Adaptations in sexual reproduction - What environmental changes make sex advantageous?
Does parasite-host coevolution explain why sex exists?
Parasite-host coevolution is almost universal in nature. Humans are parasitized by
• bacteria (such as Escherichia coli pictured opposite)
All species, including bacteria, are parasitized by viruses. A number of empirical studies have found that, in taxa in which both sex and asex is found, sexual reproduction is commoner where parasite pressure is higher; this work tentatively supports the theory, although there are often alternative interpretations of the results.
The parasitic, like the mutational, theory, requires certain preconditions, and these need to be tested too. It requires:
• that parasitism can be found in almost all species;
• a certain kind of multilocus gene-for-gene parasite-host relationship;
• that the frequencies of these genes oscillate through time.
The parasitic theory of sex remains an attractive, and coherent hypothesis, but it is uncertain whether it identifies the main factor maintaining sex in nature.
The question of why sex exists remains an 'outstanding puzzle': evolutionary biologists are not confident the question has been satisfactorily answered. Maybe, as Maynard Smith once said, “some crucial aspect of the problem has been overlooked”, and we need some radically new idea that has not yet been put forward or lies unappreciated. The subject has moved fast in the past 20 years and the hope now is that this crucial question will be answered in the next decade or so of research.