CONSTRAINTS ON ALLELE SIZE AT MICROSATELLITE LOCI - IMPLICATIONS FOR GENETIC DIFFERENTIATION

Microsatellites are promising genetic markers for studying the demogra phic structure and phylogenetic history of populations. We present the oretical arguments indicating that the usefulness of microsatellite da ta for these purposes may be limited to a short time perspective and t o relatively small populations. The evolution of selectively neutral m arkers is governed by the interaction of mutation and random genetic d rift. Mutation pressure has the inherent tendency to shift different p opulations to the same distribution of alleles. Hence, mutation pressu re is a homogenizing force, and population divergence is caused by ran dom genetic drift. In case of allozymes or sequence data, the diversif ying effect of drift is typically orders of magnitude larger than the homogenizing effect of mutation pressure. By a simple model, we demons trate that the situation may be different for microsatellites where mu tation rates are high and the range of alleles is limited. with the he lp of computer simulations, we investigate to what extent genetic dist ance measures applied to microsatellite data can nevertheless yield us eful estimators for phylogenetic relationships or demographic paramete rs. We show that predictions based on microsatellite data are quite re liable in small populations, but that already in moderately sized popu lations the danger of misinterpretation is substantial.