DNA sequences for the gene encoding mitochondrial cytochrome oxidase I in a group of rodents (pocket gophers) and their ectoparasites (chewing lice) provide evidence for cospeciation and reveal different rates of molecular evolution in the hosts and their parasites. The overall rate of nucleotide substitution (both silent and replacement changes) is approximately three times higher in lice, and the rate of synonymous substitution (based on analysis of fourfold degenerate sites) is approximately an order of magnitude greater in lice. The difference in synonymous substitution rate between lice and gophers correlates with a difference of similar magnitude in generation times.

The close correspondence often observed between the taxonomy of parasites and their hosts has led to Fahrenholz's rule, which postulates that parasites and their hosts speciate in synchrony. This leads to the prediction that phylogenetic trees of parasites and their hosts should be topologically identical. We report here a test of this prediction which involves the construction of phylogenetic trees for rodents and their ectoparasites using protein electrophoretic data. We find a high degree of concordance in the branching patterns of the trees which suggests that there is a history of cospeciation in this host-parasite assemblage. In several cases where the branching patterns were identical in the host and parasite phylogenies, the branch lengths were also very similar which, given the assumptions of molecular clock theory, strongly suggests that the speciation of these hosts and ectoparasites was roughly contemporaneous and causally related.

The development of polymerase chain reaction-based methods for assessing the genotypes of small individual organisms will promote groundbreaking investigations of the genetic architecture of parasite populations. Both quantitative genetic models and general knowledge of parasite natural history are useful for making general predictions about the distribution of genetic variation over geographic space. However, designing experimental studies to assess relationships between specific life history variables and patterns of genetic structure in natural populations will be challenging. Traditional biochemical-genetic methods have already been used to study a limited number of parasite populations, and inferred patterns of genetic structure are distinctly different between certain species. Some of these differences in genetic architecture may be explained by parasite or host factors that either promote or retard the dissemination of life cycle stages over geographic space. Many additional empirical studies are needed to characterize basic features of parasite populations, including the spatial distribution and group size of random mating populations and levels of gene flow among parasite subpopulations.Since the publication of On the Origin of Species (Darwin, 1859), the principal focus of studies in microevolution has been selection, although Darwin was aware that accident and interbreeding can oppose natural selection. Classical studies in ecological genetics, sensu Ford (1964), have documented and quantified the effects of natural selection on conspicuous phenotypic variation within populations. Subsequent studies of natural selection in wild populations of plants and animals (Endler, 1986) have proved invaluable for documenting additional examples and providing empirical evidence for theoretical predictions. Current studies in ecological genetics have, by necessity, become considerably more inclusive. This is because a thorough understanding of microevolution will occur only by considering, in an historical context, the effects of stochastic events, gene flow, natural selection, mutation, and mating systems on the metapopulation (the spatially separate, temporally extant, "interconnected" subpopulations of a species). Much of the theoretical groundwork for metapopulation studies was published prior to the advent of molecular genetic methods by the famous quantitative evolutionary geneticist Sewall Wright. It may be surprising to many parasitologists that Wright was a Master's student of Henry Baldwin Ward and published the first of his 211 scientific papers (Wright, 1912), on the morphology of the trematode Microphallus opacus! Wright's subsequent pioneering quantitative and theoretical studies in microevolution emphasized the importance of unpredictable changes in allelic frequencies due to stochastic events in finite populations (genetic drift), and modification of allele frequencies in populations due to the movement of gametes or individuals (gene flow). Wright's theoretical results have now received ...

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