Sexual selection is related to natural selection in that they both deal with methods that the gene pool sorts itself into new generations.
Natural selection can generally be thought of as evolutionary processes giving favor to whoever is best fit to survive in a given environment.
Sexual selection is more about processes that determine who is best fit to attract a mate.
Sexual selection is controlled by the members of a species, as opposed to natural events. When a female chooses a male as a mate because of the beauty of his plumage, she is exercising sexual selection. The fact that he gets to breed has nothing to do with his ability to survive, and everything to do with his looks. Sexual selection, not natural selection.
There are two ways that sexual selection show up.
1. Members of one sex determine reproductive distinction by competing amongst themselves for opportunities to mate. Otherwise known as intrasexual selection.
2. Members of one sex create reproductive distinction by preferring certain individuals as mates. Otherwise known as intersexual selection.
- Female choice. All the power resides in the female, who chooses whoever she mates with. Peacocks are a good example of this. Female peacocks, which are plain and unattractive, choose the male peacock with the prettiest tail display. This does not benefit natural selection. Those flamboyant tails really stick out and ought to turn the bird into a giant billboard with ‘Lunch’ written all over it. But the female chooses, and the female gets what she wants. In humans, most of the power also seems to lie with the women.
Why do males get the ornaments?
- Females are the limiting factor – they invest more in offspring than males and many are unavailable for fertilization during the time devoted to offspring. Females don’t have the time to devote to developing ornaments.
- Males tend to exist in excess numbers and therefore develop ornaments for attracting females or fighting each other. There ends up being too many males, and they have to do something to make sure they get to spread their genes.
Little extra tidbits:
Sexual dimorphism - The physical differences between men and women
Sexual selection produces these differences. Quite simply, we want mates of the opposite sex to look like they are members of the opposite sex. It’s interesting to note that most of the physical markings that show up that determine sex, only show up after puberty. It adds emphasis to the idea that these differences are only there for sexual purposes.
Sex role reversal
Some species reverse the traditional roles of sexual selection. If the roles are reversed, it makes sense that the power of sexual choice is also reversed. This time, the male has to be choosy about who he mates with.
- Male ostriches sit on eggs most of the time, instead of the females.
- Male seahorses are the ones that actually carry the fertilized eggs around.
- AlexZander reminds me: "You might add that Male Penguins care for eggs too -- on the tops of their feet. Freaky penguins." I'd forgotten about this one, thank you!
So, the next time you add a little bit of you to the gene pool, ask yourself, was it natural selection you just succeeded at? Or sexual selection? Or maybe a little bit of both.
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