The second hypothesis regards the advantages that sexual reproduction provides in dealing with other organisms. When two or more species evolve in response to each other, this is known as
coevolution. Every species must coevolve with its predators and/or prey and/or hosts and/or parasites. Evolutionary “arms races” result, because predators must evolve to cope with evolutionary changes in their prey, prey species must evolve in response to evolutionary changes in their predators, host species must evolve in response to evolution of their parasites, and parasites must evolve in response to evolution of their hosts.
In other words, every species’ evolution is influenced by the demands imposed by the other species with which it interacts. For example, if a host species evolves a more effective defense against its parasites, that will create selection pressure for the parasites to evolve more effective ways to circumvent their host’s defenses. That, in turn, will create selection pressure for the host to evolve even more effective defenses – which will create selection pressure for the parasites to defeat
those defenses. And so on, and so on.
According to this hypothesis, the advantage of sexual reproduction is that it greatly speeds up the rate at which evolution occurs, thus making it far easier for species to respond to evolutionary changes in their prey, predators, parasites, or hosts.
In an asexually-reproducing population, since there is no sexual recombination of genes, the only source of new gene combinations is the occasional mutation. Therefore, asexual populations inevitably evolve far more slowly than do sexual populations of the same species.
Because species must constantly evolve in order to keep pace with other species, they’re caught in a situation similar to that faced by the Red Queen in
Alice in Wonderland, who told Alice, “Now
here, you see, it takes all the running you can do, to keep in the same place.” So this hypothesis – namely, that sexual reproduction is favored because it allows species to quickly evolve in response to changes in other species – is known as the “Red Queen hypothesis.” The advantage to the individual in producing genetically-diverse offspring is, again, that this maximizes the chances that at least some of those offspring will happen to inherit gene combinations that make them well-adapted to deal with their predators/prey/hosts/parasites.
According to the Red Queen hypothesis, the need to quickly evolve in response to other species, especially parasites, is the driving force behind sexual reproduction. The hypothesis predicts that asexual reproduction should be favored in environments where parasites are rare or absent, whereas sexual reproduction should be favored in environments where parasites are common. Field studies support this hypothesis, too. In populations that can switch between sexual and asexual reproduction, whether they reproduce sexually or asexually tends to be strongly correlated with parasite densities.

The freshwater snail species Potamopyrgus antipodarum
(Phylum Mollusca). In parasite-free environments, asexual
populations out-reproduce and replace sexual populations.
In parasite-filled environments, sexual populations out-reproduce
and replace asexual populations.