Sex costs half your genes, so why did evolution keep it?
On paper, sexual reproduction is a bad deal. An asexual population, where every individual can bear young, should grow much faster, and a sexual parent passes on only half of its genes. Biologists call this the two-fold cost of sex, and explaining why sex persists anyway is one of evolution's classic puzzles.
The mechanics are shared across plants, animals and fungi. Specialised cells undergo meiosis: DNA is copied, matching chromosomes pair up and swap similar but not identical stretches, and two rounds of division yield cells with a single set of chromosomes. When two such gametes fuse at fertilisation, the resulting zygote carries two sets, half from each parent. That shuffling is the payoff. It raises genetic diversity and makes it harder for harmful mutations to pile up.
Other proposed benefits include faster adaptation to changing surroundings, coping with competition, repairing DNA and hiding damaging mutations. All have some support, but population size appears to decide how useful sex really is, with larger populations cashing in on its benefits faster. Newer models suggest a basic advantage for slowly reproducing, complex organisms. The oldest fossil evidence of sexual reproduction in eukaryotes dates from the Stenian period, about 1.05 billion years ago, and it is thought to have originated in an ancient eukaryotic ancestor. Bacteria and archaea never adopted it, though some of their gene-swapping tricks are loosely similar.
Sex also unleashes sexual selection, which has been described as a powerful force absent from asexual populations. Usually females choose and males compete, sometimes through combat or display, and a feedback loop called Fisherian runaway can exaggerate traits to extremes. Peacocks drag cumbersome tails, lions with dark full manes attract lionesses despite overheating, and female deer favour large antlers.
Strategies vary enormously. Insects, more than two-thirds of all animal species, typically transfer a sperm packet that the female stores until she is ready. Most fish release eggs for external fertilisation, but guppies bear live young and male seahorses carry developing offspring in a pouch. Some fish are both sexes at once, others switch sex during life. Plants alternate between a spore-making diploid generation and a gamete-making haploid one.
Source: Sexual reproduction
