Adaptive evolutionary theory argues that organisms with larger effective
population size (Ne) should have higher rates of adaptive evolution and
therefore greater capacity to win evolutionary arm races. However, in
some certain cases species with much smaller Ne may be able to survive
beside their opponents for an extensive evolutionary time. Neutral
theory predicts that accelerated rates of molecular evolution in
organisms with exceedingly small Ne is due to the effects of genetic
drift and fixation of slightly deleterious mutations. We test this
prediction in two obligate social parasite species and their respective
host species from the bee tribe Allodapini. The parasites (genus
Inquilina) have been locked into a tight coevolutionary arm races with
their exclusive hosts (genus Exoneura) for ~15 million
years, even though Inquilina exhibit Ne that are an order of magnitude
smaller than their host. In this study, we compared rates of molecular
evolution between host and parasite using nonsynonymous to synonymous
substitution rate ratios (dN/dS) of eleven mitochondrial protein coding
genes sequenced from transcriptomes. Tests of selection on mitochondrial
genes indicated no significant differences between host and parasite
dN/dS, with evidence for purifying selection acting on all mitochondrial
genes of host and parasite species. Several potential factors which
could weaken the inverse relationship between Ne and rate of molecular
evolution are discussed.
Population Genetics, Molecular Evolution, and the Neutral Theory. Selected Papers of Motoo Kimura. Edited by Naoyuki Takahata. University of Chicago Press, Chicago. 1994. 686 pages. Price: Cloth US$80.00 £63.95 Paper US$29.95 £23.95. ISBN 0 226 43562 8.
