I am more or less familiar with the evolution theory based on mutations.

Now, starting with a nonsexual being, how did the first organism that reproduces sexually come to exist.

  • $\begingroup$ Your question is too broad. It won't be possible to provide a precise answer to such a question. Please add details to your question to make it more precise. $\endgroup$ – WYSIWYG Mar 22 '14 at 4:55
  • $\begingroup$ I think this is answerable - you need to look at fungi though. animals did not invent sex. will try to put up an answer tomorrow if someone else doesn't. $\endgroup$ – shigeta Mar 22 '14 at 5:36
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    $\begingroup$ related: Evolution from Mating types to different sexes $\endgroup$ – fileunderwater Mar 22 '14 at 19:24

Big topic yes, but lets hit some high points.

First, sexual reproduction is a no brainer in evolution. In its basis, sex only requires a segment of the chromosome to be heterogamous - that it will not be recombined. In male humans this is both X and Y chromosomes, in other animals it is the female which has heterogamy. Its not the entire chromosome either - the pseudo-autosomal regions of X and Y in humans do recombine.

Sexual reproduction allows some individuals to combine their genomes by donating some but not all chromosomes to offspring. The heterogamous regions are always donated by one parent and the same sex is not allowed to combine with others.

Sexual reproduction is found in plants, single celled eukaryotes as well as animals. The forces that drive the emergence of sex show up frequently enough and are persistent enough for sexual chromosomes to have evolved hundreds of times in plants.

Even in animals, where asexual reproduction is very rare there are multiple configurations of sex determining chromosomes.

Sex goes back to a very ancient trait in eukaryotes related to the haploid / diploid life cycle. In fungi, who are related to the original metazoans, sex shows up when its needed. There the combination of haploid phases may be limited by more than one combination of specific elements. Fungi can have many genders (mating types) - each of may only allow mating to specific types. Others like Saccharomyces allow recombination between any two different mating types.

The earliest single celled eukaryotes related to animals show mating types. The various sexual characteristics related to sexual reproduction, such as placentas, eggs, sperm form and function are all embellishments from sexual selection which vary tremendously from one form of animal to another. The chromosomal functions which became sexual traits are ancient and predate metazoans. They are not necessarily conserved though - they are simple enough to reproduce that they change their characteristics often in evolutionary history.

I should say that all this doesn't apply to bacteria and eubacteria. Why? Because bacteria are much older and have their own systems of gene transfer and recombination with their own rules. They usually have a single copy of their genome (I'd like to hear about exceptions - I think I know of maybe one) which makes quite gender different. The do have something called 'sex' where a pili gene can be found on a plasmid which allows the bacteria to transfer DNA directly, which is like a sex determining locus. But a bacteria may catch gender like a cold - it can not have it, it can get it, it can lose it.

Bacteria are also capable of transferring a single gene or a segment of DNA by various means, including pili, plasmids, phage, and simply taking in the naked DNA that they find laying around.

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  • $\begingroup$ I’m not sure I would agree with “no brainer”. As far as I know, sexuality has only evolved once. Also, could you expand a bit on the heterogamy argument? In particular, wouldn’t the same apply to plasmids? Yet we don’t see anything particularly resembling sexual selection emerge from that. /EDIT: Your answer might actually contradict my first assertion (I should have read the full post first) – I’m still interested in an answer to the second. $\endgroup$ – Konrad Rudolph Mar 23 '14 at 13:07
  • $\begingroup$ you can argue the terminology, but i do have references saying that sex has evolved independently several times in plants etc, so i'll leave that to you and the authors or to post to your answer. I've added a section on bacteria. sex in bacteria is more of an analogy than a correspondence from what i know. The question focused on animals originally. $\endgroup$ – shigeta Mar 23 '14 at 13:36
  • $\begingroup$ Actually your text already convinced me, concerning the number of times sex evolved (hence the edit). $\endgroup$ – Konrad Rudolph Mar 23 '14 at 13:39

This is a very broad field, with many complementing, overlapping, and contrasting theories. I suggest that you read this part of the Wikipedia article on this topic:


Here is what I got from the Wikipedia article (this is just one hypothesis, read the article for all the others):

There are many advantages to sexual reproduction, two main ones are greater genetic variation and recombinational DNA repair (occurs during meiosis).

There are certain species that can reproduce both sexually and asexually. They choose to reproduce sexually during times when the environment is uncertain (changing). This provides evidence that greater variation is one reason for sexual reproduction.

Asexual organisms only have one copy of each gene, therefore if this copy is mutated in a bad way, they are out of luck. Organisms that sexually reproduce have two copies of each gene, when one goes bad, they can use the other copy to repair the original copy, during meiosis.

As you may know, stress has many harmful effects, one being that it leads to DNA damage. There are certain species of bacteria who, in response to stress, uptake/integrate nearby bacteria cell's DNA into their own. They can then use this new copy to repair their own copy in case they experience DNA damage.

This "trait" of up taking/integrating other's DNA may have been evolutionary selected for (because these organisms repaired their DNA, they survived and passed on their genes), therefore eventually leading to organisms that sexually reproduce.

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    $\begingroup$ I’ve had to edit your answer, “just one theory” sounds a bit too similar to the “just a theory” creationist canard for comfort. I’m wary that it might feed the common misconceptions about what makes a scientific theory. $\endgroup$ – Konrad Rudolph Mar 23 '14 at 13:05
  • $\begingroup$ I fully support your edit, that was a nice catch :) Thanks Konrad. $\endgroup$ – MeLikeyCode Mar 25 '14 at 18:36

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