coevolution ftw

"The Red Queen hypothesis proposes that coevolution of interacting
species (such as hosts and parasites) should drive molecular evolution
through continual natural selection for adaptation and
counter-adaptation. Although the divergence observed at some
host-resistance and parasite-infectivity genes is consistent with
this, the long time periods typically required to study coevolution
have so far prevented any direct empirical test. Here we show, using
experimental populations of the bacterium Pseudomonas fluorescens
SBW25 and its viral parasite, phage Φ2, that the rate of molecular
evolution in the phage was far higher when both bacterium and phage
coevolved with each other than when phage evolved against a constant
host genotype. Coevolution also resulted in far greater genetic
divergence between replicate populations, which was correlated with
the range of hosts that coevolved phage were able to infect.
Consistent with this, the most rapidly evolving phage genes under
coevolution were those involved in host infection. These results
demonstrate, at both the genomic and phenotypic level, that
antagonistic coevolution is a cause of rapid and divergent evolution,
and is likely to be a major driver of evolutionary change within
species." -- Michael J. Benton, Antagonistic coevolution accelerates
molecular evolution, Nature 464, 275-278 (11 March 2010) abstract

Cf. Leigh Van Valen. (1973). "A new evolutionary law". Evolutionary
Theory 1: 1—30.