Changing Views on Speciation

Our understanding of speciation processes is constantly changing. Two important concepts that have influenced thinking over the long term are the isolation and the genic views of speciation. However, neither of these views is fully compatible with our current understanding of speciation processes. The fact that many species with overlapping ranges remain distinct despite hybridisation and introgression suggests that species are not co-adapted gene pools that have evolved in allopatry and are protected from merging by reproductive isolation, as the isolation view of speciation assumes. Speciation can occur without geographic isolation, and alleles of some genes may be exchanged between species, while others may not, as suggested by the genic view of speciation. However, in contrast to the genic view, the alleles of genes underlying differential adaptation may not have opposite fitness effects. Rather, the traits under selection may be determined by effects of many genes. The facts that individual genes have usually only small effects on adaptive traits, the same trait can be achieved by different combinations of alleles, homologous traits may be determined by different gene regulatory underpinnings, new traits resulting from transgressive segregation may be essential for differential adaptation, and that non-genic traits can cause speciation, indicate that traits rather than individual genes are the most relevant units of speciation. The development of strong barriers that allow diverging populations to coexist may require a coupling of barrier effects, which can be facilitated by several factors, such as the pleiotropic effects of many genes or structural variants that reduce recombination, which are not considered in the genic view. Considering traits rather than single genes as units of speciation, and considering factors above the level of genes that contribute to the coupling of barrier effects, distinguishes the trait view of speciation from the genic view.