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According to Darwinism there are two mechanisms through which a living thing changes:

  1. Natural selection and
  2. Sexual selection.

Turkey Cock's Plumage:

Darwin gave the example of entering a farmyard and being struck by the magnificence of a turkey cock's plumage. Might not, he asked, a turkey hen feel something of the same and choose to mate preferentially with this male bird? In this way fowls of striking feather leave more offspring and, by heredity, elaborate plumage evolves in.

Callipygous Women:

A second example given by Darwin was of an African tribe who line their women up in a rank. The man squints down the rank and the woman who sticks out the furthest behind (here Darwin lapsed into Latin, serving Victorian delicacy) is taken as the bride. In this way the interesting problem of the human female's callipygousness (Bottom, which see) was, he believed, resolved.

The form of the argument is:

  1. Men like callipygous women.
  2. Hence men mate preferentially with callipygous women.
  3. Hence callipygousness becomes more common among women.

The main problem is that it is not made clear - we are not given a "logic" - what it is that transforms statement 1 (Is it significant that i does not appear to be a proposition - that is it is unclear whether it is true or false?) into proposition 2. We are not, in other words, told why liking something implies mating with it.

It may be relevant that this may not be the same as saying the argument is untrue.

To get round this problem Run Away Sexual Selection (which see) was devised.

Peacock's Tail:

The Peacock's tail is perhaps the most famous example of alleged sexual selection; although Darwin himself felt he could not deal with it, saying he left the problem to future generations.

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.

This has been a node your homework node.

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