Coadapted genomes and selection on hybrids: Fisher's geometric model explains a variety of empirical patterns

Natural selection plays a variety of roles in hybridization, speciation and admixture. Most research has focused on two extreme cases: crosses between closely-related inbred lines, where hybrids are fitter than their parents, or crosses between effectively isolated species, where hybrids suffer severe breakdown. But many natural populations must fall into intermediate regimes, with multiple types of gene interaction, and these are more difficult to study. Here, we develop a simple fitness landscape model, and show that it naturally interpolates between previous modeling approaches, which were designed for the extreme cases, and invoke either mildly deleterious recessives, or discrete hybrid incompatibilities. Our model yields several new predictions, which we test with genomic data from Mytilus mussels, and published data from plants (Zea, Populus and Senecio) and animals (Mus, Teleogryllus and Drosophila). The predictions are generally supported, and the model explains a number of surp...