Lots of organisms reproduce sexually, but plenty don't. Many organisms reproduce asexually, without sex. Given that sex is expensive and some organisms reproduce just fine without it, why do it? Evolutionary biologists ponder this question because at first glance, sex doesn't seem like a very fit thing to do:
i A sexual organism is effectively throwing away half its genes when it reproduces. When an organism reproduces sexually, only half its genes get passed to its offspring; the other half come from the other parent. Asexual organisms, on the other hand, pass on all their genes. So from a fitness perspective, it would seem that asexual reproduction is better (read, more fit), hands down.
i All other things being equal, the sexual individuals will soon be overrun, and sexual reproduction will be eliminated. For sex, you need males. Asexual females don't have to make any males, so they produce all daughters, which in turn produce all granddaughters, which produce great-granddaughters, and so on. A sexual female that produces the same number of offspring will make only half as many daughters as the asexual female. She spends the other half of her energy making sons. Because her daughters will, on average, make only half as many daughters as the daughters of the asexual female, the original sexual female will end up with only one quarter as many grandchildren as the asexual female, one eighth as many great-grandchildren, and so on.
But sexual reproduction hasn't been eliminated, of course, which means that reproducing sexually must have one or more fitness advantages. So what are these advantages? Well, the truth is that evolutionary biologists just aren't sure. Many ideas have been suggested, and probably more that we haven't thought of yet will be suggested. The following sections look at the current ideas.
The ideas outlined in the following sections are not intended to be mutually exclusive; they could be working together simultaneously to maintain sexual reproduction. The question "Why have sex?" may well have many answers.
Why males don't count
Males don't contribute to population growth; they of the other, and vice versa. Each sexual female are needed so that the females can reproduce, ends up with four grandchildren, but that doesn't but they are otherwise a waste of resources, and mean there are eight grandchildren; there are still that's why they don't get counted. Suppose that only four total, and only two of these are female.
two sexual females each make one daughter and The population has gone from two females to two one son. The daughter of one mates with the son females. That's why the males don't get counted.
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