Migration and the Origin of Species

Abstract

The earth is inhabited by a breathtaking number of different species. Currently, 1.7 million species have been described by scientists, and the number of undescribed species is expected to exceed this figure by at least one order of magnitude. All living and extinct species derive from a common ancestor, and the process by which an existing species gives rise to new species is called speciation. Understanding speciation is one of the defining problems in evolutionary biology. Although Charles Darwin named his seminal book On the Origin of Species (1859), he actually did not develop a theory of speciation but rather derived how evolutionary change ensues from the occurrence of heritable variation and natural selection. It was only during the modern synthesis of the 1930s that evolutionary biologists developed a clearer picture of the speciation process. The early ideas put a strong emphasis on the role of migration in speciation. Especially Ernst Mayr (1942) pushed the idea that speciation requires the split of an ancestral population into two or more spatially isolated subpopulations, which then evolve to become distinct species. Mayrs reasoning was that migration and subsequent interbreeding between immigrants and residents result in the exchange of genetic material. This exchange tends to homogenize the two subpopulations and therefore counteracts divergence and speciation. In this chapter, I will refer to migration in subdivided population as gene flow because this term directly refers to the biological relevant effect of migration in sexually reproducing plant and animal populations. Although still prominent, Mayrs strict view is not as dominant as it used to be, and nowadays, the possibility of speciation in the presence of gene flow is supported both by empirical evidence and by mathematical models. In this chapter, I will give an overview of speciation theories with an emphasis on the role of gene flow.