On the theoretical and empirical framework for studying genetic interactions within and among species.

We present a quantitative genetic (QG) interpretation of the Bateson-Dobzhansky-Muller (BDM) genetic model of speciation in order to unify the theoretical framework for understanding how the genetic differentiation of populations is associated with the process of speciation. Specifically, we compare the QG theory of joint scaling with the Turelli-Orr mathematical formulation of the BDM model. By formally linking the two models, we show that a wealth of empirical methods from QG can be brought to bear on the study of the genetic architecture of hybrid phenotypes to better understand the connections, if any, between microevolution within populations and macroevolution in the origin of species. By integrating the two theories, we make additional novel predictions that enrich the opportunities for empirically testing speciation genetic theory or facets of it, such as Haldane's rule. We show that the connection between the two theories is simple and straightforward for autosomal genes but not for sex-linked genes. Differences between the two approaches highlight key conceptual issues concerning the relevance of epistasis to evolution within and among lineages and to differences in the process of speciation in hermaphrodites and in organisms with separate sexes.