Geography of Speciation

by

Erica L. Larson, Scott A. Taylor

• LAST REVIEWED: 27 October 2022
• LAST MODIFIED: 27 October 2022
• DOI: 10.1093/obo/9780199941728-0142

Introduction

Speciation is the process of a single lineage splitting into two or more daughter lineages. Geography is key to this process. When populations are spatially isolated, they no longer exchange genes and can diverge. A byproduct of this divergence is the evolution of intrinsic reproductive barriers—traits that prevent species from interbreeding and signal the completion of speciation. Allopatric speciation is the origin of new species in spatially isolated populations. Allopatric speciation is the simplest and easiest model of speciation because there is no gene flow between populations that can erode divergence. Given enough time, allopatric speciation is expected, and it is widely agreed to be the most common mode of speciation. Indeed, allopatric speciation was described by Coyne and Orr in Speciation (2004) as “so plausible that it hardly seems worth documenting.” Speciation can of course occur in other geographic contexts, with differing levels of gene flow. A single panmictic population can be split into two isolated populations through disruptive selection, known as sympatric speciation, or new species can arise from populations that do not directly overlap, but are in close enough proximity to exchange genes, known as parapatric speciation. These three geographic modes represent different points on a continuum of spatial isolation that determines the connectivity, or amount of gene flow, among diverging populations. However, these are hardly discrete categories, and the term “speciation-with-gene-flow” (also “divergence-with-gene-flow”) is often used to encompass any geographic model of speciation other than strict allopatry. This article will provide an overview of the various geographic contexts for speciation, with a focus on cases where spatial isolation ranges from complete (allopatric speciation) to some form of contact between diverging populations.

General Overviews

The geography of speciation is a central focus of speciation research. As a result, most major texts on speciation devote some sections or chapters to the topic. By far the most comprehensive overview is Coyne and Orr 2004. There have been many advances in the field of speciation since then, but it remains an excellent resource to understand the history of the field. Dobzhansky 1937 is one of the earliest discussions of the geography of speciation and emphasizes the semantic difficulties of defining speciation in terms of geography alone. Mayr 1942 and Mayr 1963, somewhat dogmatically, canonize the predominant role of allopatry in speciation, presenting extensive evidence for its importance in many organisms. Grant 1981 extends these syntheses by providing an in-depth review of speciation in plants. Since these foundational books, there have been the anthologies Otte and Endler 1989 and Rice, et al. 1998, which provide updates on major topics, including the geography of speciation. Avise 2000 outlined the study of genetic variation in a spatial and phylogenetic context and provided the methodological foundation for many contemporary students of the geography of speciation. Two recent books, Price 2008 and Nosil 2012, are narrower in their focus but are the most recent overviews of the field.

• Avise, J. C. 2000. Phylogeography: The history and formation of species. Cambridge, MA: Harvard Univ. Press.

  DOI: 10.2307/j.ctv1nzfgj7

  While this book is not focused on speciation per se, it was a major advance in the study of genetic variation over space. Avise documented the concordance between past and present geographic barriers and genetic discontinuities within and among species. In many of the examples, genetic discontinuities coincide with geographic features in many different species, suggesting general biogeographic modes to allopatric speciation.

• Coyne, J. A., and H. A. Orr. 2004. Speciation. Sunderland, MA: Sinauer Associates.

  A comprehensive review of the speciation literature. The discussion of the geography of speciation is thorough, with a chapter dedicated to allopatric and parapatric speciation and a separate chapter dedicated to sympatric speciation.

• Dobzhansky, T. 1937. Genetics and the origin of species. New York: Columbia Univ. Press.

  Provides an overview of the study of the geography of speciation. Dobzhansky emphasized the semantic difficulties of defining speciation in terms of geography and suggested that speciation may be better defined by a balance of selection and gene flow. This book was subsequently updated in a second edition in 1982.

• Grant, V. 1981. Plant speciation. New York: Columbia Univ. Press.

  DOI: 10.7312/gran92318

  A definitive review of speciation in plants, including a discussion of allopatric speciation.

• Mayr, E. 1942. Systematics and the origin of species. New York: Columbia Univ. Press.

  This book laid out extensive arguments for why allopatric speciation is important in many different organisms and emphasized the major role of geographic isolation in speciation.

• Mayr, E. 1963. Animal species and evolution. Cambridge, MA: Harvard Univ. Press.

  DOI: 10.4159/harvard.9780674865327

  An updated version of Mayr 1942 in which the author expands his discussion of the geography of speciation and provides more examples and adamant arguments for the predominance of allopatric speciation.

• Nosil, P. 2012. Ecological speciation. New York: Oxford Univ. Press.

  DOI: 10.1093/acprof:osobl/9780199587100.001.0001

  The most recent book on speciation, but focused specifically on the role of ecology in speciation. A good resource for a fairly recent perspective on the field.

• Otte, D., and J. A. Endler, eds. 1989. Speciation and its consequences. Sunderland, MA: Sinauer Associates.

  An overview of the field of speciation. Many of the chapters of this book are classic papers in the field of speciation and it is a good resource for a perspective on the history of the field.

• Price, T. 2008. Speciation in birds. Greenwood Village, CO: Roberts.

  While this book is focused entirely on birds, it covers many of the major topics in speciation. Particularly relevant to the geography of speciation are the chapters on geographical variation and geographic isolation and island endemism.

• Rice, W. R., D. J. Howard, and S. H. Berlocher, eds. 1998. Endless forms: Species and speciation. New York: Oxford Univ. Press.

  An updated overview of the field of speciation since Otte and Endler 1989. This collection is dedicated to Guy Bush, whose contributions to the study of sympatric speciation in the apple maggot fly, Rhagoletis pomonella, challenged Mayr’s view that allopatric speciation is the only important mode of speciation. There are many important chapters addressing the geography of speciation.