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Why aren't animals diverse in phenotype?

Why aren't animals diverse in phenotype?


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I am not comparing a cat with leopard.

I am just saying that we humans are all one type of creature and we are diverse (I am not saying we are class of mammals and phylum of etc and kingdom etc, because my religion doesn't believe in it).

So consider the class of cats they are one type of species so why aren't they diverse in phenotype like us?

Why do other animals, plants, unicellular organisms not have diversity in their phenotype and how they can recognize each other, like a bird always brings food to his offspring and it can't make mistake by giving it to other offspring of its own species?

So can I say they aren't diverse because of in their meiosis division their chromosomes don't cross over and random assort (alignment)?


In your question, your assumption that animal species are less diverse phenotypically than humans is wrong. I am sure you will appreciate @terdon's answer to this post and @rg255 answer to this post.

Don't forget that we are good at detecting differences among humans (because we evolved for this purpose). We are doing much worse at telling apart animals from other species just because we have not evolved for this purpose. This is the reason why we tend to see human faces when looking at clouds but we rarely see sheep faces! Several studies (here and here) showed that sheep are able to recognize each other (and we even know the number of neurones needed to remember one face). They are probably better at telling two sheep apart than telling two human apart.

Another interesting fact is the so-called cross-race effect. We, humans, are better at recognizing faces of people from our own ethnic group than faces of people from other ethnic groups. For example, a Japanese is very good at Japanese faces recognition but not good at recognizing European faces. Same is true the other way around.

As @user568459 said in the comments: some people are not able to recognize faces. This is due to a cognitive disease called Prosopagnosia (also called face blindness). Those suffering from this disease are not better at recognizing sheep faces than human faces.


So consider the class of cats they are one type of species so why aren't they diverse in phenotype like us?

There is no good definition I think of what is phenotypic diversity (no accurate and objective index to measure it) but at first sight I would tend to think that cats are more diverse than humans. One of the main features one would probably raise when talking about human diversity is skin color. And in terms of color, cats are much more diverse than humans. You may think of an extraordinary diversity when thinking of Norwegian that are taller than Indonesian (I may not have chosen the two extremes) by several centimeters on average but think about cats! The average cat weight 4 to 5 kg but some cats weight less than 2 kg and some other (like the coon cat) weight more than 10 kg (World Record: 21.3 kg). Imagine a human ethnic group that would on average weight 5 times more than another ethnic group! And think also about cats' hair length or tail shape! Humans vary in terms of facial feature (lips size, nose shape, etc.) so do cats. Some look like their face was smashed against a wall while others have a long muffle. Again I welcome you to have a look to this post.

how they can know each like a bird always brings food to his offspring and it can't make mistake by giving it to other offspring of it's own species?

As I said above humans evolved to recognize their own. Many species also evolved in order to recognize their own. In some species individuals use smell rather than visual features in order to recognize each other (odor is also a kind of phenotypic variation). But still some species are poor to recognize each other. For a bird, it seems rather easy to not feed the wrong individual as all their offspring are usually together in the same nest. However you might be interested the lifestyle of the cuckoo who parasites nests of other bird species. Cuckoos' babies and particularly the inner beak resemble to the babies of the species they parasite and often the parents (often the mother only is involved in feeding the young) get fooled and feed the cuckoo.

So can i say they aren't diverse because of in their meiosis division their chromosomes don't cross over and random assort (alignment)?

No, you can't say that! Because they are diverse and because for many of the species you may think about, cross-over does occur. Their genetic diversity as well as their phenotypic diversity is as high than in humans. There is nothing extraordinary about humans (except their brain and the related fact that we predigest our food by cooking it) compare to other lineages. And there is nothing extraordinary to have one extraordinary feature (such as a big brain) that you can't find in other lineages! Many lineages are extraordinary in some sense.


I'd like to add that the variations in individuals within a species are a fundamental observation upon which modern biology is standing. Darwin wrote at least 2 chapters of Origin of Species demonstrating how animals and plants have a lot of individual variations:

Darwin's argument involved four steps. First, he noted the wide variation between many types of living organisms: between species of plant, fish, bird and mammal; and also between different family groups within the same species; and also between different exemplars within the same family. Wherever we look - and during his five-year voyage on The Beagle in his early twenties, there were few places on earth where Darwin had not looked - we see differences between organisms: different sizes, different colours, different features, different behaviours and, of course, different survival rates. The theory of natural selection takes variation of species and within species as its starting point.

Before that era (he was not the only one who made this argument) pretty much everyone believed that all animals of a given species were the same.

It took quite a while before everyone was convinced, but an entire generation of scientists was won over.

If you need a modern confirmation of this, look into cattle breeding. Individual phenotypes make some bulls worth many thousands of dollars/euros. Others nothing. Lastly, now that we understand that the basis of inheritance is in the DNA, we can quantitate the specific mutation rates in various genes and segments of DNA and show that they are comparable from species to species. These vary somewhat, but humans are not exceptional.


The Institute for Creation Research

God gifted His living creatures with the ability to adapt to new or changing environments. Genetic diversity in adaptation refers to variation within created kinds of organisms. For example, consider the wide variety of dogs&mdashthey come in all shapes, colors, and sizes. Humans also exhibit a large amount of variation. Observable variation in the appearance of different kinds of creatures is referred to as phenotype. Phenotypic diversity is largely based on an organism&rsquos genetic makeup (genome). The genome exhibits variation in DNA sequence called genetic diversity.

Genetic diversity is an important feature of adaptation, as evidenced by the fact that animals experience the accumulation and expression of harmful mutations during inbreeding (mating of close relatives). Inbreeding lowers the genetic diversity in a population and makes the creatures less robust and adaptable. Even among some types of plants that have self-fertilizing flowers, significant levels of out-crossing&mdashwhere pollen is transferred via wind, insects, etc.&mdashstill occur and contribute to the enhancement of genetic diversity.

Genetic diversity is related to different parts of an organism&rsquos genome. When genomes are compared within created kinds, certain portions are very stable and remain very similar among individuals, while other parts of the genome are extremely variable. Clearly, genetic variability is part of God&rsquos design for plants and animals, but it is employed as an engineered system with limitations. These systems of genetic variability are just beginning to be understood they involve not only diversity in actual DNA sequence, but also diversity in heritable chemical modifications to the DNA (methylation) and in the proteins that package the DNA (acetylation). This type of heritable variation is called epigenetic modification. It does not actually change the base sequence of DNA, but influences its function and adds another important aspect to genetic variation.

The difference between simple traits and multigenic inheritance associated with complex traits has caused some confusion among creationists. Simple inheritance generally refers to traits largely controlled by one or just a few regions in the genome. Examples of this type of inheritance include things like eye color, hair color, etc. A recent creationist article on coat color in deer shows how this type of variability functions in nature. 1

However, as discussed in the previous article in this series, 2 most expressed traits are related to adaptations associated with biologically complex responses. These adaptations involve networks of many genes, referred to as quantitative traits, and are studied by complex DNA mapping experiments across multiple environments. For this type of data, complicated statistical models are employed they enable the identification of multiple genomic regions and the percentage of variability that mapped points along chromosomes contribute to a certain trait.

Another question surrounding genetic variability is the type of genomic DNA sequence features underlying its function. A variety of creation scientists, including Jean Lightner, Todd Wood, Peter Borger, and others, have presented data and models involving the genetic diversification of created kinds via transposable elements and other types of non-protein-coding DNA. These sequences appear to offer the most opportunities for models of genetic diversity and the diversification of created kinds. Scientists have characterized these portions of the genome as containing an extremely rich storehouse of functional features that regulate many aspects of gene expression. 3

Biology researchers at ICR are currently reviewing creationist and secular literature on non-coding DNA to determine new venues of research into the field of genetic diversity and the role it plays in adaptation.

  1. Catchpoole, D. 2012. Dear deer: when white &lsquomutants&rsquo have a selective advantage. Creation. 34 (1): 28-31.
  2. Tomkins, J. 2012. Mechanisms of Adaptation in Biology: Molecular Cell Biology. Acts & Facts. 41 (4): 6.
  3. Shapiro, J. A. and R. von Sternberg. 2005. Why repetitive DNA is essential to genome function. Biological Reviews. 80 (2): 227-250.

* Dr. Tomkins is Research Associate at the Institute for Creation Research and received his Ph.D. in Genetics from Clemson University.

Cite this article: Tomkins, J. 2012. Mechanisms of Adaptation in Biology: Genetic Diversity. Acts & Facts. 41 (5): 8.


Why biodiversity is key to our survival

E. coli bacteria. Credit: Eric Erbe, digital colorisation by Christopher Pooley, USDA

Diversity, be it genetic, morphological, behavioural or ecological, is at the heart of many controversies. It fascinates us or worries us, depending on the context. But what is biological diversity? How useful is it, how is it generated and what are the foreseeable consequences of reducing it?

The incredible diversity of life

The life sciences have only recently begun to imagine the true extent of the diversity of life forms and the difficulty of quantifying it. Recent estimates of total eukaryotic diversity range from 1 to 5 × 10 7 species . Although only about ten thousand species of prokaryotes have been described, mainly because only a small number of bacteria can be grown in the laboratory, indirect molecular approaches (without culture) based on the analysis of DNA extracted from the environment suggest that there may be 109 or more prokaryotic species. However, even these already astronomical figures do not reflect the real diversity of life forms.

First, genotypic diversity within the same prokaryote species can be incredibly high. Members of one bacterial species share parts of their genome encoding essential metabolic and informational functions (called the core genomes), but often carry unique, strain-specific sequences for adaptation to local environmental pressures. In the case of the bacterium Escherichia coli, the core genome represents only 6% of the genes present in 61 sequenced strains.

Secondly, the phenotypic diversity of living forms is greater than their genotypic diversity. Biological entities may exhibit complex life cycles with multiple states of differentiation and display phenotypic plasticity. This can confer the capacity to anticipate predictable seasonal changes or react to unpredictable changes by remodelling physiological processes to compensate for the potentially negative effects of changing conditions.

Third, genetically and morphologically identical individuals can also express considerable behavioural diversity. While behavioural variation among individuals in eusocial insect societies (queen and various workers) has been described since antiquity, the existence of individual behavioural specialization is now well documented throughout the animal kingdom.

Darwin proposed that species diversity might increase the productivity of ecosystems due to the division of labour among species, suggesting that each species is unique in how it exploits its environment. It thus follows that species-rich systems can exploit resources more efficiently than species-poor systems (known as the complementarity effect).

Diversity is also thought to make ecosystems, species and populations more resilient to environmental stresses. A large number of species may imply a certain level of functional redundancy: the loss of one species has a smaller effect in a diverse system than in a species-poor one (known as the insurance effect). Genotypic or phenotypic diversity within one population of the same species may also improve resistance to environmental change. For example, it is well documented that the diversity of a population can increase its resistance to epidemics.

Diversity could also favour the emergence of complex collective behaviours, including in organisms without a nervous system, as demonstrated by the cooperative division of labour in certain species of bacteria. This allows groups of bacteria to assume mutually incompatible tasks and acquire new functions. In this way, multicellular cyanobacteria gain the ability to simultaneously perform photosynthesis and nitrogen fixation even though these two tasks are incompatible, as the oxygen produced during photosynthesis permanently damages the enzymes involved in nitrogen fixation.

How is diversity generated?

The neo-Darwinian theory of evolution proposes that biological diversity is the consequence of genetic accidents (mutations and recombinations of genes, for example) that occur spontaneously and randomly, without regard for their usefulness. However, the magnitude of adaptive gains from diversity suggests that partial control of its generation may be beneficial to the survival of biological systems. In support of this hypothesis, numerous examples of mechanisms generating individual genetic and phenotypic diversity, here called "diversity generators" (DG), have been described in systems ranging from prokaryotes to complex multicellular organisms.

While they may differ in their origin and components, these DGs share common functional properties. They contribute to the high unpredictability of the composition and behaviour of biological systems, promote robustness and cooperation among populations, and operate mainly by manipulating the systems that control the interaction of living entities with their environment.

The nature of DGs seems to depend on r/K reproductive strategies. Organisms with short generation time and large populations (r strategy) have reactive DGs, such as horizontal gene transfer and SOS systems. They generate diversity in response to environmental stresses and participate in the well-known Red Queen dynamic, where competitors must constantly evolve to survive: "Now here, you see, it takes all the running you can do to keep in the same place" (Through the Looking-Glass, Lewis Carroll, 1871).

The emergence of complex multicellular organisms, with a long reproductive life cycle and smaller populations (K strategy), has favoured the selection of a new class of DGs such as mandatory sexual reproduction and generation of a large adaptive immune repertoire, which act in anticipation of stress. Sexual reproduction, through the process of meiosis, allows for significant mixing of alleles between the parents and thus great genetic diversity for the offspring. Likewise, the adaptive immune repertoire is randomly generated by recombination of the genes encoding the antigen receptors within lymphocytes.

Its potential for diversity is such that an individual randomly expresses only a fraction, which ensures the maintenance of significant individual diversity of the immune response within populations. These DGs generate the distinct so-called White Queen dynamic in reference to the famous quote of the White Queen in Through the Looking-Glass: "Sometimes I've believed as many as six impossible things before breakfast." This metaphor seems particularly appropriate because the activity of these DGs is based on random phenotypic diversification, which is rarely adaptive at the individual level and favours the population (impossible things), and anticipates stress (before breakfast).

The existence of DGs leads us to consider evolution as a much more dynamic process and to give a new meaning to chance. If, as Einstein said, "God does not play dice," biological entities seem to do so frequently, which would partly explain their great adaptability and survival. The ubiquity of DGs in living organisms also confirms that diversity is essential for adapting to environmental stress and that regulated self-generation of diversity must be considered as a fundamental trait of biological systems.

It is urgent to reconsider the importance of diversity, which is more than just icing on the cake. It is both a property of living organisms and a necessary condition for their survival.

Education and fundamental research are both subject to an increasing number of evaluation criteria. While these controls were initially developed to optimise the outcomes, they also lead to standardisation. Yet, we should perhaps ask ourselves: is it wise to homogenise the intellectual formation of individuals and research activities, while diversity is a source of robustness, synergy and complexity in all living systems?

Global population growth will require sustained food production during the 21st century. However, the industrialisation of agriculture over the past 50 years has led to a dramatic fall in the diversity of agricultural products. Plants and animals have been intensively selected for strength and productivity. While this strategy led to good results over the short term, it is reasonable to doubt the ability of standardised populations to resist future climate changes that will likely lead to the emergence of new pathogens. Each particular genotype/phenotype is optimised for one given set of environmental conditions and only individual diversity can guarantee the adaptation of populations to unpredictable changes in their environment.

Finally, the importance of diversity in ensuring the robustness of biological systems suggests that decreasing the diversity of natural ecosystems could, in the near future, lead to their sudden disruption, which would further hamper our ability to maintain stable food production.

This article is republished from The Conversation under a Creative Commons license. Read the original article.


Dog breeds are artificial and potentially temporary

So if breeds are that similar to one another in their genomes, how are the vast differences maintained? The obvious answer is the mating pattern we impose on our dogs – we keep breeds separate by preventing interbreeding between them.

The fact humans keep them apart is crucial here. Species are commonly defined as “groups of interbreeding natural populations that are reproductively isolated from other such groups”. This requires hybrids between distinct species to either be non-viable (such as the proposed “humanzee”), or for their offspring to be infertile like most mules, or the more exotic “ligers”. In both these cases there would be complete reproductive isolation between the two groups, whether they be humans and chimps, lions and tigers, or Labradors and poodles.

Labrador (right) + poodle = the fluffy and fertile labradoodle (left). Bildagentur Zoonar GmbH / shutterstock

Yet two entirely different dogs will produce perfectly fertile offspring, and many modern breeds in fact originated in this way. Of course in some cases other factors might make mating very tricky. A female Chihuahua would have trouble naturally delivering a male Great Dane’s offspring, for instance. But though some breeds would never mate with each other without human intervention, middle-sized breeds could provide the link between extremely large and small dogs.

Street dogs are a vivid illustration of this point – they show how the distinct gene pools of dog breeds can rapidly mix once the restrictions of artificial breeding are removed. Moscow’s famous feral dogs have existed separate from purebred pets for at least 150 years now. In this time they have largely lost features like the spotty colouration that distinguish one breed from another, or the wagging tails and friendly behaviour towards humans that distinguish dogs from wolves.

Left to their own devices, street dogs soon stop looking like distinct breeds. Andrey, CC BY

So genetic exchange would still be common among dog breeds, were they allowed to reproduce freely. In that sense, dog breeds would not be classified as separate species under most definitions. If those Chihuahuas and Great Danes don’t look like the same species right now, it’s only because humans are constantly maintaining a barrier between them.


Conditional plasticity

Responses to predation or variation in food resources have provided some of the best examples of conditional responses to local environmental conditions. The conditional response may be dichotomous or it may be graded so that it is proportional to the degree of the challenge providing what is termed ‘the norm of reaction’. Some plastic responses induced in early life may have delayed benefits, so that their primary or only adaptation is expressed at a much later stage in the life cycle. Such anticipatory responses rely on a cue in early life predicting some characteristic of the future environment. The implication of many such examples is that environmental induction provides a forecast about the conditions of the world that the individual will subsequently inhabit (Bateson, 2001). In mammals, the best route for such a forecast may be via the mother. Vole pups (Microtus pennsylvanicus) born in the autumn have thicker coats than those born in spring the cue to produce a thicker coat is provided by hormonal signals from the mother before birth (Lee and Zucker, 1988). The potential benefit of doing so was termed the predictive adaptive response by Gluckman and Hanson (2006).

Nutrition during development may affect the individual’s preparedness for the nutritional environment when it is adult. Saastamoinen et al. (2010) found that undernourished larvae of an East African butterfly (Bicyclus anynana) had more strongly developed thoracic musculature after pupation, enabling them to fly more strongly as adults and potentially to reach more favourable environments. When pregnant mother rats (Rattus norvegicus) were given restricted diets, their offspring were smaller when they were born, but if these offspring were then given plentiful food they became much more obese than the offspring of mothers given an unrestricted diet (Jones and Friedman, 1982). This observation was followed by further extensive work on rats in many laboratories. Offspring born to undernourished rats developed increased appetites (Vickers et al., 2000). Even though the undernourished rats are more sedentary when kept in standard laboratory cages (Vickers et al., 2003), their behaviour differs in another striking way from the control animals. When given a choice between pressing a lever to obtain food and running in a wheel, they are significantly more likely to run in the wheel (Miles et al., 2009). This finding suggests that these offspring of undernourished mothers may attempt to find more reliable sources of food in a natural environment.

In human biology extensive studies of the effects of maternal nutrition on the offspring’s outcome characteristics have shown how maternal condition affects body composition, metabolic control, neuronal reserve, kidney size, reproductive maturation and behaviour (McMillen and Robinson, 2005). Human children with lower birth weights are likely to enter puberty early (Sloboda et al., 2007), birth weight being taken to be a proxy for poor intrauterine nutrition. The individual relatively undernourished in early life has a preference for high-fat foods, a higher set-point for satiety and a smaller somatic phenotype—a suite of characteristics that well are adapted to limited food resources in adult life. If human foetuses respond to nutritional cues provided by their mothers, then those individuals who experience cues that indicate a plentiful environment should be adversely affected if they encounter famine later in life. Indeed, the evidence suggests that people who enjoyed a plentiful environment in early life may be at greater risk during periods of prolonged famine than those who experienced relatively lower levels of nutrition in utero. In concentration camps and the worst prisoner-of-war camps, many reports have indicated that the physically large individuals died first while at least some of the small individuals survived (Bateson, 2001). In an Ethiopian population suffering from famine, high birth weight of babies who had had mothers on a higher plane of nutrition was associated with a ninefold higher risk of rickets, which carries fitness costs during reproduction, particularly for women (Chali et al., 1998). Children born smaller are less likely, in a famine, to develop kwashiorkor, the form of infant malnutrition with high mortality that involves a lower ability to mobilise substrates. In contrast, the low-birth-weight children respond to severe undernutrition by developing marasmus, which is associated with much lower mortality (Jahoor et al., 2008).

Maternal forecasting by induction of a specific developmental trajectory is thought by many researchers to be important in human biology (for example, Bateson, 2001 Gluckman and Hanson, 2004 Sandman et al., 2012). The individual benefits, it is argued, by adjusting the trajectory of his or her development so that the developed phenotype is most likely to match the anticipated environment. In general, a cue from the mother suggesting a future environment with relatively scarce resources leads to a more economical body form and a bias towards insulin resistance, thereby capturing the higher-energy fat-dense foods when they are available (Gluckman et al., 2010). A full discussion of the predictive adaptive response in humans is provided elsewhere (Bateson et al., 2014). Although the evidence provides strong grounds for supposing that humans exhibit conditional plasticity, extreme nutritional impoverishment of the mother can have long-term effects that are maladaptive (Gluckman and Hanson, 2004).


Conclusions

The recognition of the key role of founder genotypic and heritable phenotypic diversity for successful establishment has important implications for different areas and calls for some changes in policy and management. For instance, conservation programs that use reintroductions and translocations to vitalize or restore declining and locally extinct populations and species should focus at least as much on founder diversity as on propagule pressure and degree of environmental match between the habitat occupied by the source population and the properties at the introduction site. From the perspective of invasive species management, an increased focus on the role of diversity may help improve our ability to identify and protect against potential harmful invaders. Substantial research has attempted to identify traits and ecological characteristics that typify invasive species and properties that make environments susceptible or resistant to colonization and invasion. The results reported here suggest that founder diversity may influence the ability of invasive species to establish and subsequently spread outside of their native community, as well as the ability of pathogens and parasites to colonize and invade the environment constituted by their hosts. It therefore seems likely that an exchange of ideas, methodological approaches, and insights of the role of diversity for establishment in different contexts may further our knowledge, vitalize future research, and improve management plans in different areas.


Phenotype

Our editors will review what you’ve submitted and determine whether to revise the article.

Phenotype, all the observable characteristics of an organism that result from the interaction of its genotype (total genetic inheritance) with the environment. Examples of observable characteristics include behaviour, biochemical properties, colour, shape, and size.

The phenotype may change constantly throughout the life of an individual because of environmental changes and the physiological and morphological changes associated with aging. Different environments can influence the development of inherited traits (as size, for example, is affected by available food supply) and alter expression by similar genotypes (for example, twins maturing in dissimilar families). In nature, the influence of the environment forms the basis of natural selection, which initially works on individuals, favouring the survival of those organisms with phenotypes best suited to their current environments. The survival advantage conferred to individuals exhibiting such phenotypes enables those individuals to reproduce with relatively high rates of success and thereby pass on the successful genotypes to subsequent generations. The interplay between genotype and phenotype is remarkably complex, however. For example, all inherited possibilities in the genotype are not expressed in the phenotype, because some are the result of latent, recessive, or inhibited genes.

One of the first to distinguish between elements passed from one generation to the next (the “germ” plasm) and the organisms that developed from those elements (the “soma”) was German biologist August Weismann, in the late 19th century. The germ plasm later became identified with DNA, which carries the blueprints for the synthesis of proteins and their organization into a living body—the soma. Modern understanding of phenotype, however, is derived largely from the work of Danish botanist and geneticist Wilhelm Ludvig Johannsen, who in the early 20th century introduced the term phenotype to describe the observable and measurable phenomena of organisms. (Johannsen also introduced the term genotype, in reference to the heritable units of organisms.)

This article was most recently revised and updated by Kara Rogers, Senior Editor.


Why Can Some Animals Be Domesticated, But Not Others?

Why can some animals be domesticated while others can't? Is there any evolutionary reason to it? originally appeared on Quora: the knowledge sharing network where compelling questions are answered by people with unique insights.

Answer by Suzanne Sadedin, Ph.D. in evolutionary biology, on Quora:

I doubt there's an animal that couldn't be domesticated with effort, but some are certainly easier than others. Here are some traits that facilitate domestication:

  • Fast growth rate. Animals that grow and mature quickly are easier to breed selectively, and are more profitable for farmers.
  • Hardy/flexible. Humans aren't always reliable caretakers, so domestic animals are usually capable of surviving in a wide range of conditions, eating garbage and going without food or water for some time.
  • Social. Farmers typically raise animals in groups, so domestic animals need to be comfortable with that to breed well in captivity.
  • Group mind. Animals that follow the herd are easy to control, as every politician knows.
  • Low fear. Nervous species are easily stressed in captivity, making them susceptible to disease, slow to grow and hard to breed.
  • Low aggression. Fighting lowers productivity, and might endanger the farmer.
  • Learning. Animals that remember routines and respond to training are easy to manage.

These traits can be bred in, but wild forms of most domestic species already possess most of them to some extent. Wild cats are solitary and fearful wolves and wild boar are aggressive. Their domestic forms have been selected to vastly reduce these traits. In the case of cats and dogs, this selection was probably at first natural. Populations found a niche living on the fringes of human settlements, where social and harmless individuals were more successful. Later, people consciously bred them to enhance their convenient features.

In the 1950s, a Russian scientist named Dmitri K. Belyaev started farming silver foxes. Wild foxes are solitary, independent and very shy. Within a few generations, he had produced foxes with startling new colours that were completely docile, friendly, and trainable.

Belyaev's experiment suggests that many species which haven't been domesticated could be. We just haven't tried. Domestication is usually done to fill a useful niche in human society. If a domestic animal fits that niche already, it's more practical to import it than to domesticate a new one.

Some readers have been confused about the relationship between domestication and taming. Domestication is the evolutionary process that occurs when humans selectively breed living organisms over many generations to accentuate their useful traits. Taming is the process of making a particular individual animal comfortable with humans. Many tame animals (like pet crows) are not domestic, and many domestic animals (like farm chickens) are not tame. As Belyaev's experiment shows, even species that are naturally very difficult to tame can still be domesticated with effort.

Though it's theoretically possible to domesticate anything, the difficulty involved may have profound historical impact. Domesticating any animal requires a concerted effort spanning multiple human generations, and large mammals in particular are often dangerous and slow to breed. In Guns, Germs and Steel, Jared Diamond argues that technological development in the Americas was slower because people there lacked suitable pulling and riding animals. Perhaps, had a visionary farming family dedicated its dynasty to breeding a heat-tolerant, ride-on llama, the invading Spaniards would have met with a very different fate.

This question originally appeared on Quora. Ask a question, get a great answer. Learn from experts and access insider knowledge. You can follow Quora on Twitter, Facebook, and Google+. More questions:


The scope of development

All organisms, including the very simplest, consist of two components, distinguished by a German biologist, August Weismann, at the end of the 19th century, as the “ germ plasm” and the “ soma.” The germ plasm consists of the essential elements, or genes, passed on from one generation to the next, and the soma consists of the body that may be produced as the organism develops. In more modern terms, Weismann’s germ plasm is identified with DNA ( deoxyribonucleic acid), which carries, encoded in the complex structure of its molecule, the instructions necessary for the synthesis of the other compounds of the organism and their assembly into the appropriate structures. It is this whole collection of other compounds (proteins, fats, carbohydrates, and others) and their arrangement as a metabolically functioning organism that constitutes the soma. Biological development encompasses, therefore, all the processes concerned with implementing the instructions contained in the DNA. Those instructions can only be carried out by an appropriate executive machinery, the first phase of which is provided by the cell that carries the DNA into the next generation: in animals and plants by the fertilized egg cell in viruses by the cell infected. In life histories that have more than a minimal degree of complexity, the executive machinery itself becomes modified as the genetic instructions are gradually put into operation, and new mechanisms of protein synthesis are brought into functional condition. The fundamental problem of developmental biology is to understand the interplay between the genetic instructions and the mechanisms by which those instructions are carried out.


Animal Evolution and Diversity - Invertebrates Get More Complex

The rest of the animal kingdom fits nicely into the next two big branches of the animal family tree: the protostomes and the deuterostomes. In this section, we'll cover one of the three protostome phyla: Mollusca. What's new in the protostomes? Their bodies have distinct sections, in addition to a head and complex organ systems.

All the animals we've discussed so far, as well as mollusks, annelids, and arthropods (discussed in this and the next sections), are invertebrates. They have no backbone. Spineless invertebrates. This distinguishes them from fish, reptiles, amphibians, birds, and mammals.

The Mollusks

Mollusca is from the Latin word for "soft." Step on a clam or oyster shell at the beach and this may not seem obvious. Ouch. Mollusks are soft-bodied creatures, but most have a shell for protection. A few have internalized the shell or even lost it through further evolution.

Mollusks live in water. Familiar ones include clams, oysters, mussels, scallops, squid, octopi, and snails. Most species are marine, but a few live in freshwater, like the snails and slugs.

That's Not a Blob, It's a Body

Besides a shell, these animals all have a common body plan with a few basic parts: a foot, a visceral mass, a mantle, and a mantle cavity. The foot is one big muscular part on the bottom of the animal. The name gives it away it is generally used for movement. The visceral mass has most of the organs and the head. ("Viscera" is Latin for "internal organs.") The mantle is the tissue above the visceral mass and it secretes the shell. There is a space under the mantle called the mantle cavity (that's an easy one at least). This space has gills for breathing and doubles as the end of the digestive system.

Mollusks have an open circulatory system. This means it has no blood vessels. Nada. Instead of flowing through vessels, fluid carrying food and oxygen moves around the visceral mass, which holds most of the internal organs. Each organ is floating in its food and oxygen source instead of having it delivered through vessels. Gills take oxygen from water, which then goes into the internal fluid. Wastes are filtered back out through the mantle cavity by organs that function like little kidneys.

It might be hard to tell sometimes, but mollusks usually have a head with a nervous system rattling around in there. Nerve cords run to key areas like the internal organs and the foot. The head has sensory organs: eyes to see and tentacles to sense chemicals and move.

They have a mouth, too. Mollusks eat by scraping. They use a radula, which is a long organ covered with little structures similar to teeth. The radula is pulled across a surface to remove anything edible. It isn't the most precise way to get food, but mollusks are apparently not very picky.

Mollusks reproduce sexually. Some species are hermaphrodites and some have separate genders. Most mollusks go through a few different forms as they grow, starting as little swimming larvae called trochophores that will eventually sink to the floor and morph into the adult form. Little squid and octopus, however, come out as miniature adults. Can you say, "adorable?"

Types of Mollusks

There are eight classes of mollusks, and we'll cover four examples here: Gastropoda (garden snails and slugs), Bivalvia (clams, oysters, and mussels), Polyplacophora (chitons), and Cephalopoda (squids and octopuses).

Gastropods

Snails and slugs are examples of Gastropoda. Most live in the ocean, but some live in fresh water, or even on land. These mollusks sport either a single hard shell (versus the segmented plates of the chiton) or no shell at all, like slugs and nudibranchs (sea slugs). This is a very large and diverse phylum.

Gastropods are upside down, as compared to most animals. As a gastropod embryo develops, it goes through a process called torsion, where one side of the body grows faster than the other does. This causes the end of the gastropod to be above the head and the mantle cavity, which secretes the heavy shell, over the center of the body.

Bivalvia

Familiar, tasty mollusks are from the class Bivalvia. Bivalve means "two shells." The animals of this phylum have a hinged, two-part shell and include clams, oysters, scallops, and mussels.

Bivalves are a bit of an exception to the cephalization that characterizes most mollusks. Bivalves don't have a head. They feed by filtering out food from the water in which they live. They actually eat with the same structure used for breathing: the gills. Tiny particles of food get caught in the gills and cilia near the gills move the particles into the mouth.

Bivalves generally sit still. They can move slowly using their muscular foot, which can be stuck out between the shells. They can also move by "flapping" their shells, like a startled Pac-Man. Most of the time, though, bivalves clam up. How convenient. Powerful muscles keep the shells together. Some bivalves even anchor themselves to something, including boats and other animals.

Polyplacophora

Polyplacophora are also called chitons. Chitons live in tidal areas of the ocean and eat by scraping algae off rocks. They are oval-shaped with a shell made of eight overlapping plates. They look a little like an oval, armored space ship.

Cephalopoda

Squids, octopi, cuttlefish, and nautiluses are mollusks in the class Cephalopoda, which means, "head feet." The tentacles (which are attached to the head…hence the name) are highly modified from the basic mollusk foot. They surround the mouth, which looks like a beak.

Cephalopods are the least mollusk-like of the mollusks. Their shell has been internalized or completely lost in all but the nautilus. Cephalopods also have a closed circulatory system. They have distinct heads, which hold the most complex brains of the invertebrates. Octopuses have been observed problem solving and even using tools. They use their tentacles to explore their environment and can make their way through human-constructed mazes.

Cephalopods are hunters—highly effective carnivores designed for capturing prey. They use tentacles for swimming and for moving along the ocean floor. The tentacles are covered with suckers on the bottom side. They're used to catch and pull prey into the octopus' beaked mouth (om nom nom and all that).

These aren't slow mollusks. Cephalopods can put on a big burst of speed by taking water into the mantle cavity and pushing it out for a little jet propulsion. They sort of spit themselves forward. Since they move so fast, it's good that they also have eyes and other sense organs that give them the ability to detect light and sound. Wasting time and energy slamming into rocks or the ground wouldn't exactly help their hunting prospects.

Cephalopods are tricked out like the Bat-mobile. Most have an ink sac and can shoot a cloud of ink to distract predators. They can also change color and texture in an instant, to match their surroundings. Being able to become the color and texture of the wall behind you in the blink of an eye is a great party trick. If discovered, you can always buy some time behind a blinding inky cloud.

Cephalopods have male and female forms and reproduce sexually. They lay eggs that are protected by the female until hatched. Many cephalopods live fast, short lives that end after reproducing.

Brain Snack

Check out some facts about the octopus. Who says learning isn't hilarious?

Nudibranchs are hermaphroditic marine mollusks. One species was discovered to have a disposable penis.