Founder-event Speciation Research Articles

Island hopping inDrosophila: patterns and processes

Radiation of Drosophila along the Hawaiian archipelago has resulted in an astounding array of diversity. The speciation in this group corresponds well to the geological history of the region and colonization events appear to have been a major contributing factor. Although much less impressive in terms of diversity, Drosophila have also radiated throughout the Caribbean islands. In contrast to the pattern exhibited in Hawaii, major changes that distinguish the species in the Caribbean are not always coupled to colonization events. The patterns of speciation for these two island groups are compared and contrasted in light of founder effect speciation models.

Founder Effect Speciation: A Theoretical Reassessment

We propose a series of simple models of founder effect speciation. In these models, the resulting reproductive isolation (as measured by the proportion of inviable hybrids or the strength of the barrier to gene exchange between populations) can be very high and can evolve with a high probability on the time scale of dozens or hundreds of generations. In developing our theoretical framework, we utilize Dobzhansky's idea that strong selection against hybrids between two genotypes can occur simultaneously with the existence of a chain of genotypes that connect those two and differ only weakly in fitness among themselves. The mathematical models that we have studied are closely related to the verbal schemes of Mayr's "genetic revolutions," Carson's founder-flush process, and Templeton's genetic transilience. For appropriate parameter values, our theoretical models demonstrate that founder effect speciation is plausible; its importance becomes an empirical question.

The Evolution of Human Populations: A Molecular Perspective

Human evolution exhibits repeated speciations and conspicuous morphological change: fromAustralopithecustoHomo habilis, H. erectus,andH. sapiens;and from their hominoid ancestor to orangutans, gorillas, chimpanzees, and humans. Theories of founder-event speciation propose that speciation often occurs as a consequence of population bottlenecks, down to one or very few individual pairs. Proponents of punctuated equilibrium claim in addition that founder-event speciation results in rapid morphological change. The major histocompatibility complex (MHC) consists of several very polymorphic gene loci. The genealogy of 19 human alleles of theDQB1locus coalesces more than 30 million years ago, before the divergence of apes and Old World monkeys. Many human alleles are more closely related to pongid and cercopithecoid alleles than to other human alleles. Using the theory of gene coalescence, we estimate that these polymorphisms require human populations of the order ofN= 100,000 individuals for the last several million years. This conclusion is confirmed by computer simulations showing the rate of decay of the polymorphisms over time. Computer simulations indicate, in addition, that in human evolution no bottlenecks have occurred with fewer than several thousand individuals. We evaluate studies of mtDNA, Y-chromosome, and microsatellite autosomal polymorphisms and conclude that they are consistent with the MHC result that no narrow population bottlenecks have occurred in human evolution. The available molecular information favors a recent African origin of modern humans, who spread out of Africa approximately 100,000 to 200,000 years ago.

Founder-effect speciation theory: failure of experimental corroboration.

The theory of founder-effect speciation proposes that colonization by very few individuals of an empty habitat favors rapid genetic changes and the evolution of a new species. We report here the results obtained in a 10-year-long and large-scale experiment with Drosophila pseudoobscura designed to test the theory. In our experimental protocol, populations are established with variable numbers of very few individuals and allowed to expand greatly for several generations until conditions of severe competition for resources are reached and the population crashes. A few random survivors are then taken to start a new population expansion and thus initiate a new cycle of founding events, population flushes, and crashes. Our results provide no support for the theories proposing that new species are very likely to appear as by-products of founder events.