How many genetic changes create new species?

From the introduction: The formation of new species generates biodiversity and is often driven by evolution through natural selection. However, the number of genetic changes involved in speciation is largely unknown. Many theoretical models predict that if speciation occurs without geographic isolation, it will be driven by a small number of genes. The logic is that only the few genes that experience the strongest natural selection can overcome the homogenizing effect of genetic mixing (i.e., gene flow) to diverge between populations. However, empirical studies in plants and animals now suggest that speciationeven with gene flowinvolves differentiation in surprisingly many genetic regions. This is thought possible because the effects of selection can become coupled across correlated genes such that the selection each gene experiences is much stronger than it would receive in isolation. Thus, the potential for genes to evolve collectively because of coupling may be a key to understanding speciation.