Ernst Mayr and the integration of geographic and ecological factors in speciation

Abstract

Mayr's best recognized scientific contributions include the biological species concept and the theory of geographic speciation. In the latter, reproductive isolation evolves as an incidental by-product of genetic divergence between allopatric populations. Mayr noted that divergent natural selection could accelerate speciation, but also argued that gene flow so strongly retards divergence that, even with selection, non-allopatric speciation is unlikely. However, current theory and data demonstrate that substantial divergence, and even speciation, in the face of gene flow is possible. Here, I attempt to connect some opposing views about speciation by integrating Mayr's ideas about the roles of ecology and geography in speciation with current data and theory. My central premise is that the speciation process (i.e. divergence) is often continuous, and that the opposing processes of selection and gene flow interact to determine the degree of divergence (i.e. the degree of progress towards the completion of speciation). I first establish that, in the absence of gene flow, divergent selection often promotes speciation. I then discuss how population differentiation in the face of gene flow is common when divergent selection occurs. However, such population differentiation does not always lead to the evolution of discontinuities, strong reproductive isolation, and thus speciation per se. I therefore explore the genetic and ecological circumstances that facilitate speciation in the face of gene flow. For example, particular genetic architectures or ecological niches may tip the balance between selection and gene flow strongly in favour of selection. The circumstances allowing selection to overcome gene flow to the extent that a discontinuity develops, and how often these circumstances occur, are major remaining questions in speciation research.

Revision of Drusinae subfamily (Trichoptera, Limnephilidae): divergence by paraproct and paramere: speciation in isolation by integration

“…Reproductive isolation can evolve within tens, hundreds or thousands of generations (Hendry et al 2007), especially in allopatric or in founding populations. Contrary, ecological trait divergence under a balance between selection and gene flow may exhibit only weak reproductive isolation (Nosil 2008). …”

“…Genes under divergent selection are usually different from the genes causing reproductive isolation. There are some empirical evidences that ecological divergent selection promotes the evolution of reproductive isolation (Nosil 2008). However it is rather an incident then a rule.…”

Union of phylogeography and landscape genetics

“…95, 99, and 101). But how different abiotic conditions must be to ensure divergent selection even in the face of gene flow, and at what spatial scale, is an open and interesting question (102).…”

Filogenia do Complexo Drosophila Buzzatii (Grupo Repleta): Inferências de Análises Multilocus Mitocondriais e Nucleares.

“…As áreas de ocorrência atual das espécies do clado AB, com diversos pontos de sobreposição (Figura 2), tornam difícil o entendimento do contexto geográfico dos processos de especiação que deram origem a estas espécies, e podem ter ocorrido tanto em alopatria, com posterior contato secundário entre as linhagens, quanto na presença de áreas em simpatria, mediante diferenciação ecológica (Losos & Glor, 2003;Nosil, 2008).…”

“…No entanto, tem crescido a quantidade de dados sugerindo cenários em que a diversificação ocorreu na presença de fluxo gênico entre espécies desse gênero (Machado et al 2007), bem como em outros grupos de insetos (Nosil, 2008).…”