Evolution - Why Sex?
- males cannot bear young and, in most species, are no help in raising young
- females only pass on only half of there genes but must expend time, effort, and energy
- why bother with sex?
In pools in Mexico - 40% of minnows are heavily infected with a pathogen. Those reproducing asexually were more affected than those reproducing sexually.
Red Queen theory - from Alice in Wonderland - Red Queen said "you must run as fast as you can to stay in the same place." Alice said "Isn't it strange, we are running as fast as we can and getting nowhere."
i.e., you must keep evolving to keep up with rapidly evolving competitors
- after a drought, parasites infected sexually reproducing fish but the clones were not affected.
- sexually reproducing fish became inbred because come from a small population; therefore the parasites targeted them.
- sex provides diversity which is an immense challenge to competitors
- natural selection explained why beneficial traits become popular. It did not explain things such as a peacock's tail - heavy, conspicuous, energy to grow, definitely a survival disadvantage. Any elaborate ornamentation is hard to explain using natural selection. Usually seen on males even though natural selection operates equally on both males and females.
- explained by sexual selection - males compete with each other and females choose the best male because they invest far more in reproduction
- the idea is to have offspring that will survive to reproduce
- offspring have the greater chance of survival if the female chooses a male that sticks around rather than simply choosing the best genes. The result is monogamy. Some evolutionary forces favor monogamy but others do not.
- organisms must balance genetic quality with stability of the male. Songbirds are extremely monogamous but 40% of chicks are not sired by the male who raises them.
- in some species the male gives parental care so the female can produce more eggs. This behavior is often accompanied by females which are larger, aggressive, compete and fight - a complete role reversal. The successful female destroys the eggs of the vanquished female so her male can mate with the winner female.
- infanticide in some species is solved by females mating with all males so he never knows which offspring are his.
- chimpanzee females forage alone for food so they don't form bonds to one another.
- bonobos monkeys live in an area with lots of food on the ground so females can bond together rather than live alone. Female solidarity allows them to dominate the males and therefore they are less violent.
- humans were driven out of Africa by the same drought so they had to forage as nomads therefore didn't evolve close bonds between individuals.
- drought forced gorillas out of the forest so chimps were able to get food on the ground. When plants came back, gorillas didn't therefore chimp evolution branched to form bonobos.
- human females choose males based on different genes (i.e., different races etc.) by smelling t-shirts so makes sense to get different genes.
- sugar and fruit analogy - animals evolved to like sugar because it came in fruit so they got a good meal if they liked sugar
- beauty may have evolved in the same way - cues in the face give cues to fertility. e.g., small jaw and full lips means higher [estrogen]. High testosterone is good for a short term mate but nurturing, feminine qualities (indicated by a softer face) is better long term.
- ovulation changes what females find attractive
- why is the human brain so large? personality, wit, charm
- determines how a person acts and feels therefore it is important for choosing a mate. The brain is important in addition to beauty.
- eventually sex became fun and parenting rewarding so animals had more offspring and it was selected for.
- because of our large brain, we can take the urge to parent and apply it to a child with no genetic link. Humans are the only species to do this over the long term.