WEIT 2012 Chap6

HOW SEX DRIVES EVOLUTION

What is sexual dimorphism?

We start the reflexion reflexion not an English word--Pierre.brawand 15 mars 2012 à 17:51 (CET) about how sex drives evolution from the maladaptive traits for survival that we can observe in certain animals.
A maladaptive trait for survival means that this trait is an inconvenience for the animal. It can be a physical trait (as the tail of the peacock that decreases its speed) or a trait that has to do with the behavior (for example the singing tùngara frogs that attract bats and bloodsucking flies).
The evident question that follows these observations is: How could such deficiencies be the result of natural selection? Charles Darwin was very worried about this question because in fact it could show a failure in his natural selection theory. An observation will bring Darwin on the way of the answer: only males suffer from this type of elaborate trait. This is why we call it sexual dimorphism. We need to understand now how does it come sexual dimorphism to exist. The explanation of Darwin is that these traits aren't the result of a natural selection for survival, but a sexual selection in order to increase the chances of mating. In other words, the goal of these dimorphism is obviously not to increase the survival potential of an animal, but the reproductivity check that word--Pierre.brawand 15 mars 2012 à 17:51 (CET) of this one.

What are the two strategies observed for sexual selection?

There are two forms of sexual selection. The first one is the direct competition of the males, while the other is the female choosiness. We can observe that in both cases, males compete for females.

Direct competition of the males

The first one is the competition that take place between males, who struggle against each other to gain access to females and so win the right to reproduce. This male-male competition also influe largely on evolution, because each trait that will permit a male to win the struggle will be pass to the next generation. In this case, most traits will be linked to physical aspects, and selection will make that males will get stronger and stronger (or larger or everything which is needed to win the battle, according to the species) in the next generations.

Female choosiness

However, there is another way of process, which depends only on the female's choice. Male competition is still present in this kind of selection, but in an indirect way. They do not compete by struggling, but they fight with their colours, which have to be the brightest possible, or with their voices or singing, or other traits or behaviour that females could find attractive. The winner is the male that the female decide to choose.

Why sex?

Producing eggs or sperm for a being is like sacrificing 50 percent of its genetic material compared to an individual who reproduces asexually. So why do sexual reproduction still exists? To understantd this we should take an example. Imagine that there was a gene in human who led to sexual reproducion but that its mutant form enabled a female to produce eggs that develop without fertilization. (Some animals do reproduce this way.)This first mutant female would only have daughters because woman can only produce eggs containing the sexual chromosome X. Those mutant daughters would produce even more daughters. Mutant woman would not need man to mate anymore, therefore, male would become less and less and finally disapear. The final event of this is that sexual reproduction also disapears. This situation is called the "twofold cost of sex". The good point in sexual reproduction is that it produces new combiantions of genes in the offsprings.

What are females looking for when they pick a male?

This question is not that easy to answer, because even if we know that there is a link between patterns, male calls, etc. and the females choosing, we don’t know what kind of link we are dealing with. Darwin’s hypothesis is quite surprising. For him, it’s just a question of aesthetics. Females would be “charmed” by males’ features. He admits that his theory is hard to believe but, as his and his contemporaries’ knowledge is very small concerning animal’s mind, he doesn’t want to exclude this idea. Actually, he was right only about the general idea: yes, females do choose males and yes, features have something do to with it, but not the way Darwin thought it would be. In fact, it’s all a question of genes. We mustn’t forget that females always tend to spread their genes, so their goal is to spread them in the best possible way. With this in mind, we can easily see that the chosen male will have his importance too.

What do females gain by choosing a particular male?

First, they can benefit directly, for having a caring male will permit them to have a stronger and healthier offspring (the young ones are better taken care of). The second way is to choose a male that will have better genes, but it will not be an advantage for the females - that it why it is called the "indirect way", because it’s only during the next generation that the particular genes – favoured by natural selection – will be put to use.

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