Sex is ubiquitous. The vast majority of animals and plants reproduce sexually at least some of the time. Some, such as humans, can reproduce no other way. Figuring out why sex is so common, though, has been a longstanding challenge for evolutionary biologists.
The problem is that, as a reproductive strategy, sex seems wasteful. The mere fact that you have survived to adulthood means that you are reasonably well adapted to your environment, and it is not at all clear that reshuffling your genes with those of someone else will lead to anything as good, let alone better. Furthermore, a female who reproduces asexually by making diploid eggs passes roughly twice as much of her genetic material on to the next generation as does one who reproduces sexually. Overall, cloning yourself would seem to be the way to go.
Sex and recombination generate new variation faster than mutation alone, and this might facilitate adaptation to an unpredictable environment. Is this sufficient, though, to overcome the short-term costs of sex to an individual? Through the middle and late 20th century, the answer seemed to be “no.” A number of population geneticists studied models in which a “modifier gene” determines whether or not the genome that contains it undergoes recombination (one allele, or variant, of the modifier gene allows for recombination; the other prevents it). Under most conditions, recombination declined in frequency and ultimately disappeared in these models. Furthermore, sex and recombination often reduced the rate of adaptive evolution. John Maynard Smith, surveying these models, concluded that while it was possible for recombination to be favored in an unpredictable environment, the environment would have to be “unpredictable in a special and somewhat implausible sense.”
For the rest of the story: http://nautil.us/issue/7/waste/evolution-in-a-finite-world