How exactly did asexually reproducing organisms evolve into sexually reproducing ones? Why was it considered more favourable?
Sexual reproduction is a process where two cells fuse to form a diploid cell. Unicellular organisms (or even multicellular lower organisms like alga, fungi and protists) prefers to reproduce by asexual means under favourable conditions. But when the conditions become unfavourable, they opt to follow sexual reproduction. This suggests, sexual reproduction evolved to increase the chances of survival under unfavourable condition.
So how does sexual reproduction helps in increasing survival rate? By fusion of cells i.e., sexual reproduction a cell now becomes diploid and hence have two copies of its genome. This increased copy of genome is beneficial in repairing any breaks or alteration in genome, as the other copy will act as a template for repair (via homologous recombination).
To get a better hold of this, let us take an example. Let there be a population of haploid cells. This population was exposed to conditions which would cause damage to genome, specifically would cause breaks in DNA. There were two cells "Fella A" and "Fella B". They both knew that their genome has suffered damages, but neither was successful in repairing the DNA as they required an intact DNA template for repair. Suddenly "Fella B" came up with this great idea to fuse with "Fella A". This would work, reasoned "Fella B", as the position of damage in the DNA in his genome is different from the position in "Fella A". Fusion of cells was the first step towards evolution of sexual reproduction.
Later on the biological system realised importance of variation generated by sexual reproduction and hence this mode of reproduction caught on with various lineages.
Sexual vs asexual is not a binary condition it is a spectrum. Many organisms do both and/or a wide range of things in between. One of the most basic is plasmid swapping Bacterial conjunction in unicellular organisms.Even asexual selction has multiple forms of reproduction such as budding vs fission. Exchanging genes can be highly beneficial for multiple reasons including but not limited to; making infection far more difficult for parasites or giving a favorable gene a better change to spread since they are not stuck in the same genetic line forever.
Many organisms are opportunistically sexual, using sexual reproduction when they can (when they can find a mate or when resources are abundant) and using asexual selection when they cannot. this implies that there is an advantage to sexual selection even if what it is is debated, but it also shows it is an an either/or question the two forms of reproduction coexist quite well.
As for why sexual selection evolved, this is a hotly debated topic with no clear answer, it might be as simple as a way to get the benefits of gene exchange without the virus vulnerability plasmids create. It also a vague question by its very nature, because many organism engage in many different forms and combinations of forms of reproduction AND becasue many of these appear to have evolved independently the question is horribly muddled.
In short we know sexual reproduction has benefits but we can not say which ones were the deciding factors in its early evolution partially becasue there are several different instances of it evolving…
No, you've got the wrong end of the stick… asexual reproduction came from sexual reproduction, not the other way around! Asexual reproduction is a degenerate state that has benefits but is an evolutionary dead end because sexual combination is necessary to promote beneficial alleles and build new genes. See: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4231362/
The answer that says "Sexual vs asexual is not a binary condition" is misleading; yes, that is true. Sexual species will start to experiment with asexual forms if they can because that confers advantages. The ability to be parthenogenic is very useful, but since it is a dead end, the ability to reproduce sexually should not be lost.
Although sex was quite tricky to evolve in the first place, it was necessary from the very beginning. It evolved from simpler systems of gene swapping and building that were needed from the beginning.
Types of Reproduction: Sexual versus Asexual Reproduction
00:00:16.05 Although most species on the planet reproduce sexually,
00:00:19.22 why do some species still reproduce asexually?
00:00:22.14 Why is it evolutionarily beneficial
00:00:24.27 for some species to require two parents to produce offspring
00:00:28.07 and others only one?
00:00:30.28 Let's explore why different forms of reproduction exist
00:00:33.22 and what makes each one beneficial
00:00:35.25 in specific circumstances.
00:00:38.24 The two types of reproduction
00:00:40.15 are asexual and sexual.
00:00:42.01 Asexual reproduction is when an organism
00:00:45.14 makes a genetically identical clone of itself.
00:00:48.17 Most of the simplest life forms,
00:00:50.18 such as bacteria,
00:00:52.13 reproduce in this manner.
00:00:54.16 Sexual reproduction is much more complex
00:00:57.21 and involves two members of a species
00:01:00.01 coming together to produce genetically distinct offspring.
00:01:03.25 Most eukaryotic multicellular organisms,
00:01:07.17 such as humans, birds, and insects,
00:01:09.11 undergo sexual reproduction.
00:01:12.12 The biggest advantage of sexual reproduction
00:01:14.14 is that offspring are genetically distinct
00:01:18.03 from their parents and their siblings.
00:01:19.24 This allows a species to evolve,
00:01:22.11 since natural selection will favor the offspring carrying genes
00:01:24.25 that improve their survival in diverse environmental conditions.
00:01:28.16 For example, humans who carry two abnormal copies
00:01:32.07 of the hemoglobin gene
00:01:34.03 develop sickle cell anemia,
00:01:36.01 a devastating disease that results in
00:01:38.21 abnormally low levels of oxygen in the blood.
00:01:41.10 Individuals who have only one copy of the abnormal gene
00:01:44.18 have a much less severe form of the disease.
00:01:47.27 Interestingly, heterozygous individuals
00:01:50.00 are much less prone to infection by malaria,
00:01:52.06 making them more likely to survive
00:01:54.05 and have higher reproductive success
00:01:56.08 in malaria-endemic areas.
00:01:59.09 This example shows how sexual reproduction
00:02:01.14 can produce offspring
00:02:03.14 who are genetically distinct from their parents
00:02:05.15 and are better suited to survive
00:02:07.22 under certain environmental conditions.
00:02:10.02 We have been talking about genetic diversity,
00:02:12.14 but how does sexual reproduction
00:02:14.19 generate this diversity in the first place?
00:02:17.03 During meiosis, which produces the sperm and the egg
00:02:19.29 needed for sexual reproduction,
00:02:22.09 there are two key steps that produce genetic diversity.
00:02:25.11 First is recombination between homologous chromosomes.
00:02:29.09 Recombination mixes maternal and paternal alleles,
00:02:32.07 and results in novel gene combinations
00:02:34.17 on each chromosome in each generation.
00:02:37.18 The second is the independent assortment of chromosomes,
00:02:40.29 which generates unique combinations of chromosomes
00:02:44.11 in each gamete.
00:02:46.20 During fertilization,
00:02:48.24 one genetically unique sperm
00:02:50.26 and one genetically unique egg
00:02:54.02 randomly combine to form an offspring
00:02:56.01 that is genetically distinct from its parents.
00:02:58.01 While genetic diversity arising from sexual reproduction
00:03:01.00 has been critical for the survival of many species,
00:03:04.07 mating is an energy consuming, slow, and risky process.
00:03:08.02 To improve the chance of selecting the best partner,
00:03:11.03 and therefore the best alleles,
00:03:13.04 some species have evolved highly sophisticated courtship behaviors.
00:03:18.10 Male peacocks, for example,
00:03:20.03 grow long, iridescent tail feathers
00:03:22.20 and perform an elaborate dance to attract a mate.
00:03:26.00 But this process requires energy
00:03:27.29 and can attract predators.
00:03:30.01 In addition, the generation of gametes for sexual reproduction,
00:03:33.05 and the subsequent gestation and incubation,
00:03:36.07 take time and energy.
00:03:37.27 For example,
00:03:40.19 emperor penguins must sit on their eggs for over two months,
00:03:43.14 and elephants can be pregnant for almost two years.
00:03:49.02 While asexual reproduction
00:03:51.03 does not produce genetically distinct offspring,
00:03:53.07 it has important advantages
00:03:55.07 that allow for species survival.
00:03:57.11 One, it is energy efficient
00:03:59.15 and does not require attracting a mate.
00:04:02.02 Two, it is usually fast.
00:04:04.10 Some bacteria can reproduce
00:04:06.12 in as little as twenty minutes.
00:04:08.17 And three, it produces offspring that are genetically identical,
00:04:12.14 an advantage in stable environmental conditions.
00:04:16.24 For example, bacteria that carry
00:04:19.04 an ampicillin antibiotic resistance gene
00:04:22.07 will continue to grow in patients prescribed this antibiotic.
00:04:25.16 Conversely, because asexual reproduction
00:04:28.22 results in genetic clones,
00:04:30.20 these bacteria would all die if the patient
00:04:32.20 was treated with a different antibiotic.
00:04:35.26 Rare, random mutations in the genome
00:04:38.13 do allow asexually reproducing organisms
00:04:41.23 to evolve diversity.
00:04:44.17 However, this is a slow process,
00:04:46.27 and there is no mechanism to separate harmful and beneficial mutations
00:04:50.17 for propagation to future generations.
00:04:54.04 Both sexual and asexual reproduction
00:04:56.20 provide important methods
00:04:59.07 for species survival and adaptation,
00:05:01.01 allowing the millions of species on Earth today
00:05:04.00 to evolve and survive in their current forms.
00:05:09.05 This video has been provided to you by Youreka Science
00:05:11.22 and iBiology, bringing the world's best biology to you.
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Asexual reproduction is the primary mechanism of reproduction for the vast majority of organisms on the planet however, there are multiple ways that asexual reproduction occurs. Some organisms can reproduce using several of these methods.
Cloning is perhaps the most well-known mechanism of asexual reproduction, in part due to its appearance in multiple sci-fi series. However absurdly it is portrayed in sci-fi, cloning does occur in some asexual organisms and even in the larva of some sexual organisms.1 What cloning does is completely duplicate the genome of an organism into its offspring. Thus, when cloning occurs, the offspring are all exact copies of the adult, with few genetic differences.
Binary fission is yet another mechanism of asexual reproduction, which is regarded as a form of cloning. This method of reproduction is mostly restricted to prokaryotes, organisms that lack a defined nucleus. Essentially, in binary fission, the organism makes a complete copy of itself, then splits in half. Binary fission is only possible in very small organisms, such as prokaryotic mycoplasmas.2
While not quite the same as cloning, parthenogenesis is another method of asexual reproduction that periodically occurs in some sexually reproducing organisms. In this mode of reproduction, the organism reproduces alone, giving rise to one or more clones of itself, without the need to mate. This has been observed in numerous vertebrates,3 and is believed to be the source of the infamous marbled crayfish.4
A third method of asexual reproduction is called budding. Budding takes place when an adult organism produces a clone of itself that “buds” off the side of the adult organism. Some organisms will undergo budding as well as normal sexual reproduction, such as hydras.5 Other organisms, like yeasts, rely almost exclusively on budding.6
Another method of asexual reproduction is fragmentation. In fragmentation, the organism splits itself, or is split by an outside force, into multiple parts, all of which then regrow the remainder of the body. This could be considered a strange method of cloning as it does produce identical clones of the parent organism. The most well documented examples among animals come from starfish, which are famous for being able to be chopped into pieces and regrow whole organisms from a single arm and a piece of the central disk. However, other organisms do this as well. Some worms also reproduce by fragmentation.7
Vegetative propagation is a well-known method of reproduction, particularly in plants. It involves a part of the plant branching off from the rest of the plant and eventually growing into an entirely new plant. In some cases, the two plants remain attached and form long chains of connected plants, each with all the properties of an individual plant but connected in some way either above or below ground. Mosses are known to undergo this process.8 Numerous plants, as well as some algae, also undergo vegetative reproduction in one form or another.9 The most common forms of this are frequently used in gardening, as cuttings pieces broken or cut from a plant which grow into a new plant.
Spore production is another mechanism of asexual reproduction. Spores may be thought of as essentially “seeds” that are produced asexually and designed to be spread through a wide area. Spores are the choice method of reproduction of many fungi,10 but some plants, particularly ferns and mosses, also use this method of reproduction.
Evolution of sexuality: biology and behavior
Sexual reproduction in animals and plants is far more prevalent than asexual reproduction, and there is no dearth of hypotheses attempting to explain why. Even bacteria and viruses, which reproduce by cloning, engage in promiscuous horizontal gene exchange ("parasexual reproduction") on such short time scales that they evolve genotypic diversity even more rapidly than eukaryotes. (We confront this daily in the form of antimicrobial resistance.) The host-parasite and host-pathogen arms race purports to explain the prevalence of sexual reproduction, yet there are over a dozen other hypotheses, including the proposition that sexual reproduction purges the genome of deleterious mutations. An equally daunting challenge is to understand, in terms of evolutionary logic, the jungle of diverse courtship and mating strategies that we find in nature. The phenotypic plasticity of sex determination in animals suggests that the central nervous system and reproductive tract may not reach the same endpoint on the continuum between our stereotypic male and female extremes. Why are there only two kinds of gametes in most eukaryotes? Why are most flowering plants, and few animals, hermaphroditic? Why do male animals compete more for access to females than the other way around in most animals that have been studied?This review presents more questions than answers, but an extraordinary wealth of data has been collected, and new genetic techniques will provide new answers. The possible relevance of these data to human sexuality will be discussed in a future article.
Peacock with a highly ornamented…
Peacock with a highly ornamented tail which, like the male quetzal's tail, evolved…
Sexual Reproduction Challenges Evolution, Affirms Creation
A review of the scientific literature reveals that the evolution of sexual reproduction has been described as
“controversial,” 1 and as a “paradox” 2 and a “mystery.” 3 A 2002 article in Nature
Reviews Genetics states that “at least 20 theories had been proposed to explain the widespread occurrence of
sexual reproduction” 4 and a 2017 article in Trends in Ecology and Evolution observes that
“many of the plausible hypotheses for sex have restrictive assumptions.” 5 Moreover, there is no hard
evidence for any of the twenty or so hypotheses. 6
Yet the most perplexing question from an evolutionary perspective is: why did advanced life-forms evolve
sexuality at all? Why didn’t we evolve based on asexual reproduction? To use a mythological image: why aren’t we
all Amazon women?
It should be noted that asexual reproduction can be much more complex than simple cell division (mitosis).
An advantage of sexual reproduction via meiosis
is the exchange of genetic material however, this can also be done asexually. For example, the phenomenon of parasexuality describes
the direct transfer of genetic material among bacteria 7 and, as first reported in 1953, 8
it is common in fungi. 9 Even some higher life-forms reproduce asexually via apomixis,
which is reproduction by special generative tissues without fertilization that is, without female and male
union. The term apomixis includes parthenogenesis
(in which a new individual develops from an unfertilized egg) in some plants, invertebrates, and even
The cell biology of some of these asexual reproductive mechanisms is similar to sexual reproduction in the
transfer of genetic material it’s just that distinct “male” and “female” forms are lacking. Since the theory of
evolution often focuses on finding transitional forms, these asexual reproductive mechanisms might be
viewed in that way.
However, the scientific literature suggests that these asexual processes could be a barrier to the
development of sexuality, rather than a transition into it. Articles in the Journal of Heredity and
Integrative Zoology, respectively, emphasize that sexual reproduction offers no advantage in the context
of natural selection, and that it is less efficient than asexual reproduction:
Nobody has attributed any benefit of amphimixis
[sexual reproduction] to the parents who are engaged in it, and the supposed beneficiaries are the offspring.10
[The] inherent costs [of sex] have made its maintenance difficult to explain. The most famous of these is the
twofold cost of males, which can greatly reduce the fecundity [reproductive capacity] of a sexual population,
compared to a population of asexual females. 11
A 2018 article article in Trends in Genetics summarizes the problems:
The issue of the evolution of amphimixis involves three problems: (i) how it originated some 2 billion years ago
(ii) how it manages to outcompete obligate apomictic clones that keep emerging in some, although not all,
amphimictic species and (iii) how its gradual evolution does not turn amphimixis into apomixis or something
similar. . . . Problem (iii) is the toughest one. Amphimixis can disappear not only abruptly, . . . but also
These and other journal articles taken together reveal an interesting picture:
- There is no consensus among evolutionary biologists about the development of sexual reproduction, and many of
the hypotheses are structured as restrictive “just so stories” requiring a cascade of unlikely events.
- Most—if not all—life-forms could be configured for asexual reproduction.
- Sexual reproduction is more “expensive” than asexual reproduction it is more difficult and requires more
- From the point of view of natural selection, there is no benefit to sexual reproduction. To the contrary,
asexual reproduction offers reproductive advantage over sexual reproduction.
- The real question may be how do we keep sexual reproduction—however we acquired it. Why do we and
other sexual life-forms not devolve into the less “expensive” condition of asexuality?
Which Model Makes More Sense?
In light of all the above, the evolution of sexual reproduction by random, spontaneous processes makes no
sense. Assuming natural selection as the mechanism, evolution should have stopped with some form of asexual
apomixis. Even if a life-form had happened to evolve sexual reproduction, it would have disappeared because
there was no reproductive advantage.
There must be a different answer . . . and that answer may be found in
God created man in His own image, in the image of God He created him male and female He created them (Genesis
The LORD God said, “It is not good for the man to be alone I will make him a helper suitable for him” (Genesis
Sexual reproduction is not for efficiency it offers no reproductive advantage. Why does it exist? It may be
for companionship and partnership, as ordained by the creator-God for humans—and for many animals as well. It
seems most plausible that it came from a loving God—not as the product of an evolutionary processes.
This topic was the idea of my 17-year-old grandson, Jeff Obermeyer, Jr., who
deserves special credit for his insight.
Events in sexual reproduction
Events in sexual reproduction: After attainment of maturity, all sexually
reproducing organisms exhibit events and processes that have remarkable
fundamental similarity, even though the structures associated with sexual
reproduction are indeed very different. The events of sexual reproduction
though elaborate and complex, follow a regular sequence. Sexual
reproduction is characterised by the fusion (or fertilisation) of the male and
female ganmetes, the formation of zygote and embryogenesis. For convenience
these sequential events may be grouped into three distinct stages namely,
the pre-fertilisation, fertilisation and the post-fertilisation events.
Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are three main categories of life cycles: diploid-dominant, demonstrated by most animals haploid-dominant, demonstrated by all fungi and some algae and the alternation of generations, demonstrated by plants and some algae.
Many plants reproduce asexually as well as sexually. In asexual reproduction, part of the parent plant is used to generate a new plant. Grafting, layering, and micropropagation are some methods used for artificial asexual reproduction. The new plant is genetically identical to the parent plant from which the stock has been taken. Asexually reproducing plants thrive well in stable environments.
Plants have different life spans, dependent on species, genotype, and environmental conditions. Parts of the plant, such as regions containing meristematic tissue, continue to grow, while other parts experience programmed cell death. Leaves that are no longer photosynthetically active are shed from the plant as part of senescence, and the nutrients from these leaves are recycled by the plant. Other factors, including the presence of hormones, are known to play a role in delaying senescence.
Sexuality, not extra chromosomes, benefits animal, biologists find
Most animals, including humans, have two copies of their genome -- the full set of instructions needed to make every cell, tissue, and organ in the body.
But some animals carry more than two complete sets of the genome, referred to as polyploidy. Biologists have long wondered whether these extra chromosomes help or hinder those species.
In a study involving multiple generations of a freshwater snail in New Zealand, researchers at the University of Iowa found that polyploidy appears to be neither an asset nor a drawback for females bearing offspring without the help of a male. Instead, it's the snails' sexuality that creates an advantage: Asexual females, the study found, grew twice as fast during the late juvenile phase and reached reproductive maturity 30 percent faster than female snails that mated with males.
That in itself raises fundamental biological questions: If asexual females grow faster and bear children much more quickly than sexual females, what's the purpose of sex, and why is it the dominant method of reproduction in the animal world?
"When we did the study, we thought polyploidy would be bad for asexuals, but we didn't find any evidence of that," says Maurine Neiman, associate professor of biology at the UI and corresponding author on the paper, published in the journal Ecology and Evolution. "This is making the role of sex even harder to explain."
The mud snails, Potamopyrgus antipodarum, live in lakes and streams all over New Zealand. They were also discovered in Idaho in 1987 and have since spread to the Great Lakes and farther east to Chesapeake Bay, according to the U.S. Department of Agriculture, which classifies the animals as an invasive species.
Sexual and asexual females are known to live in the same lakes in New Zealand, although they also exist separately in other lakes. Males will mate with either, but their genes are not passed on in encounters with asexual females. Those factors, and the snails' abundance, made them a good species in which to test the effects of polyploidy.
The UI team compared sexually reproductive female snails with only two copies of their genome to asexual females carrying three and four copies. Together, they produced enough generations to occupy 1,500 cups -- one cup per snail.
"When we began, we thought the project would take six to nine months," says Katelyn Larkin, who earned her bachelor's and master's degrees in biology at the UI and has worked in Neiman's lab since she was a sophomore. "Instead, it took more than three years. We learned that these snails grow at a snail's pace."
The asexual females, regardless of the number of genomic copies they possessed, grew faster and reached reproductive maturity quicker than the sexual females, the researchers discovered. In human terms, it'd be as if the asexual females could produce children at 13 years of age, whereas the sexual females wouldn't reach reproductive age until age 18. Couple that with the fact that asexual females produce only female offspring, and you wonder why sexual female snails still exist.
"It's not only that (the asexuals) are not making males. The asexual daughters are growing up faster too," Neiman notes.
Before her study, Neiman thought the asexual females would bear extra costs for each additional genome because they would be chock full of metabolically expensive ingredients like RNA and proteins. Such is the case with plants, like wheat. But there was no difference in growth rate, shell length, or time to reproductive maturity between asexual females with three genomic copies and those with four, ruling out that theory.
Instead, the surprise lay in the fact the asexual females didn't seem to pay any price whatsoever for having those extra genomes. In fact, there was no discernible disadvantage at all.
So, why do sexual female snails exist, and how do they survive when they're co-existing and competing with asexual females?
The answer may lie in part to a parasitic worm that preys upon the snails. The asexual females are more vulnerable because their offspring's genomes are exact replicas of their own, making them easier to target and wipe out. The sexual females, because they mate, inherit a separate, distinct genomic set that diversifies the gene pool and thus makes them better able to withstand parasitic attacks.
Still, sexual females have been found alongside asexuals in lakes without the parasitic worms, which muddies the whole idea that genetic diversity is the sole reason why sexual snails persist.
Neiman seems to like it that way.
"You could argue that our genome is the most important thing we have, yet we don't know why humans have two copies when a lot of organisms do fine with one, or three, or more," she says. "This research speaks to that question."
Evolution of sexual reproduction from asexual - Biology
Three common systemic human fungal pathogens — Cryptococcus neoformans, Candida albicans and Aspergillus fumigatus — have retained all the machinery to engage in sexual reproduction, and yet their populations are often clonal with limited evidence for recombination. Striking parallels have emerged with four protozoan parasites that infect humans: Toxoplasma gondii, Trypanosoma brucei, Trypanosoma cruzi and Plasmodium falciparum. Limiting sexual reproduction appears to be a common virulence strategy, enabling generation of clonal populations well adapted to host and environmental niches, yet retaining the ability to engage in sexual or parasexual reproduction and respond to selective pressure. Continued investigation of the sexual nature of microbial pathogens should facilitate both laboratory investigation and an understanding of the complex interplay between pathogens, hosts, vectors, and their environments.