WEIT Chap6

First of all, sex is good.

HOW SEX DRIVES EVOLUTION

Why sex?

If sex between males and females didn't exist and the females could clone themselves by asexual reproduction, males would disappear completely and the evolution of species would end with them. Sex exists to allow animals to survive by adapting them to the environment that is constantly changing. The most important element is that sexual reproduction, by the random combination of genes, produces variability and thanks to this variability, some individuals are better adapted to the environment than others.

Example: If a species had the possibility of reproducing without sex (the female produces offspring on her own), the male of that species would disappear because his genes would become useless. (it exists a species of lizards in which it happens!) Then, if an illness appears and affects only that species, all the members will probably die because of the lack of variability in the genes (except if a strong mutation occurs: in this case, the different alleles would probably vary enough to counter the illness).

Why two sexes?

Jerry A.Coyne doesn't give any biological explanations for this question. In fact, he just says that "the question of the number of sexes is a messy theoretical issue". We can't really imagine more sexes because evolution made our sexes "compatible" to reproduce together. The differences between the males and the females gametes are also due to evolution. Furthermore, we can just imagine if there were three sexes or more... Today, with two sexes there are a lot of difficulties to find the good partner to reproduce. More sexes would be harder, would take more time, and evolution would be completely unsettled.

What is sexual selection?

Sexual selection is a natural process that occurs in animals just before mating: it's a selection that increases an individual's chance of getting a mate. There are two forms:

1) Male-male competition (The Law of Battle): It's a struggle between the males for the possession of females. Two males of the same species fight out, and the goal is to drive off the other male in order to win access to females. This kind of selection produces (allows the developpment of) armaments, like stronger weapons, larger body size or anything that helps a male win physical contests.

2) Female choice: This is a choice made by females. They "compare" males to each others and recognize the ones which have the most impressive physical features; for example, females birds prefer males with big tails, bright colors etc. And females frogs prefer males with complex calls. In those cases, the "armaments" that males deploy are different.

What is sexual dimorphism?

Sexual dimorphisms are traits that differ between males and females of a species. (ex: tails, colors, songs, etc). Through years, biologists have found that sexually dimorphic traits in males, even if they are developped in order to seduce females (bright colors, long tails, etc), show negatives aspects: during the seduction phase, males don't pay enough attention to predators (they are more vulnerable!) and they are more susceptible to be attacked by predators than females because of their bright features. (These features are nearly always restricted to males).

Why choose ? And why is it the role of females?

Females choose in terms of two aspects: the direct benefit and the indirect benefit. In direct benefit, the female is picking a male who will help her produce more or healthier young during the act of child care. In indrect benefit, females choose a male who has better genes than those of other males.

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