Arunas L. Radzvilavicius in Aeon:
At its heart, sex is a process of genetic mixing: it creates unique sets of genes and trait combinations different from either of the two parents. In eukaryotes (organisms such as animals and plants), the molecular machinery of recombination deliberately breaks the chromosomes into chunks, only to reunite the pieces of maternal and paternal origin into novel permutations that are then passed on to the progeny: a remarkable act of molecular wizardry, perfected over billions years of evolutionary tinkering.
But it is not the molecular workings of recombination that captivates biologists the most, it’s the fact that genetic mixing in the form of sex has evolved in the first place, in spite of it being a cumbersome and costly endeavour. Evolutionary theorists agree that cloning, in many ways, is a more efficient mode of reproduction, which, in a world governed by the rules of natural selection, should readily outcompete sex. An asexual female, for example, would produce twice as many offspring as the sexual one, avoiding the burden of bearing males or searching for suitable mating partners.
Sex is unknown in bacteria – the simplest and most ancient living cells on Earth – that reproduce by simply splitting into two. Evolving considerably later, eukaryotes are built of much larger and awfully complex cells, their insides full of organelles and membranous labyrinths buzzing with sophisticated molecular machinery and cargo-transport networks. Unlike bacteria, very few eukaryotic species revert to strict asexuality, and those that do seem to be relatively short-lived on the evolutionary timescale. Sex is costly, but it also appears to be essential for the long-term survival of complex life.
Some of the most talented theorists have striven to understand why. Myriad explanations made their way into science journals and textbooks – from the earliest proposals that sex generates variation and speeds up adaptation, to mathematical models demonstrating that gene shuffling bolsters resistance to parasites and slows down the accumulation of hazardous genetic defects. But even with the overwhelming amount of attention the problem has received over the years, it is still considered unsolved.