Published online by Cambridge University Press: 14 April 2009
Geographic populations are not necessarily equivalent to genetic populations. Electrophoretic isozyme analysis shows that in the biennial, Verbascum virgatum, not only is spatial distribution a factor creating isolated populations, but time also is an isolating mechanism. Thus, in the biennial situation there is a possibility that selection can be rejected as the probable cause of variation, much as can random drift in cases involving spatial distributions. The magnitudeof difference infrequency of alleles between the odd and even year colonies studied was too large to be explained by random drift in populations of the size and short duration of those we observed. Similarly, it is unlikely that random fluctuations in selection intensity on one age class would produce a difference as large as that observed. It is possible, however, that variation was introduced (mutation or founder) when the population was much smaller and that the difference was trapped at a relatively high level when the population rapidly increased in size. Simulations and algebraic theory do not refute this idea. They also show that colony differentiation can occur with migration rates considerably greater than previously predicted.