Given that animals reproduce sexually - by mixing genes, why do we have two distinct sexes - male and female? Surely it would be an advantage for any member of the species to be able to be mate with any other instead of being only limited to half of the population.
Suppose there is a single gender, and reproduction is a free for all. Suppose that both parties produce eggs, which grow into fully fledged creatures. Suppose that two eggs must (or can) merge to produce a fully fledged individual. Now consider what will happen with such a system.
For simplicity let's assume that a whole group of animals (of the same species) release their genderless eggs into some kind of primeval soup, that the eggs find another egg, and begin to form a multi-celled creature. Let's also assume the eggs have a certain optimal size - a trade-off between chance of survival, and resource levels. Some of the animals who contribute to the egg soup will cheat. Some of them will instead of producing one egg of the optimal size, produce many many eggs of much smaller size. Lets look closely at this to see why.
Suppose that everybody (except you) produces one optimal sized egg. Imagine what will happen if you produce eggs which are 1000 times as small. You can probably produce 1000 of these with the same effort. When these are dumped into the primal soup, you are likely to impregnate 1000 other eggs, whereas everybody else will have only impregnated one. True the combined eggs that your mini-eggs produce will only have about half the size of all the others, and therefore less chance of surviving, but you have produced 1000 times as many. Obviously your genes (the ones that code for cheating) will spread far more successfully than the others (all other things being the same).
Of course, given the choice of whether to join with a large "fair" egg, or a smaller "cheating" egg, the logical choice is to merge with the large one, but what if there is no way off telling the size of the egg until the merge has happened - and then it is too late. The exploited egg is not going to suicide just because it has been exploited. It will make the best of a bad lot, and try to survive, otherwise it will be even less successful.
There is a natural balance here though. If your smaller eggs merge with another smaller egg, then the chances of survival are nearly zero, if the vast majority of eggs are cheating eggs, then there is no longer any point in producing them, because there will be very little chance of finding a large egg to exploit. In this case you will be better off producing larger eggs, knowing that you will be exploited, but at least knowing that you will have a successful union. So we see half the population producing small eggs, or sperm (we call them males), and the other half producing large eggs (which we call females).
Hence being male is fundamentally exploitative. In some cases the roles of male and female have been reversed, and the male cares for the young, but these many natural factors are strong enough for exploitation to occur in many cases.
While this discussion has been very simplistic, the reader should be able to convince himself that similar forces exist in many aspects - people who steal, and people who are honest. Very aggressive, and very passive people, and so on. The complexity of human society is great.
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