Sexual reproduction is the creation of a new member of the species by mixing the genes of the individuals. Sexual, as opposed to asexual reproduction offers many advantages, one of the main ones which seems to be that it speeds up the ability to adapt. If two new genes are present in the population (say one for greater strength, and one for greater speed) sexual reproduction allows these to merge instead of competing. Many of the details are complicated and beyond the scope of this work, but some discussion is relevant. The answers rely on Selfish Gene Theory.
Suppose you have a good gene A, and your partner has a good gene B (these may be for greater speed, and greater strength, or whatever). If both of you reproduced asexually then all of your offspring will have the A gene, and all of your partner's offspring will have gene B. If however you reproduce asexually then one quarter of your joint offspring will have gene A only, one quarter will have B only, one quarter will have neither, and one quarter will have both. Intuitively half of the offspring break even from the arrangement, one quarter will lose out, and one quarter will benefit. This may seem like a break even choice, but consider the following.
The males of many species keep harems of females. Ninety percent of males may never even copulate, the remainder of males produce dozens, or even hundreds of offspring. So the top 10 percent of the males produce 90 percent of the children. A small improvement in the genetic structure can have a huge impact in the success of the animal in reproduction. Writing off one quarter of the children for the possible overwhelming success of another quarter makes good genetic sense.
It also provides an interesting, though possible inconclusive insight into taking of favorites - the practice of some parents in allocating far more resources (time, food, education, love etc) to some children than others. The runts effectively get written off.
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