In defense of founder-flush theories of speciation

Founder-flush models of speciation rely on the assumption that genetic evolution is in some way different in a population that is founded by a small number of individuals and that subsequently grows rapidly. That assumption is supported by the analysis of several population genetic models of the founder-flush demographic process. The basic ideas are that a newly founded population represents a random sample from the parent population and that, during the period of rapid growth, genetic drift is much weaker than would be expected, particularly for low-frequency alleles. As a consequence, the probability of a loss of a neutral lineage present in a single copy is quite low, and the probability of a fixation of advantageous alleles, including alleles with additive, recessive, and epistatic effects, is much higher than in populations that remain of constant size. Furthermore, the probability of a peak shift during the founding event is reasonably high provided that the second peak is substantially higher than the peak occupied by the parent population. Thus, there is no reason to dismiss founder-flush models of speciation because of their intrinsic implausibility or inconsistency with standard population genetics theory. The same kind of analysis suggests a test of whether a population has had a history consistent with a founder-flush model. Sampling during the founding event would lead to a strong negative disequilibrium between low-frequency nucleotides, and, if the sites are closely enough linked, then that disequilibrium would be preserved during the period of growth. Hence, closely linked sites would be expected to exhibit substantially more negative disequilibrium than would the same sites in the ancestral population.