Reproductive isolation (RI) is a core concept in evolutionary biology. It has been the central focus of speciation research since the modern synthesis and is the basis by which biological species are defined. Despite this, the term is used in seemingly different ways, and attempts to quantify RI have used very different approaches. After showing that the field lacks a clear definition of the term, we attempt to clarify key issues, including what RI is, how it can be quantified in principle, and how it can be measured in practice. Following other definitions with a genetic focus, we propose that RI is a quantitative measure of the effect that genetic differences between populations have on gene flow. Specifically, RI compares the flow of neutral alleles in the presence of these genetic differences to the flow without any such differences. RI is thus greater than zero when genetic differences between populations reduce the flow of neutral alleles between populations. We show how RI can be quantified in a range of scenarios. A key conclusion is that RI depends strongly on circumstances—including the spatial, temporal and genomic context—making it difficult to compare across systems. After reviewing methods for estimating RI from data, we conclude that it is difficult to measure in practice. We discuss our findings in light of the goals of speciation research and encourage the use of methods for estimating RI that integrate organismal and genetic approaches.
Journal of Evolutionary Biology
We are grateful to the participants of the ESEB satellite symposium ‘Understanding reproductive isolation: bridging conceptual barriers in speciation research’ in 2021 for the interesting discussions that helped us clarify the thoughts presented in this article. We thank Roger Butlin, Michael Turelli and two anonymous reviewers for their thoughtful comments on this manuscript. We are also very grateful to Roger Butlin and the Barton Group for the continued conversations about RI. In addition, we thank all participants of the speciation survey. Part of this work was funded by the Austrian Science Fund FWF (grant P 32166).
Westram AM, Stankowski S, Surendranadh P, Barton NH. What is reproductive isolation? Journal of Evolutionary Biology. 2022;35(9):1143-1164. doi:10.1111/jeb.14005
Westram, A. M., Stankowski, S., Surendranadh, P., & Barton, N. H. (2022). What is reproductive isolation? Journal of Evolutionary Biology. Wiley. https://doi.org/10.1111/jeb.14005
Westram, Anja M, Sean Stankowski, Parvathy Surendranadh, and Nicholas H Barton. “What Is Reproductive Isolation?” Journal of Evolutionary Biology. Wiley, 2022. https://doi.org/10.1111/jeb.14005.
A. M. Westram, S. Stankowski, P. Surendranadh, and N. H. Barton, “What is reproductive isolation?,” Journal of Evolutionary Biology, vol. 35, no. 9. Wiley, pp. 1143–1164, 2022.
Westram AM, Stankowski S, Surendranadh P, Barton NH. 2022. What is reproductive isolation? Journal of Evolutionary Biology. 35(9), 1143–1164.
Westram, Anja M., et al. “What Is Reproductive Isolation?” Journal of Evolutionary Biology, vol. 35, no. 9, Wiley, 2022, pp. 1143–64, doi:10.1111/jeb.14005.
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