Advantages of sex
The cost of sex is large. The problem of explaining sex is to find a compensating advantage of sexual reproduction that is large enough to make up for its cost. Several possibilities have been suggested:
• Faster evolution Sexually reproducing populations will evolve faster than a set of asexual clones, provided that the rate of favorable mutation is high enough. Thus sex may be maintained by group selection.
• Lower extinction rates The taxonomic distribution of asexual reproduction suggests that asexual forms have a higher extinction rate than sexual forms.
• Deleterious mutations The large numbers of deleterious mutations are more efficiently removed by sexual than asexual reproduction.
• Host-parasite arms race The coevolutionary arms race of parasites and hosts produces rapid environmental change, making sexual reproduction advantageous.
For these dandelions (opposite) the cost of sex probably outweighs any possible advantages: biologists believe that they have recently switched to asexual reproduction.