Why do humans (and most other animals that I know) have only two sexes? Only male and female? Of course, I am cognizant of the fact that other genders do exist and the sex column in applications these days has not only male and female. But still, the majority of humans, and majority in other animals I know, are divided into males and females. In fact, Wikipedia defines sex as "Organisms of many species are specialized into male and female varieties, each known as a sex" Further, you need a male and a female for the important act of procreation. But why is there not a male, a female and a gmale? Why is it not that a male and female always gave birth to a male, a female and a gmale gave birth to a female and a gmale and a male gave birth to a gmale? A much interesting rock, papers, scissors form of procreation!
Well, first of us let us find out if there are such cases in the animal kingdom. Of course, there are species which are not divided by gender. Of these, I think, there are two kinds. One, like bacteria, where each individual can reproduce by itself. Whenever they feel bored, I guess they just divide. Do bacteria have pleasure in this act of procreation? I do not know. Do they feel pleasure at all? But they do seem to be generally at it like bunnies.
The second should be the hermaphrodites, those that have it all. The earthworms found all over the earth are hermaphrodites. The earthworms are generally jealous of the bacteria because they cannot do it by themselves and do need a mate for reproduction. There are other hermaphrodite species, like the banana slug, which, if the need arises or if someone dares them, can reproduce by themselves.
After we have got those ones out of our way, we can come back to our main, most pressing, question. Why only two? First, why not just one? Well, for organisms which reproduce once in a while, random mutations may not provide enough variety for all the members of the species to not die off when the long winter comes. Two sexes and the mixing of gametes produces more variety and ensures survival of the species under changing circumstances.
Hmmm.... seems like there are worm species of the genus Auanema that can have three sexes. See here and here.
So? I don't know. Maybe three sexes is too complicated a dynamics, considering that species with two sexes are flourishing. Also, probably reproduction cannot be of the rock, papers and scissors kind that was described earlier. It is more natural for the offspring to have the sex of either of the parents, assuming that there is some "sex factor" that it gets from that parent that gives it the sex. (Reproduction requiring a threesome is of course too complicated for a species to survive in the natural world.) For example, a human male needs a Y chromosome that it gets from its father. It is possible to imagine scenarios where the manifested sex is that of what is missing, and hence the offspring could be of a third sex than the parents. But that does not seem to be the most likely scenario. To understand what could happen, we can make a small perturbation to our usual two sexes. Imagine there being three sexes, two of them male and the third female. The two male sexes, A and B, compete for the female sex. A small advantage on the part of the A sex could be amplified over generations leading to the extinction of the B sex. Thus, it seems that small perturbations about the two-sex state are not stable. But then why was one sex not stable? Varying conditions were not favourable for complicated organisms that needed more time to reproduce and thus did not have enough time to get changes by mutation. Thus, we might surmise that if we have an era when conditions are very volatile, then having more sexes than two would be favorable for survival. There could be an optimum number in such situation, depending on how fast the conditions are changing, for the number of sexes.
Well, those are some ideas. There are some resources I obtained while trawling the web. See this, this and this. The one useful thing from the second link seems to be the third link. The first and second links have to be looked at in more detail.
Comments
Post a Comment