WEIT 2012 Chap6
HOW SEX DRIVES EVOLUTION
What is sexual dimorphism?
We start reflecting 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 to increase its chances to mate.
- see Appendix 1
Why can we find sexual dimorphism in many monogamous species?
It would be logical that in species that are monogamous, males don't have to compete. But even then we can observe dimorphism. The answer to this problem is not to find in the dimorphism explanation, but rather in the idea of "monogamous species". Researches have shown that these species aren't actually monogamous. Numbers show that if 90 percent of birds are socially monogamous, three quarter of these species have "extra-pair" matings.
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 between males
The first one is the competition that takes 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.
- see Appendix 2
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.
- see Appendix 3
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 understand 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 wouldn't need man to mate anymore, therefore, male would become less and less and finally disappear. The final event of this is that sexual reproduction also disappears. 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.
Why is there only two different sexes? Why not three or four? In fact, two sexes is the most robust and stable strategy.
Why do the two sexes have different size and number of gametes?
Male can produce large quantity of gametes and so can father a great number of offsprings. This number is of cours limited by the number of female and theire gametes. For this reason, we can say that males compete for the eggs of the females in order to fertilize them. (This explains why the "law of battle" exists and why males developed differents features to be choosen by the females.)
Producing sperm cells and mating for male is cheap. In contrary, mating for a famale is very expensive because she produces large quantity of nutrients for the eggs and fetus but she also needs energy and time for females provide all the parental care in more than 90 percent of mammals species. (Example: The record number of children for a woman is is 69 but the record for a man is around 1042.) This is why females should be "picky". They have a limited amount of eggs that forced them to choose the best father as possible.
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, females' choice, etc, 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 are 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?
As we have seen it, females don't chose males randomly. The reason why they chose a particular male rather than an other one, is that the chosen male carries some assets (characteristics) that are beneficial for the offspring. There are two type of characteristics that each benefits to the female in a particular way. The female can benefit either directly, or indirectly.
Direct benefit
A female could pick the more caring male who will help her having more healthier young because they will be better taken care of. Indeed, if the male possesses a good territory, her offspring will be better nourished and occupy better nests. Consequently those young will have more chances to survive and it means that the young females may carry the "preference gene". In other words, the preference for males having good territories will evolve and be stronger and stronger over time.
Indirect benefit
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.
What is the link between features and benefits?
An experience showed that the female house finch of North America prefers brighter males. But why? In fact, it is proved that brighter males feed their young more often than lighter males. Why is there such a difference between them? Because bright color is a sign of good health. A male that is not in good health can carry parasites or diseases which may be dangerous for the female and her young.
In the case of the sage grouse, selection will favor genes that promote the female's preference for plain vocal sac and the male trait that make them have a plain vocal sac. In fact, female's preference for the plain vocal sac will increase and the male's vocal sac will get bigger generation after generation.
To summarize, females prefer any trait showing that a male will be a good father.
Is the female looking for direct or indirect benefits?
Another experience had been made to test the good-genes model and it was done on gray tree frogs. Female frogs prefer males whose calls are longer so researchers took eggs from different females and fertilized half of them with sperm from long-calling males and the other half with sperm of short-calling males. It appeared that offspring from long-calling males grew faster and survived better than the others. This experience shows that female frogs get no direct benefits in choosing a long-calling male but her young will be gentically superior.
It is just the same for peacocks. Males with more eyespots are carrying better genes.
How does sexual dimorphisms evolve?
Imagine a member of a species that have developed a preference for red color because what he usually eats is red like berries for example. If a mutant male appeared with a patch of red on his breast, females might prefer him because of their preexisting preference for red. If the mutant male mates, the offspring might carry the mutant gene too and be prefered by femals which an advantage. This way there would be more and more red males and the color dimorphism could evolve.
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