is limited by a population’s existing genetic variance and whatever new alleles arise through mutation and gene flow.
Natural selection is also limited because it works at the level of individuals, not alleles, and some alleles are linked due to their physical proximity in the genome, making them more likely to be passed on together (linkage disequilibrium). Any given individual may carry some beneficial alleles and some unfavorable alleles. It is the net effect of these alleles, or the organism’s fitness, upon which natural selection can act. As a result, good alleles can be lost if they are carried by individuals that also have several overwhelmingly bad alleles; likewise, bad alleles can be kept if they are carried by individuals that have enough good alleles to result in an overall fitness benefit.
Furthermore, natural selection can be constrained by the relationships between different polymorphisms. One morph may confer a higher fitness than another, but may not increase in frequency due to the fact that going from the less beneficial to the more beneficial trait would require going through a less beneficial phenotype. Think back to the mice that live at the beach. Some are light-colored and blend in with the sand, while others are dark and blend in with the patches of grass. The dark-colored mice may be, overall, more fit than the light-colored mice, and at first glance, one might expect the light-colored mice be selected for a darker coloration. But remember that the intermediate phenotype, a medium-colored coat, is very bad for the mice—they cannot blend in with either the sand or the grass and are more likely to be eaten by predators. As a result, the light-colored mice would not be selected for a dark coloration because those individuals that began moving in that direction (began being selected for a darker coat) would be less fit than those that stayed light.
Finally, it is important to understand that not all evolution is adaptive. While natural selection selects the fittest individuals and often results in a more fit population overall, other forces of evolution, including genetic drift and gene flow, often do the opposite: introducing deleterious alleles to the population’s gene pool. Evolution has no purpose—it is not changing a population into a preconceived ideal. It is simply the sum of the various forces described in this chapter and how they influence the genetic and phenotypic variance of a population.
Because natural selection acts to increase the frequency of beneficial alleles and traits while decreasing the frequency of deleterious qualities, it is adaptive evolution. Natural selection acts at the level of the individual, selecting for those that have a higher overall fitness compared to the rest of the population. If the fit phenotypes are those that are similar, natural selection will result in stabilizing selection, and an overall decrease in the population’s variation. Directional selection works to shift a population’s variance toward a new, fit phenotype, as environmental conditions change. In contrast, diversifying selection results in increased genetic variance by selecting for two or more distinct phenotypes.
Other types of selection include frequency-dependent selection, in which individuals with either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) phenotypes are selected for. Finally, sexual selection results from the fact that one sex has more variance in the reproductive success than the other. As a result, males and females experience different selective pressures, which can often lead to the evolution of phenotypic differences, or sexual dimorphisms, between the two.
Genetic Variation and Drift
Individuals of a population often display different phenotypes, or express different alleles of a particular gene, referred to as polymorphisms. Populations with two or more variations of particular characteristics are called polymorphic. The distribution of phenotypes among individuals, known as the population variation, is influenced by a number of factors, including the population’s genetic structure and the environment (Figure 1). Understanding the sources of a phenotypic variation in a population is important for determining how a population will evolve in response to different evolutionary pressures.
Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against. Acquired traits, for the most part, are not heritable. For example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not necessarily grow up to be a body builder. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child. Ultimately, heritability tells us how much phenotypic variation in a population is ulimately due to genetic differences as opposed to acquired differences.
The diversity of alleles and genotypes within a population is called genetic variance. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease.
In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances.
The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure.
Another way a population’s allele and genotype frequencies can change is genetic drift (Figure 2), which is simply the effect of chance. By chance, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting).
Figure 2. Click for a larger image. Genetic drift in a population can lead to the elimination of an allele from a population by chance. In this example, rabbits with the brown coat color allele (B) are dominant over rabbits with the white coat color allele (b). In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of .5. Only half of the individuals reproduce, resulting in a second generation with p and q values of .7 and .3, respectively. Only two individuals in the second generation reproduce, and by chance these individuals are homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele is lost.
Do you think genetic drift would happen more quickly on an island or on the mainland?
[reveal-answer q=”949142″]Show Answer[/reveal-answer]
[hidden-answer a=”949142″]Genetic drift is likely to occur more rapidly on an island where smaller populations are expected to occur.[/hidden-answer]
Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a young age before it leaves any offspring to the next generation, all of its genes—1/10 of the population’s gene pool—will be suddenly lost. In a population of 100, that’s only 1 percent of the overall gene pool; therefore, it is much less impactful on the population’s genetic structure.
Watch this animation of random sampling and genetic drift in action:
Genetic drift can also be magnified by natural events, such as a natural disaster that kills—at random—a large portion of the population. Known as the bottleneck effect, it results in a large portion of the genome suddenly being wiped out (Figure 3). In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population.
Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is likely due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital abnormalities—even cancer.
Watch this short video to learn more about the founder and bottleneck effects. Note that the video has no audio.
A YouTube element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/biowm/?p=226
Another important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes (Figure 4). While some populations are fairly stable, others experience more flux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other populations of the same species some distance away. Even a population that may initially appear to be stable, such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and out of the group not only changes the gene structure of the population, but it can also introduce new genetic variation to populations in different geological locations and habitats.
Check Your Understanding
Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.
12.4: Mutations and Evolution - Biology
All of the alleles that exist within a species are known as the gene pool. When mutations or genetic leakage occur, new genes are introduced into the gene pool. Genetic variability is essential for the survival of a species because it allows it to evolve to adapt to changing environmental stresses. Certain traits may be more desirable than others and confer a selective advantage&mdashan advantage that allows for the individual to produce more viable, fertile offspring. In this section, we will consider genetic diversity and mutations, leakage, and genetic drift, which cause changes to the alleles present in the gene pool.
A mutation is a change in DNA sequence. New mutations may be introduced in a variety of ways. Ionizing radiation, such as ultraviolet rays from the sun, and chemical exposures can damage DNA substances that can cause mutations are called mutagens. DNA polymerase is subject to making mistakes during DNA replication, albeit at a very low rate proofreading mechanisms also help prevent mutations from occurring through this mechanism. Elements known as transposons can insert and remove themselves from the genome. If a transposon inserts in the middle of a coding sequence, the mutation will disrupt the gene.
Flawed proteins can arise in other ways without an underlying change in DNA sequence, as well. Incorrect pairing of nucleotides during transcription or translation, or a tRNA molecule charged with the incorrect amino acid for its anticodon, can result in derangements of the normal amino acid sequence.
The major types of nucleotide-level mutations are discussed in great detail in Chapter 7 of MCAT Biochemistry Review, so we offer just a brief overview here of each type.
Many mutations occur at the level of a single nucleotide (or a very small number of nucleotides). These mutations are shown in Figure 12.3 and are summarized below.
Figure 12.3. Common Nucleotide-Level Mutations
Point mutations occur when one nucleotide in DNA (A, C, T, or G) is swapped for another. These can be subcategorized as silent, missense, or nonsense mutations:
·&emspSilent mutations occur when the change in nucleotide has no effect on the final protein synthesized from the gene. This most commonly occurs when the changed nucleotide is transcribed to be the third nucleotide in a codon because there is degeneracy (wobble) in the genetic code.
·&emspMissense mutations occur when the change in nucleotide results in substituting one amino acid for another in the final protein.
·&emspNonsense mutations occur when the change in nucleotide results in substituting a stop codon for an amino acid in the final protein.
Frameshift mutations occur when nucleotides are inserted into or deleted from the genome. Because mRNA transcribed from DNA is always read in three-letter sequences called codons, insertion or deletion of nucleotides can shift the reading frame, usually resulting in either changes in the amino acid sequence or premature truncation of the protein (due to the generation of a nonsense mutation). These can be subcategorized as insertion or deletion mutations.
Chromosomal mutations are larger-scale mutations in which large segments of DNA are affected, as demonstrated in Figure 12.4 and summarized below.
Figure 12.4. Common Chromosomal Mutations
·&emspDeletion mutations occur when a large segment of DNA is lost from a chromosome. Small deletion mutations are considered frameshift mutations, as described previously.
·&emspDuplication mutations occur when a segment of DNA is copied multiple times in the genome.
·&emspInversion mutations occur when a segment of DNA is reversed within the chromosome.
·&emspInsertion mutations occur when a segment of DNA is moved from one chromosome to another. Small insertion mutations (including those where the inserted DNA is not from another chromosome) are considered frameshift mutations, as described previously.
·&emspTranslocation mutations occur when a segment of DNA from one chromosome is swapped with a segment of DNA from another chromosome.
Consequences of Mutations
Mutations can have many different consequences. Some mutations can be advantageous, conferring a positive selective advantage that may allow the organism to produce more offspring. For example, sickle cell disease is a single nucleotide mutation that causes sickled hemoglobin. While the disease itself is detrimental to life, heterozygotes for sickle cell disease usually have minor symptoms, if any, and have natural resistance to malaria because their red blood cells have a slightly shorter lifespan&mdashjust short enough that the parasitic Plasmodium species that cause malaria cannot reproduce. Thus, heterozygotes for sickle cell disease actually have a selective advantage because they are less likely to die from malaria.
On the other hand, some mutations can be detrimental or deleterious. For example, xeroderma pigmentosum (XP) is an inherited defect in the nucleotide excision repair mechanism. In patients with XP, DNA that has been damaged by ultraviolet radiation cannot be repaired appropriately. Ultraviolet radiation can introduce cancer-causing mutations without a repair mechanism, patients with XP are frequently diagnosed with malignancies, especially of the skin.
One important class of deleterious mutations is known as inborn errors of metabolism. These are defects in genes required for metabolism. Children born with these defects often require very early intervention in order to prevent permanent damage from the buildup of metabolites in various pathways. For example, in phenylketonuria (PKU), the enzyme phenylalanine hydrolase, which completes the metabolism of the amino acid phenylalanine, is defective. In the absence of this enzyme, toxic metabolites of phenylalanine accumulate, causing seizures, impairment of cerebral function, and learning disabilities, as well as a musty odor to bodily secretions. However, if the disease is discovered shortly after birth, then dietary phenylalanine can be eliminated and treatments can be administered to aid in metabolizing any additional phenylalanine.
Genetic leakage is a flow of genes between species. In some cases, individuals from different (but closely related) species can mate to produce hybrid offspring. Many hybrid offspring, such as the mule (hybrid of a male horse and a female donkey), are not able to reproduce because they have odd numbers of chromosomes&mdashhorses have 64 chromosomes and donkeys have 62, so mules, with 63 chromosomes, cannot undergo normal homologous pairing in meiosis and cannot form gametes. In some cases, however, a hybrid can reproduce with members of one species or the other, such as the beefalo (a cross between cattle and American bison). The hybrid carries genes from both parent species, so this results in a net flow of genes from one species to the other.
Genetic drift refers to changes in the composition of the gene pool due to chance. Genetic drift tends to be more pronounced in small populations. The founder effect is a more extreme case of genetic drift in which a small population of a species finds itself in reproductive isolation from other populations as a result of natural barriers, catastrophic events, or other bottlenecks that drastically and suddenly reduce the size of the population available for breeding. Because the breeding group is small, inbreeding, or mating between two genetically related individuals, may occur in later generations. Inbreeding encourages homozygosity, which increases the prevalence of both homozygous dominant and recessive genotypes. Ultimately, genetic drift, the founder effect, and inbreeding cause a reduction in genetic diversity, which is often the reason why a small population may have increased prevalence of certain traits and diseases. For example, branched-chain ketoacid dehydrogenase deficiency (also called maple syrup urine disease) is especially common in Mennonite communities this implies a common origin of the mutation, which may be a very small original population.
This loss of genetic variation may cause reduced fitness of the population, a condition known as inbreeding depression. On the opposite end of the spectrum, outbreeding or outcrossing, is the introduction of unrelated individuals into a breeding group. Theoretically, this could result in increased variation within a gene pool and increased fitness of the population.
MCAT Concept Check 12.2:
Before you move on, assess your understanding of the material with these questions.
1. What are the three main types of point mutations? What change occurs in each?
2. What are the two main types of frameshift mutations?
3. What are the three main types of chromosomal mutations that do NOT share their name with a type of frameshift mutation? What change occurs in each?
4. Why would genetic leakage in animals be rare prior to the last century?
5. Why is genetic drift more common in small populations? What relationship does this have to the founder effect?
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Evolution & Scoliosis
Evolution, as it pertains to biology, is a gradual modification of living things over time. This change happens through populations of organisms. Evolution is the most important concept in biology because all living things are by-products of evolution.
The process takes place through population genetics, when allele frequencies change. This can happen in numerous ways. These are known as the Four Forces of Evolution:
1. Mutations – When a new genetic variation is created in a gene pool.
2. Gene flow – When organisms move into or out of a population. Both the population they leave and the one they enter can change.
3. Genetic drift – A random change in allele frequencies that occurs in a small population.
4. Natural selection – When there are differences in fitnesses among members of a population. Some individuals will pass more genes to the next generation, which causes allele frequencies to change.
In this entry, I will be relating my project topic, Scoliosis, to the concept of evolution. I have three aspects to share as a result of my findings.
1. A Gene Associated Idiopathic Scoliosis Identified
During previous research, I came across a study by Japanese scientists that linked the cause of scoliosis with a genetic explanation. This experiment showed that a certain gene, GPR126 on chromosome 6, increases susceptibility to scoliosis when present. This is the first gene to ever be linked with scoliosis. More research is required in order to prove the connection.
This study of genes associated with scoliosis closely ties in with evolution, since evolution is a study of genes.
For more information on scoliosis and genes, please visit my “Scientific Articles” post: https://scoliosisandbiology.wordpress.com/2013/12/08/scientific-articles/ (Article #3)
2. Evolution Of The Human Spine
Scoliosis is a disorder of the human spine. If we look more closely at the evolution of the spine itself, we can see why abnormalities and disorders are actually quite common. According to an article on Sciencemag.org, “evolution doesn’t ‘design’ anything…it works slowly on the genes and traits it has on hand, to jerry-rig animals’ and humans body plans to changing habitats and demands.” The article discusses several issues of the human body’s anatomic design that can cause problems for some people. One example of this is the structure of the spine. The human spine, which is essentially the anchor of the body, has an “S” shape. Due to this natural curvature, any weight imposed on the spine (weight of head, pressure from carrying things, or impact sports such as gymnastics or football) can lead to problems, such as general back pain or slipped discs.
This article begs the question, could scoliosis be a by-product of poor evolutionary construction? Though the article doesn’t mention scoliosis, one theory on the cause or worsening of the abnormality is too much pressure placed on the spine (for example, young children carrying heavy backpacks), which causes a curvature.
3. Scoliosis In An Orangutan Experiment
I decided to look into further detail on the issue of scoliosis and evolution by researching if apes, who are in theory the ancestor of humans, ever develop the condition.
I came across an experiment done in the UK on scoliosis in an orangutan, a member of the great ape family. This orangutan was diagnosed with scoliosis, but in the conclusion of the study it was noted that the diagnosis was unlikely. From the background data, we can see that no case of scoliosis has ever been reported in an ape. Also the study found some factors that were atypical, including that the ape was male and the curve was short.
Although more research would be needed to confirm, we can see that scoliosis is not nearly as common in apes as it is in humans. This makes the connection to evolution that it was most likely something that occurred during the evolution of apes to human, providing more evidence to the second point made in this post.
Challenge 4: How do mutations cause viral evolution?
Mutations involve changes to the sequence of an organism’s genetic code. As you have learned, viruses typically mutate more rapidly than human cells do. This is because human cells have mechanisms to proofread the genome and also mechanisms to repair a sequence if an error is detected. Mutations can vary in severity from having zero consequence to majorly altering a protein and its function. Mutations can involve the substitution of one DNA base to another, a G for an A for instance. Or mutations can involve the insertion of additional DNA bases or the deletion of existing DNA bases. Once a mutation occurs, if it changes the function of a resulting protein, a virus or organism is then changed. Because cells and viruses interact with the environment or surrounding cells, this change is either going to give the mutated cell or virus an advantage, allowing it to thrive more easily in its environment, or will make it disadvantaged, making it more difficult to survive. This is a process called natural selection. If the mutation confers an advantage, the mutated sequence then spreads within a population and if the mutation confers a disadvantage, the mutated sequence dies out.
Consider the following scenarios (actual or hypothetical) and decide if the mutation in SARS-CoV-2 virus will be detected in an increasing portion of the population of viruses or will not be detected. Explain your answer choice.
Introduction to Part II: The biology of language evolution: anatomy, genetics and neurology
This article focuses on the evolution of language along with its anatomy, genetics, and neurology. The concepts of instinct and innateness are actually quite useful for describing behaviors that routinely characterize all members of species or at least all species members of specific sex and age classes. Thus, they tend to be favored by scientists with a primary focus on the distinctive behaviors of individual species. To many developmental biologists and developmental psychologists, however, instinct and innateness are fallacious concepts because all behaviors develop through gene-environment interactions. The solution to this dilemma, in Fitch's view, is to abandon the terms “instinct” and “learning” in favor of other terms that more accurately describe the phenomena in question, such as “species-specific” or “species-typical” to describe behaviors routinely displayed by all members of a species, and “canalization” to explain the species-typical gene-environment interactions that produce behavioral regularities. From this perspective, language is a species-specific human behavior that is developmentally canalized via interactions of genes and predictable environmental impacts such as typical adult-infant interactions. In sum, evidence indicates that language evolution probably demanded changes in multiple interacting genes and involved expansions in multiple parts of the brain, as well as changes in the vocal tract and thoracic spinal cord.
Some of us have long assumed that instinct versus learning controversies met their demise back in the 1960s with publications such as ‘How an instinct is learned’ (Hailman 1969) and the insightful behavioural analyses of Robert Hinde (1966). Not so. In Fitch's view (Chapter 13), these controversies continue, both because they reflect interdisciplinary gaps and because of the tendency of scientists to black‐box issues not of their own immediate concern. Concepts of instinct and innateness are actually quite useful for describing behaviours that routinely characterize all members of species or at least all species members of specific sex and age classes. Thus, they tend to be favoured by scientists with a primary focus on the distinctive behaviours of individual species. To many developmental biologists and developmental psychologists, (p. 134) however, instinct and innateness are fallacious concepts because all behaviours develop through gene–environment interactions. The solution to this dilemma, in Fitch's view, is to abandon the terms ‘instinct’ and ‘learning’ in favour of other terms that more accurately describe the phenomena in question, such as ‘species‐specific’ or ‘species‐typical’ to describe behaviours routinely displayed by all members of a species, and ‘canalization’ (Waddington 1942) to explain the species‐typical gene–environment interactions that produce behavioural regularities. From this perspective, language is a species‐specific human behaviour that is developmentally canalized via interactions of genes and predictable environmental impacts such as typical adult–infant interactions.
Although all animals display species‐specific behaviours, most also exhibit behavioural plasticity in response to learning and/or in response to environmental conditions that may directly impact on brain development or physiological status (West‐Eberhard 2003). Some animals can even, if subject to unusual rearing conditions, develop behaviours not considered typical of their species. Great apes reared in human homes or subject to language‐training experiments, for example, develop a number of behaviours not found in wild apes. In other words, dissimilar phenotypes (i.e. observable behaviours and characteristics) can develop from similar genotypes (i.e. genetic endowment), a phenomenon termed phenotypic plasticity. As Számadó and Szathmáry (Chapter 14) note, phenotypic plasticity plays important evolutionary roles. Specifically, those phenotypes which prove adaptive and the genes that facilitate their development are subject to positive selection, hence, increase in the population (see also West‐Eberhard 2003). Ultimately, these phenotypes may become fixed in the population (Baldwin effect Baldwin 1902). If the genes producing them also become fixed, genetic assimilation will have occurred (Waddington 1953).
Each species occupies physical environments that can change in response to numerous external events such as climate change, earthquakes, or volcanic eruptions. Species, however, also modify and create their own environments, and hence the selective pressures that impinge upon them, a process termed niche construction (Odling‐Smee et al. 2003). Although external environmental events have undoubtedly influenced human evolution, niche construction has arguably played an even greater role in shaping the selective forces that help mould the modern human mind, and perhaps the human body as well, because our lineage has repeatedly created and adapted to new technological, cultural, and linguistic environments. It is sometimes thought that genetic change is too slow for our genes and brain to have adapted to selective pressures posed by ever‐changing languages and cultures. Számadó and Szathmáry counter this argument by presenting numerous examples of rapid genetic change in humans and other species. They also argue that the pace of language change, like technological change, was probably considerably slower during Pleistocene times than it is today. The result of the combined processes of potentially rapid genetic change and an earlier, (p. 135) somewhat slower, pace of language change is that genes, languages, and the brain have co‐evolved, and to some extent may be continuing to do so. On the one hand, genes and brains enable language on the other, language change selects for further, linguistically‐conducive, changes in genes and brains.
12.1 Developmental plasticity and genes
Számadó and Szathmáry (Chapter 14) also suggest that some biological systems, such as the immune system, are specifically adapted to enable rapid responses to environmental change. They suggest, for example, that the brain has been specifically shaped by selection to function as a rapid responder to linguistic change (and we would add cultural change as well). This postulate draws clear support, not only from our species' well‐recognized learning and problem‐solving capacities, but also from the plasticity that characterizes all developing and mature mammalian brains. First, during early developmental periods, all mammalian brains routinely overproduce neurons those neurons that fail to achieve full functionality are subsequently pruned (Edelman 1987). In humans, neuronal production primarily occurs prenatally, as does much neuronal pruning. Similarly, all mammalian brains overproduce synapses during certain periods of development. Again, those that fail to achieve full functionality are later pruned. Our species typically overproduces synapses in the first several postnatal years and again just prior to puberty. One unexpected result is that the typical human adolescent has more synapses than most adults, at least in the frontal lobes (Blakemore and Choudhury 2006). Although the production and pruning of neurons and synapses is primarily a maturational phenomenon, these processes never truly cease. New cortical synapses continue to be produced and pruned throughout life, and a region of the brain concerned with declarative and episodic memory (Zito and Svoboda 2002), the hippocampus, continues to produce new neurons throughout life (Eriksson et al. 1998).
These processes have demonstrable functional effects. For example, in rats, final adult brain size as well as performance on laboratory learning exercises varies depending on experience during the maturational process (Bennett et al. 1964). Similarly, humans who practise particular skills such as piano‐playing or taxi‐driving develop enlarged neural structures pertinent to those tasks (Amunts et al. 1997 Maguire et al. 2000). Language‐related functional reorganizations are also known to occur in humans in relationship to environmental inputs. For example, in congenitally deaf subjects who master sign language at a young age, regions of the temporal lobe that normally mediate auditory functions become more attuned (p. 136) to visual input, including visual gestures (Neville 1991). Similarly, the visual neocortex of congenitally blind subjects assumes tactile functions, if such individuals master Braille at a young age (Sadato et al. 1998). Even literacy changes brain functions, and may, in fact, sharpen the neural perception of phonemes (Dehaene et al. 2010). Recognition of the environmentally‐induced developmental plasticity of mammalian brains helps explain why chimpanzees, bonobos, and other apes, reared from infancy in human homes, can, within limits, develop protolanguage‐like behaviours, whereas wild apes and/or apes captured in adulthood usually cannot.
Brain plasticity, of course, has its limits. All brains of a given species strongly resemble each other in overall structure and function. This must reflect considerable genetic programming. As Számadó and Szathmáry note, numerous genes impact on brain development, and these genes appear to evolve at a rapid pace, thereby potentially impacting rapid evolutionary changes in behaviour. Diller and Cann (Chapter 15) focus on specific genes thought to influence the evolution of language and the brain. FOXP2, a regulatory gene, helps determine when and where other genes are expressed. In humans, certain FOXP2 mutations produce orofacial dyspraxia (possibly by disrupting motor sequencing behaviours), some language deficits, and mal‐development of several neural structures (Lai et al. 2003). In other animals, depending on the species, FOXP2 may exhibit increased or decreased activity during periods of vocal learning. Hence, although no evidence indicates that FOXP2 directly controls for vocal behaviour, the gene does, apparently, impact on the development and functions of neural structures that do. Specific human mutations in the FOXP2 gene were once thought to have occurred in the last 120,000 years. Re‐evaluations of the genetic data now suggest a much earlier date of about 1.8 to 1.9 million years ago (Diller and Cann, Chapter 15).
Diller and Cann also review variants of two additional genes that, when mutated in modern humans, result in microcephaly (microcephalin and ASPM). Dysfunctional mutations in these genes result in abnormally small brains. Hence, it has been suggested that both played key roles in the evolutionary enlargement of the brain. Brain development, however, is a complex process involving hundreds, possibly thousands, of genes. Functional disruptions in any of these can cause developmental neural pathologies. This does not mean that earlier, different mutations in the same genes caused increased brain size, only that normal, fully‐functional genes are needed for brain development. Other evidence cited by Diller and Cann, however, indicates that certain variants of ASPM and microcephalin have increased in frequency in the last 37,000 and 5800 years respectively. Some have interpreted this to mean that these genes are currently experiencing positive selection for their roles in brain function or development, but after reanalysing the data, Diller and Cann conclude that the increased gene frequencies could equally well represent genetic drift. In their view, in‐depth analysis also fails to support reports of correlations between the distribution of these genes and tonal (p. 137) languages. Ultimately, Diller and Cann conclude that language evolution is likely to have resulted from interactions of a multiplicity of genes, rather than from a single mutation in a ‘magic’ language gene. In sum, despite increasing research in this area, our understandings of the genetic basis of language and of human‐specific neural developmental pathways remain vague.
Even though human children speak in full sentences by the time they are about 2½ years old, most research on the neurological basis of language focuses on the anatomy of the adult brain and, then, mostly on brain size or on the anatomy of neocortical structures, some of which reach full functionality only in adolescence or later. Brain size, both absolute and relative to body size, did increase steadily from about 2,000,000 to 300,000 years ago (Mann, Chapter 26). It is likely that these size increases were functionally adaptive otherwise, they would have been selected against. Large brains, after all, are metabolically expensive (Aiello and Wheeler 1995). Specific language‐related neural structures have also increased in size in human evolution, as delineated in a number of the chapters in this section. Consequently, increased brain size almost certainly contributed to the evolution of language. However, no one‐to‐one correlation exists between language and overall brain size, and no specific brain size Rubicon separates the linguistically capable from the linguistically inept. Indeed, given that microcephalics do not entirely lack linguistic abilities (Diller and Cann, Chapter 15) it is clear that overall brain size is not the sole determinant of language capacity.
Most investigators have worked on the assumption that language evolution primarily involved the neocortex, either the differential expansion of neocortical areas and connections already present in non‐human primates and/or the addition of new neocortical structures. Gibson (Chapter 16) takes a somewhat different stance. Following on from Gibson and Jessee (1999) and P. Lieberman (1991, 2000, 2002), she reminds us that lesions in structures such as the cerebellum and basal ganglia often produce speech and language deficits. These areas have greatly expanded in human evolution and they mature earlier than many areas of the neocortex (Gibson 1991). In addition, a greater percentage of descending cortical fibres terminate directly on brainstem and spinal cord motor neurons in humans than in monkeys and apes, providing for finer control of lip, tongue, and finger movements (Kuypers 1958). Consequently, neural areas and connections other than those confined to the neocortex deserve far greater scrutiny from the language origins community.
(p. 138) Donald (Chapter 17) also emphasizes the role of neural circuitry involving the basal ganglia, cerebellum, and neocortical areas (especially the premotor and dorsolateral prefrontal cortex). These circuits enable procedural learning, that is, the acquisition of motor skills that require much practice, including those needed for mimesis and tool‐making, both of which, in his view, preceded language evolutionarily (see also Arbib, Chapter 20). Donald further notes that although mimesis is, in large part, a sensorimotor function, the social contexts in which it is used are amodal. Hence, once mimesis became an integral part of human behaviour, through, for example, mime, it would have selected for enhanced amodal cognitive capacities, such as those needed for language and mediated by the inferior parietal and frontal lobes (see also Wilkins, Chapter 19).
Most vertebrate brains exhibit functional lateralization (Rogers and Andrew 2002). In humans, the left hemisphere controls the right arm and hand and, as Hopkins and Vauclair (Chapter 18) note, it is also dominant for language and speech in 96% of right‐handed and 70% of left‐handed individuals. Although it has long been known that individual primates prefer specific hands, until recently it was assumed that population‐wide preferences for the right hand were a uniquely human trait. Indeed, the coincidence of two left hemisphere‐controlled, largely species‐specific human behaviours (right‐handedness and language) has led to hypotheses that cerebral lateralization, language, and right‐handedness evolved together in a causally interconnected manner (Corballis 1993 Crow 2004). Such views long received support from studies indicating that monkeys and apes fail to display population‐level handedness in simple manual reaching tasks. More recently, however, Hopkins' group has found population‐wide right‐handedness in captive chimpanzees, when they were tested on complex manipulative tasks requiring that an object be held in one hand and manipulated in the other (Hopkins 1995).
In Chapter 18, Hopkins and Vauclair also report that captive chimpanzees, bonobos, gorillas, and baboons all exhibit population‐wide biases for the use of the right hand for communicative gestures. In contrast, judging by asymmetrical facial expressions, the majority of vocalizations and facial expressions in non‐human primates are controlled by the right hemisphere. The few exceptions, controlled by the left hemisphere, include marmoset twitters and the novel raspberry sounds and extended food grunts made by some captive chimpanzees. Hence, lateralization in non‐human primates may be greater for communicative gestures than for manipulative behaviours, and for voluntary, as opposed to emotional, vocalizations. These findings suggest that left‐hemisphere dominance for speech and language may have been preceded evolutionarily by left‐hemisphere dominance for voluntary gestures and vocalizations in other primates.
In humans, language and handedness are usually thought to be accompanied by differential expansion of some left‐hemisphere areas, in comparison to similar areas on the right. A literature review by Hopkins and Vauclair finds that Broca's (p. 139) area is somewhat inconsistently expanded in the left hemisphere in both apes and humans. In contrast, the left temporal plane is usually expanded not only in humans, but in apes as well. The left Sylvian fissure is also somewhat longer than the right in both apes and humans. This fissure, which separates the temporal lobe from the parietal and frontal lobes, is surrounded by neocortical areas known to have language functions. In sum, anatomical and behavioural data indicate that neural asymmetry is neither unique to humans nor a specific language specialization. That great apes and monkeys exhibit greater lateralization with respect to gestural usage and voluntary vocalizations may, however, provide clues to possible behavioural precursors to speech and language.
Wilkins (Chapter 19) addresses the anatomy and functions of Broca's area, the POT (parieto‐occipito‐temporal junction), the inferior parietal lobe, and tracts that interconnect these areas. Since one of her aims is the delineation of ape/human neural differences potentially visible in the fossil record, her primary focus is on those species differences that can be seen on the external surface of the brain. This is a critical point, because historically three different parameters have been used to identify neural regions: external anatomy, internal cellular architecture (cytoarchitecture), and function. The three do not always provide identical results. For example, a Broca's area homologue was identified in monkey and ape brains via cytoarchitecture as early as the 1940s (von Bonin and Bailey 1947 Krieg 1954), but most investigators continued to insist, based on external morphology, that Broca's area was unique to humans until the discovery, in the 1990s, of mirror neurons, in what many now accept as the monkey homologue of Broca's area (see Arbib, Chapter 20). Similarly, rhesus monkey and chimpanzee brains contain cytoarchitectonic areas that these earlier neuroanatomists considered homologous to the human POT. Externally, however, the anatomy of the POT region is quite different in apes and humans. In apes, the lunate sulcus separates the occipital lobe from the parietal and temporal lobes, while all three lobes merge in the human brain. Wilkins accepts that much of the parietal cortex and Broca's area have homologues in monkey and ape brains, but she considers the POT to be uniquely human. Nonetheless, she concludes from fossil evidence that the POT evolved early in our lineage, prior to speech. These findings present an evolutionary quandary. The so‐called language areas of the human brain apparently evolved long prior to language. From this, she concludes that language evolution involved exaptation, that is, the re‐appropriation of pre‐existing functions to new uses.
For the most part, Wilkins focuses on the spatial functions of the parietal lobes and their interactions with Broca's and other motor areas in the frontal lobe. Specifically, she notes that that the human POT plays an active role in the formation of modality‐free conceptual structures (see also Coolidge and Wynn, Chapter 21 Donald, Chapter 17). In her view, these functions represent a natural expansion of primate posterior parietal lobe functions, which include the construction of modality‐neutral spatial concepts and the spatial orientation of arm and hand (p. 140) movements. In non‐human primates, these posterior parietal functions are coordinated with motor functions of the frontal lobes to produce object‐related actions. She hypothesizes that the expansion (or emergence) of the POT permitted the enhanced spatial analyses required for the coordination of arm, hand, and thumb movements with respect to tool use and throwing (see also Calvin 1985). Since many linguistic structures are spatially and thematically organized, POT expansion also provided the necessary conceptual structure for critical components of the language function hence, in her view, spatial skills that developed initially in tool‐using situations were later co‐opted for language.
Arbib (Chapter 20) also pursues issues of primate/human neural homologues and neural exaptations. He accepts that the human Broca's area is homologous with similar areas in monkeys and apes, but notes that it has no obvious vocal functions in other species. In contrast, neural areas that do mediate primate vocalizations have no known linguistic role in the human brain. Rather, he posits that mirror neurons found in Broca's area of non‐human primates served as the foundation stones upon which imitation, gesture, and language were built. These neurons fire when a monkey executes a particular manual action and when it observes another individual performing the same action. Mirror neurons thus provide an essential language function—parity that is, assuring that communicator and recipient have similar perceptions. In Arbib's view, mirror neurons serve as essential components of language and imitation, but are not, by themselves, sufficient to mediate behaviours which require the hierarchical integration of multiple actions and concepts. Since the earliest mirror neurons to be identified were related to manual actions, Arbib adopts a gestural model of language origins and delineates how such a system may have evolved. More recent research indicates that mirror neurons are also found in the inferior parietal lobe may represent oral movements as well as manual and may also be of an audiovisual nature. Hence, the mirror neuron story continues to unfold.
Coolidge and Wynn (Chapter 21) focus on the neurological and cognitive correlates of indirect speech, that is, intentionally ambiguous utterances that must be interpreted with regard to social context and that are used primarily in situations that require diplomacy. In their view, indirect speech requires working memory, executive control structures, and theory of mind. Working memory, in turn, is composed of phonological storage capacity, a visual spatial sketchpad and an episodic buffer which allows the contents of phonological storage and the visuospatial sketchpad to be simultaneously held in conscious thought, manipulated, and combined and recombined with respect to each other. Hence, it facilitates the construction of complex plans and mental models. Executive functions monitor these activities via selective inhibition and attention. Since indirect speech is a product of multiple interacting cognitive components, it must also be a product of multiple interacting regions of the brain. In particular, Coolidge and (p. 141) Wynn note the involvement of the inferior parietal and superior temporal lobes and the dorsolateral frontal cortex.
Taken as a whole, the chapters in this section indicate that nearly all higher neural processing centres play some role in the mediation of language or speech. The complexity of the neural interactions required for indirect speech, in particular, suggests that whatever neural changes may have been needed to initiate and sustain protolanguage and/or the language of human infants, fully developed ‘diplomatic’ language capacities reflect the interactions of much of the neocortex. In her paper on ape language (Chapter 3), Gibson notes that great apes fall short of humans in their ability to construct linguistic, technical, and other hierarchies, and it is widely accepted that many aspects of language, most strikingly syntax, are hierarchically structured. Consequently, it would seem of prime interest to determine which neural areas mediate hierarchical abilities. Greenfield (1991) assigned that role to Broca's area. Arbib (Chapter 20) follows her lead. Wilkins (Chapter 19) remarks that the POT is structured to automatically create mental hierarchies. Coolidge and Wynn (Chapter 21) do not use the term ‘hierarchical’, but they do suggest that the ability to hold multiple images in mind in order to combine and recombine them is mediated by the dorsolateral frontal lobe. That the various authors in this section assigned hierarchical processing or components thereof to different parts of the neocortex would appear to validate earlier suggestions by Gibson that the creation of linguistic and other hierarchies, like indirect speech, requires the interactions of multiple cortical processing areas (Gibson 1996a Gibson and Jessee 1999).
Although changes in brain function almost certainly played central roles in language evolution, as MacLarnon notes (Chapter 22), other critical anatomical changes occurred as well. For example, the larynx is lower in humans than in apes and the oral cavity is differently structured. Together, these changes allow humans to produce sounds not readily produced by apes. This vocal tract reorganization may have been facilitated by bipedalism, diet, or a combination of the two. In addition, humans have far greater neural control of their breathing than do apes, and, unlike apes, they have no laryngeal air sacs. MacLarnon suggests that increased control of the respiratory apparatus involved expansion of the numbers of neurons in the thoracic spinal cord. While it is impossible to determine laryngeal position from fossils, the very few hyoid bones that have been found suggest that modern human laryngeal structure, including absent air sacs, may have been (p. 142) present in the common ancestor of Neanderthals and anatomically modern humans, but not in australopithecines. Similarly, fossil vertebrae indicate that both Neanderthals and anatomically modern humans had achieved the modern size of the thoracic spinal cord, but Homo erectus had not (see Wood and Bauernfeind, Chapter 25, for a contrary view on the thoracic cord).
In sum, evidence indicates that language evolution probably demanded changes in multiple interacting genes and involved expansions in multiple parts of the brain, as well as changes in the vocal tract and thoracic spinal cord. Given our current understandings of exaptation, niche construction, the Baldwin effect, and neural plasticity, neural changes probably built upon precursor neurobehavioural functions in non‐human primates and occurred over a lengthy period of time. In some cases both neural and vocal changes may have occurred in response to the selective pressures exerted by language and culture, as opposed to strictly external environmental circumstances see also Bickerton (2009a). Nothing that we know about genetic or neural functions would suggest that language arose in response to a sudden mutation or the sudden appearance of a new neural module.
Kathleen R. Gibson is Professor Emerita, Neurobiology and Anatomy, University of Texas Houston. Her co-edited books include, with Sue T. Parker, Language' and Intelligence in Monkeys and Apes (CUP 1990) with Tim Ingold, Tools, Language, and Cognition in Human Evolution (CUP 1993) with Paul Mellars, Modelling the Early Human Mind (McDonald Archaeological Institute 1996) and, with Dean Falk, Evolutionary Anatomy of the Human Neocortex (CUP 2001). She is the co-editor with James R. Hurford of the series, Oxford Studies in the Evolution of Language.
Maggie Tallerman has spent her professional life in northeast England, at Durham then Newcastle University, where she is currently Professor of Linguistics. Her edited and authored books include Language origins: Perspectives on evolution (OUP, 2005), Understanding syntax (Hodder/OUP, third edition 2001), and The syntax of Welsh (co-authored with Borsley and Willis CUP, 2007). She started working on evolutionary linguistics in case a guy on a train asked her where lanugage came from, though some think her real work is on Welsh.
7.3 Does sex lead to fewer mutations?
Mutations are a fact of life. All organisms possess DNA that is different from the DNA they inherited, and this altered DNA, or mutation, is the ultimate source of all genetic variation. Some have suggested that one of the benefits of sex is its ability to rid the body of harmful (or deleterious) mutations.
Sex and DNA repair
Mutations arise from random errors in DNA replication: by the insertion or deletion of mobile genetic elements, or through the effects of various mutagens (e.g., chemical compounds or radiation) that damage DNA. However, most of the damage that occurs to a cell’s DNA is repaired. The cell has numerous tools that it can use to fix the damage that can occur.
Mutagens such as radiation can cause single- and double-strand breaks in the double helix of DNA. If there is damage to only one DNA strand, the other strand can be used as a template for repair. More problematic is when both strands of DNA break when this happens there is not a simple way to re-synthesize the lost DNA. In cells that are diploid (cells that have two copies of each chromosome), there is a complementary chromosome, or homologue, available–but in order to be useful, it must be close to the damaged DNA.
Chromosome pairs are close together in the first stage of meiosis, in which crossing over between the chromosomes occurs. In fact, many of the cellular tools used to repair double strand breaks in DNA are the same ones that facilitate crossing over (homologous recombination) during meiosis. So, if meiosis repairs damaged DNA, and meiosis is essential to sexual reproduction, perhaps sex exists because of the repair benefits of meiosis itself.
Sex and Ratchets
A ratchet works because it rotates in one direction, but not the other. Similarly, once a mutation occurs within a population, it is extremely unlikely to un-occur. Because mutations are relatively rare and occur randomly in an organism’s genome, there is very low probability that there will be a “back mutation” or a second mutation in the exact same spot to undo the first mutation. Thus, over time, a lineage will likely accumulate increasing numbers of mutations, some of them harmful. This idea is often called “Muller’s ratchet,” after Hermann Joseph Muller, who hypothesized it, and the simple machine, the ratchet (figure 7.5).
Figure 7.5 The ratchet works because it only rotates in one direction. This comes in handy when hoisting heavy objects, tightening screws, and loosening bolts. But what could this possibly have to do with sex?
According to Muller’s ratchet hypothesis for the existence of sex, sexual reproduction is better able to eliminate harmful mutations from the genome. Creating a variety of gametes and combining those gametes with another individual (who would presumably have a different collection of mutations) results in some of the offspring carrying more harmful mutations than others. Presumably, the individuals or gametes with fewer harmful mutations survive and reproduce more successfully. In this way, through sex, a lineage has an opportunity to shuffle its genetic material, and produce some offspring with fewer harmful mutations (and the most beneficial ones). Thus, the shuffling of material that occurs during sexual reproduction essentially rotates the ratchet backwards.
Evidence supporting Muller’s ratchet
A testable prediction associated with Muller’s ratchet is: Mutations accumulate more rapidly in asexually reproducing organisms than in sexual organisms. To investigate this hypothesis, scientists study organisms that occur in both sexual and asexual forms. One such animal is the microscopic water flea (Daphnia figure 7.6), which can be maintained in clonal or sexual lineages.
A comparison of strains of water fleas that reproduce asexually with those that occasionally have sex revealed that the asexual strains have more mutations. Specifically, the asexual water fleas have a higher proportion of substitution mutations that result in amino acid changes. This comparison supports the hypothesis that sexual reproduction reduces the accumulation of potentially harmful (or deleterious) mutations. Similar observations have been made in sexual and asexual lineages of freshwater snails.
Mutations Make Evolution Irreversible: By Resurrecting Ancient Proteins, Researchers Find That Evolution Can Only Go Forward
A University of Oregon research team has found that evolution can never go backwards, because the paths to the genes once present in our ancestors are forever blocked. The findings -- the result of the first rigorous study of reverse evolution at the molecular level -- appear in the Sept. 24 issue of Nature.
The team used computational reconstruction of ancestral gene sequences, DNA synthesis, protein engineering and X-ray crystallography to resurrect and manipulate the gene for a key hormone receptor as it existed in our earliest vertebrate ancestors more than 400 million years ago. They found that over a rapid period of time, five random mutations made subtle modifications in the protein's structure that were utterly incompatible with the receptor's primordial form.
The discovery of evolutionary bridge burning implies that today's versions of life on Earth may be neither ideal nor inevitable, said Joe Thornton, a professor in the UO's Center for Ecology and Evolutionary Biology and the Howard Hughes Medical Institute.
"Evolutionary biologists have long been fascinated by whether evolution can go backwards," Thornton said, "but the issue has remained unresolved because we seldom know exactly what features our ancestors had, or the mechanisms by which they evolved into their modern forms. We solved those problems by studying the problem at the molecular level, where we can resurrect ancestral proteins as they existed long ago and use molecular manipulations to dissect the evolutionary process in both forward and reverse directions."
Thornton's team, which included UO research scientist Jamie Bridgham and collaborator Eric A. Ortlund, a biochemist at Atlanta's Emory University, focused on the evolution of a protein called the glucocorticoid receptor (GR), which binds the hormone cortisol and regulates the stress response, immunity, metabolism and behavior in humans and other vertebrates.
"This fascinating study highlights the value of studying evolutionary processes," said Irene Eckstrand, who oversees evolution grants at the National Institutes of Health's National Institute of General Medical Sciences. "By showing how molecular structures are finely tuned by evolution, Dr. Thornton's research will have a broad impact on basic and applied sciences, including the design of drugs that target specific proteins."
In previous work, Thornton's group showed that the first GR evolved more than 400 millions ago from an ancestral protein that was also sensitive to the hormone aldosterone. They then identified seven ancient mutations that together caused the receptor to evolve its new specificity for cortisol.
Once Thornton's team knew how the GR's modern function evolved, they wondered if it could be returned to its ancestral function. So they resurrected the GR as it existed soon after cortisol specificity first evolved -- in the common ancestor of humans and all other vertebrates with bones -- and then reversed the seven key mutations by manipulating its DNA sequence.
'We expected to get a promiscuous receptor just like the GR's ancestor, but instead we got a completely dead, non-functional protein," Thornton said. "Apparently other mutations that occurred during early GR evolution acted as a sort of evolutionary ratchet, rendering the protein unable to tolerate the ancestral features that had existed just a short time earlier."
To identify the mutations, Thornton's team prepared crystals of resurrected ancient GR proteins and took them to the particle accelerator at the Advanced Photon Source outside Chicago, where they used powerful X-rays to determine the protein's atomic structure before and after the shift in function. By comparing the precise atomic maps of each protein, they identified five specific mutations in the later version of the GR that clashed with the architecture of the earlier protein.
"Suppose you're redecorating your bedroom -- first you move the bed, then you put the dresser where the bed used to be," Thornton said. "If you decide you want to move the bed back, you can't do it unless you get that dresser out of the way first. The restrictive mutations in the GR prevented evolutionary reversal in the same way."
When Thornton's group set the five mutations back to their ancestral state, the protein could now tolerate having the seven key changes reversed, which then transformed it into a promiscuous receptor just like the its ancestor.
Despite their powerful role as a ratchet preventing reversal, the five restrictive mutations had little or no direct effect on the protein's function when they occurred. And although they must be reversed before the protein can tolerate the ancestral state, reversing them first does absolutely nothing to enhance the ancestral function. "This means that even if the ancestral function were suddenly to become optimal again, there's no way natural selection could drive the protein directly back to its ancestral form," Thornton said.
GR's evolutionary irreversibility suggests that the molecules that drive our biology today may not be inevitable products of the evolutionary process. "In the GR's case, restrictive mutations erased the conditions that previously opened up the ancestral form as an evolutionary possibility. It's likely that throughout history other kinds of restrictive mutations have taken place, closing off innumerable trajectories that evolution might otherwise have taken," Thornton speculated.
"If we could wind back the clock and allow history to unfold again, different sets of mutations, apparently inconsequential at the time, would almost certainly occur, opening up some potential paths and blocking others -- including the one that leads to the present that actually evolved in our world," he said. "If what we observed in GR evolution is a general phenomenon, then the biology we have is just one of many possible rolls of the evolutionary dice."
The National Institutes of Health, the National Science Foundation and the Howard Hughes Medical Institute supported the research.
Materials provided by University of Oregon. Note: Content may be edited for style and length.
12.4: Mutations and Evolution - Biology
Mutation is a change in DNA, the hereditary material of life. An organism's DNA affects how it looks, how it behaves, and its physiology all aspects of its life. So a change in an organism's DNA can cause changes in all aspects of its life.
Mutations are random
Mutations can be beneficial, neutral, or harmful for the organism, but mutations do not "try" to supply what the organism "needs." In this respect, mutations are random whether a particular mutation happens or not is unrelated to how useful that mutation would be.
Not all mutations matter to evolution
Since all cells in our body contain DNA, there are lots of places for mutations to occur however, not all mutations matter for evolution. Somatic mutations occur in non-reproductive cells and won't be passed onto offspring.
For example, the golden color on half of this Red Delicious apple was caused by a somatic mutation. The seeds of this apple do not carry the mutation.
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Usually, a mutation has to be expressed as some macro-functional characteristic although some may be hidden in internal systems.
Usually there are three ways in which a mutation can affect a trait of an animal mutation effects can be
A) neutral, that is, the mutation doesn't cause any change
B) negative, where the change caused by the mutation can be detrimental for the animal
C) positive, where the change caused by the mutation can give an advantage to the animal.
The last two effects play a major role in evolution when the mutation causes a positive effect, there is an increased probability that the animal (or the plant) will reproduce and pass it to their offspring (their fitness is increased). When the mutation has a negative effect, the probability of survival and reproduction of the animal is reduced (their fitness is reduced), and consequently it will be likely that this mutation will be negatively selected, thus disappearing from the population.