Darwin introduced the idea of sexual selection as a complementary to natural selection. In the living world, one sex, usually the female, invests more in reproduction process. She has standards to choose the best males to mate with. On the other hand, males can mate with many females. In order to spread one’s own genes as many as possible, the males compete with each other based on females’ standards in order to win the chance to mate with females.
The picture above shows two male red deers competing with each other. They are trying to show their strength and fitness to attract female. Some female doesn’t choose male based on the strength. For example, peacock. The male peacock that has the most beautiful and incredible tail always attracts a female peacock. Such selection has no apparent relationship with fitness, but “ornaments”. By selecting peacock with beautiful tails, a female will have “sexy sons”, who can pass on her genes in the future.
So, has sexual selection influenced the evolution of human? Although there’s not so many academic research to prove it, but it will be doubtful to say no. Some scientists have suggested that our brains are the result of sexual selection. Our ancestors viewed intelligence as one of the standards while selecting partners to mate with, so that we become smarter and smarter during the evolution.
There are lots of discussions about the maintenance of sex. Seeing many species get along perfectly well without males, why is sex still important? Unless something stops them, asexual species can grow exponentially – one, two, four, eight, sixteen …They can produce clones of themselves and some of them can have over 1 million descendants after only a few days. So, why does sexually reproducing still necessary?
The picture above shows the different between sexual and asexual reproduction. This is the main reason why sexual reproduction is important for evolution. Theoretical evolutionary research has put forward more than 25 hypotheses in the 1970s and 1980s to explain the advantages of sexual reproduction in evolution. The hypotheses can mainly be divided into two groups, both of them point out that sexual reproduction reduces the risk of extinction.
First, sex provides novel genetic variation in offspring by reshuffling genes during recombination and meiosis. Thus, it will provide opportunities for novel fast adaptations. Only sexual reproduction can reshuffle genes because the offspring will mix the feature of both parents, asexual reproduction will have offspring identical to parent. Also, sex removes negative deleterious mutations through meiosis and recombination.
Overall, although sex appears costly, it will do well to offspring for most species. Rather than asking why we need sexual reproduction, we may want to ask how those asexual species exist during the evolution.