FOUNDER-FLUSH SPECIATION IN DROSOPHILA PSEUDOOBSCURA: A LARGE-SCALE EXPERIMENT

A founder-flush-crash model of speciation has been proposed that may particularly apply to island and other colonizations. Previous laboratory experiments testing the model have given inconsistent results. We have conducted a large experiment with Drosophila pseudoobscura designed to meet the essential postulates of the model and to separately test some of the postulates. Forty-five experimental and 12 control populations have been studied during seven successive founder-flush-crash cycles, or about 50 generations. Sexual isolation tests yield significantly positive assortative mating in a few tests between pairs of experimental populations. Populations with fewer founders (N = 1 or 3) yield more significant instances of assortative mating than those with more founders (n = 5, 7, or 9), and this difference becomes statistically significant for pooled data. Only one of 15 population pairs tested three times (generations 25, 32, and 46) shows positive assortative mating in all three tests. No significant assortative mating occurs between control populations, including highly inbred ones. We conclude that although founder events may occasionally lead to the evolution of assortative mating and hence to speciation, our results do not support the claim that the founder-flush-crash model identifies conditions very likely to result in speciation events.