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How exactly did asexually reproducing organisms evolve into sexually reproducing ones? Why was it considered more favourable?

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  • $\begingroup$ The evolution of sexual reproduction is very much a field in progress. There in any case not a single simple answer to this question. A number of reviews exist on the subject. One of the reason being that the dichotomy sexual vs asexual reproduction is misleading as reproductive methods are immensely diverse. I think the question is therefore too broad. $\endgroup$ – Remi.b Mar 5 '17 at 15:55
  • $\begingroup$ You could give it a start with the wikipedia article about the evolution of sexual reproduction or maybe you could have a look at the SE posts Why asexual reproduction? and Why are not all species hermaphrodites?. $\endgroup$ – Remi.b Mar 5 '17 at 15:55
  • $\begingroup$ You will find in any intro textbook on evolution a whole chapter about the evolution of sex. You should have a look at it. $\endgroup$ – Remi.b Mar 7 '17 at 15:47
  • $\begingroup$ I agree with "JJ": Answer not good enough. $\endgroup$ – Michael Reginald Atchley May 21 '18 at 20:28
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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.

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  • $\begingroup$ But I want to know HOW the change occurred from asexual to sexual $\endgroup$ – AP2261 Mar 5 '17 at 12:50
  • $\begingroup$ @Tushar You are presenting one of the many arguments people put forward. Barton and Otto 2005 is just one example of another explanation. $\endgroup$ – Remi.b Mar 5 '17 at 15:59
  • $\begingroup$ @Remi.b: Agreed! There are many hypothesis (arguments) and to the extent I was able to understand work of Barton and Otto, it discusses "if cells gained the capability of fusion (probably for DNA repair, my view) then what factors will promote its spread to entire population". While my answer explains why cells would start fusing leading to sexual reproduction. $\endgroup$ – Tushar Mar 6 '17 at 6:57
  • $\begingroup$ @user29774: I made an edit. Does this answer your question? $\endgroup$ – Tushar Mar 6 '17 at 11:33
  • $\begingroup$ @user29774 While I consider the the current answer gives a poor representation of the diversity of explanations and diversity of mating systems, I think that if you feel like the post has answered your question, then you should accept it (and check the checkmark). $\endgroup$ – Remi.b Mar 7 '17 at 15:46
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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..

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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.

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    $\begingroup$ This answer is just plain wrong. The article you cite only claims that asexual animals are an evolutionary dead end. Animals are just one branch of the tree of life, not its root. $\endgroup$ – tel Oct 13 '18 at 23:38

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