Academic journal article Genetics

Simple Biophysical Model Predicts Faster Accumulation of Hybrid Incompatibilities in Small Populations under Stabilizing Selection

Academic journal article Genetics

Simple Biophysical Model Predicts Faster Accumulation of Hybrid Incompatibilities in Small Populations under Stabilizing Selection

Article excerpt

(ProQuest: ... denotes formulae omitted.)

SPECIATION is of great importance in generating the observed diversity of life, yet it is still poorly understood, especially at the genetic level. Two populations are said to have speciated when they have developed reproductive isolation (RI), that is, when they can no longer interbreed. A standard model of how postzygotic reproductive isolation arises is due to Dobzhansky, Muller, and Bateson (Bateson 1909; Dobzhansky 1936; Muller 1942), where so-called Dobzhansky-Muller incompatibilities (DMIs) arise due to epistatic interactions; for example, two geographically isolated lineages evolving allopatrically from a common ancestor ab can fix the allelic combinations aB and Ab, respectively, yet the hybrid genotype AB can be inviable due to the epistatic interactions between these two loci. In polygenic systems, where many loci code for an additive quantitative trait, a similar hybrid incompatibility arises; quadratic, or any nonlinear, selection induces epistasis such that divergent populations, under the action of drift, maintain different underlying allelic combinations at the many loci (Wright 1935a,b) for the same optimal trait value, which when combined in hybrids can lead to incompatibilities (Barton 1989). Although there are many examples of genes directly involved in reproductive isolation (Wu and Ting 2004), we still lack a theoretical understanding of the functional relationship between genes and their role in the development of hybrid incompatibilities and speciation dynamics. In this article, we examine an important example of such a functional relationship, the genotype-phenotype map of transcription factor-DNA binding. Using a simple biophysical model of transcription factor-DNA binding we analyze how incompatibilities can arise between allopatric lineages.

Despite many studies of the evolution of RI, very little attention has been paid to the role of population size; however, there is indirect and direct evidence that smaller populations develop incompatibilities more quickly. The observation of the large diversity of species on small young islands, such as Hawaii (Mayr 1970), or on the island of Cuba (Glor et al. 2004) and in the East African Great Lakes (Owen et al. 1990; Santos and Salzburger 2012), where in the latter two cases each one has been subject to historically fluctuating water levels and thus opportunities for allopatric speciation, suggests that smaller populations speciate more quickly. This is in contrast to lower levels of reproductive isolation observed in marine species with large ranges and population sizes, for example, the relatively small fraction of Pacific-Caribbean species pairs separated by the Isthmus of Panama a few million years ago compared to those that are not reproductively isolated (Mayr 1954, 1970; Rubinoff and Rubinoff 1971). There is also evidence that reproductive isolation arises more slowly in birds compared to mammals (Fitzpatrick 2004). Strikingly, even after ^55 MY divergence (Cooper and Penny 1997), domestic chickens (Gallus gallus)canstill hybridize with helmeted guineafowl (Numida meleagris), where estimates of the effective population size of domestic chickens range from Ne ^ 105 to 106 (Sawai et al. 2010), whereas in contrast, cichlids develop reproductive isolation as quickly as 1 2 10MY after divergence (Stelkens et al. 2010) and have relatively small population sizes [100 2 10; 000 (Oppen et al. 1997; Fiumera et al. 2000)]. This population size trend is further supported by net rates of diversification (Coyne and Orr 2004) inferred from phylogenetic trees (Barraclough and Nee 2001; Nee 2001). On the other hand, there are examples that buck this trend, such as Drosophila, which shows rapid speciation, for example, in adaptive radiations in Hawaii at large population size (Ayala et al. 1996).

Where does current theory stand in light of these observations? There are a number of theoretical models of allopatric speciation based on the Dobzhansky-Muller mechanism, which consider independent lineages evolving neutrally or under varying selection pressures on each lineage (Nei et al. …

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