duminică, 26 februarie 2012

To mate or not to mate, this is the red question, mate

"By contrast, a Red-Queen-type theory that organisms are running cyclic arms races with their parasites can explain the utility of sexual reproduction at the level of the gene by positing that the role of sex is to preserve genes that are currently disadvantageous, but that will become advantageous against the background of a likely future population of parasites.

Sex is an evolutionary puzzle.

In most sexual species, males make up half the population, yet they bear no offspring directly and generally contribute little to the survival of offspring. In fact, in some species, such as lions, males pose a positive threat to live young fathered by other males. In addition, males and females must spend resources to attract and compete for mates.

Sexual selection also can favor traits that reduce the fitness of an organism, such as brightly colored plumage in birds of paradise that increases the likelihood for an individual to be noticed by both predators and potential mates (see the handicap principle for more on this).

Thus, sexual reproduction can be highly inefficient.

One possible explanation for the fact that nearly all vertebrates are sexual is that sex increases the rate at which adaptation can occur. This is for two reasons.

Firstly, if an advantageous mutation occurs in an asexual line, it is impossible for that mutation to become fixed without wiping out all other lines, which may have different advantageous mutations of their own.

Secondly, it mixes up alleles. Some instances of genetic variation might be advantageous only when paired with other mutations, and sex increases the likelihood that such pairings will occur.

Also, in asexually reproducing organisms, especially parthenogenetic organisms, mutations conferring an advantageous allele will have to occur twice, before the advantageous allele becomes fixed in the population, resulting in a longer phase where the heterozygote for the disadvantageous allele (relative to the new advantageous allele) is fixed in the population.

For sex to be advantageous for these reasons requires constant selection for changing conditions. One factor that might cause this is the constant arms race between parasites and their hosts. Parasites generally evolve quickly because of their short life cycles.

As they evolve, they attack their hosts in a variety of ways. Two consecutive generations might be faced with very different selective pressures. If this change is rapid enough, it might explain the persistence of sex.

The Red Queen Hypothesis