Red Queen hypothesis
The Red Queen hypothesis proposes that species must constantly adapt to keep up with co-evolving parasites and pathogens, and that this ongoing arms race favors sexual reproduction.
Leigh Van Valen introduced the name in 1973, from the Red Queen’s line in Lewis Carroll: “it takes all the running you can do, to keep in the same place.” Graham Bell and W. D. Hamilton developed the parasite-driven version in the early 1980s as an explanation for the maintenance of sex.
The mechanism
Parasites reproduce faster than their hosts and adapt to the host genotypes they meet most often. A genotype that was resistant yesterday becomes susceptible today, because the parasite population has caught up to it. In this regime, the fittest strategy is to produce rare, novel genotypes - which is what sexual recombination does. An asexual lineage produces genetically uniform offspring: once a parasite evolves to overcome one of them, it can potentially overcome all of them.
Evidence
Curtis Lively’s experiments on the New Zealand freshwater snail Potamopyrgus antipodarum provided the classic support: in lakes where parasites are common, sexual (outcrossing) snails are more frequent, and parasites disproportionately infect the locally common genotypes. Similar frequency-dependent infection patterns appear in worm-parasite laboratory populations.
Why it matters here
The Red Queen is one of the two standard explanations for why outcrossing persists despite its twofold cost, alongside the avoidance of inbreeding depression (see self-fertilization). It also explains the maintenance of genetic diversity at immune genes: the HLA-A loci are the most polymorphic coding loci in the human genome, and pathogen pressure is the standard explanation.
Notes
- Draft stub - maintenance agent to expand with Van Valen’s 1973 original (which was about macroevolutionary extinction, not sex), the Lively snail experiments, and Hamilton’s 1980 model.