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Many animals species require two members — a male and a female — to sexually reproduce. Why has nature (or the process of evolution) chosen to favor the form of sexual reproduction which requires two members? Why didn't nature encourage the form of sexual reproduction which involved three members?

The reason for sexual reproduction being a better choice over asexual reproduction is justified by saying that sexual reproduction involves mixing of genetic information from two living beings and hence if one set of them had an error, the other would compensate for it. With the same kind of reasoning, we can argue that sexual reproduction involving more than two organisms coming together would cause better genetic variation, shuffling of mutations, etc. and thereby produce a better offspring.

However, most of the organisms as we see today have evolved to reproduce sexually using two beings of the same species.

Consider an animal species which has 24 pairs of chromosomes.

Sexual Reproduction Type 1:

There are two sexes: male and female. During the event of sexual reproduction, the male transfers 24 chromosomes to the female where the chromosomes from both the sexes are mixed to produce an offspring.

Sexual Reproduction Type 2:

There are three sexes: male, female and [insert word]. During the event of sexual reproduction, each member transfers 16 chromosomes to the female where it gets mixed to produce an offspring.

We don't observe sexual reproduction of type 2 (or it is rare). Doesn't the type-2 form of sexual reproduction cause better mixing of genetic information than type-1 form of sexual reproduction? Despite the aforementioned advantage, nature has chosen to encourage the type-1 form of sexual reproduction. Why is the type-1 form of sexual reproduction more common than the type-2 form of sexual reproduction?

This question can be extended to "Why has evolution favored sexual reproduction involving two mates than sexual reproduction involving $n \space (n > 2)$ mates?

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    $\begingroup$ i think it's an interesting question $\endgroup$
    – Filipe Rocha
    Mar 25, 2017 at 11:49
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    $\begingroup$ Are you asking about species with more than two sexes/mating types or situations where more than two sexes are involved in a single reproductive event? $\endgroup$
    – fileunderwater
    Mar 25, 2017 at 12:29
  • $\begingroup$ @fileunderwater I edited the question. Is it clear now? $\endgroup$
    – Yashas
    Mar 25, 2017 at 12:39
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    $\begingroup$ @kmm I don't think so - the dynamics behind the evolution of sexes and multiple mating types is an active field of research. $\endgroup$
    – fileunderwater
    Mar 25, 2017 at 17:44
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    $\begingroup$ Because it's a lot easier to get two sexes together than three? Especially in the more primitive types of mating, where eggs/sperm are simply released into the water. (Or pollen into the air.) $\endgroup$
    – jamesqf
    Mar 25, 2017 at 18:11

3 Answers 3

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Let's break it down: The chance that two individuals of distinct gender meet at one location in space time is tremendously higher than for three. We could do some math here (assuming biased random walks of points in three classes on a 2d surface etc.) but that is not necessary I guess. As far as we know sexual reproduction evolved long before complex sensory systems and/or brains (namely 1250 million years versus 500 million years) that allow for active search behavior beyond chemotaxis. Therefore a random walk would be a reasonable first approximation for organism behavior to simulate the situation back then. Having two genders seemed to have been a good trade-off between the chance to meet and advantages in terms of mixing the genetic information. Note that this answer might not be valid for organisms being able to reproduce in both ways, sexual as well as asexual.

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  • $\begingroup$ Your first sentence ("chance... meet at one location in space time...") makes me think of the pair, two as a number, as a mathematical maximum of compatibility if the premise were one and only one rule of "dominance". Underlying question: What is the mechanism that safeguards existence and identity of one and the same sex (best answered in some logical context)? $\endgroup$
    – Peter Bernhard
    Mar 3, 2022 at 6:41
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Quite simply two mates is simpler both functionally and logistically. It is basic statistics you are far more likely to find 1 individual from 50% of the population once, than 2 individuals each representing 33% of the population at the same time. That's three factors making two sexes statistically easier. Also most sexual organisms do not have chromosome counts divisible by 3, some don't even have numbers evenly divisible by 2.

There is still debate about why sex is favored at all, so it is difficult to go into greater detail for other possibilities, but it is far more likely for two individuals to cross paths than three or more, that alone would be enough for a selective pressure favoring two over three most of the time. And of course not now most sexual organisms are built to handle that 2 individual mating they inherited and could not mechanically handle a third.

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    $\begingroup$ Can you please further on the claim that two mates is simpler both functionally and logistically and eventually back up your claim with a citation? If you google the question why is there two sexes, you'll get plenty of popular science article to inspire an answer. I don;t if they cite much trusty sources though. $\endgroup$
    – Remi.b
    Mar 26, 2017 at 17:30
  • $\begingroup$ "... likely to find 1 individual from 50% of the population once, than 2 individuals each representing 33% of the population..." - I think you should state your assumption that there is non-compatibility. I guess you do NOT assume that there is compatibility of any of those sexes with each and every other. $\endgroup$
    – Peter Bernhard
    Mar 3, 2022 at 12:59
  • $\begingroup$ @PeterBernhard I do not understand what you mean, the OP specifies 2 mates, over 3 mates, encountering 1 individual will always be more likely than encountering 2 individuals at the same time, unless you resort to special pleading. $\endgroup$
    – John
    Mar 3, 2022 at 20:47
  • $\begingroup$ @Remi.b its just math, P(A) will always be more likely than P(A and B) or P(A)(A) $\endgroup$
    – John
    Mar 3, 2022 at 20:55
  • $\begingroup$ Ah, now I now why you do not understand. I did not understand your premise. Your premise is that any one gender - e.g. of three - does NOT fit any other. What you are assuming is that A matches B, but does not match C, right? - That ain't necessarily so. Every one of three might match any other of the box. That's what I meant without knowing. $\endgroup$
    – Peter Bernhard
    Mar 4, 2022 at 12:17
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Though some would disagree, sexual reproduction has distinct advantages over clonal reproduction specifically because it provides a way to shuffle homologous blocks of genes within a population. But increasing the number of parents required beyond two would not provide enough greater "shuffling" power to compensate for the logistical impediments to n-parent matings.

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  • $\begingroup$ I find "... shuffle homologous blocks of genes within a population ..." unobtrusively alluding at the clone, the clan; "the population" is "my universe". Imagine different diploid "races" that know no interaction between them, then imagine a mitotic individual mingling within itself. With diploid non-sexual reproduction there is mingling of genes - the "problem" is the clone that stemes from Adam mingling only Adam's genes, not even Eve's, no mating. Nothing specific to mating, mingling, will happen in diploid cell. Any separate clone will "stay like that". $\endgroup$
    – Peter Bernhard
    Mar 3, 2022 at 18:25
  • $\begingroup$ Currently unable to delete my commente above which is not bad : "shuffling" of genes does not happen in one individual of a population, although logically it could ("imagine"), as those beings are all diploid in their cells. So what is the advantage of refraining from shuffling in mitosis? $\endgroup$
    – Peter Bernhard
    Mar 4, 2022 at 11:02

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