article published yes Nicholas H Barton author 4880FE40-F248-11E8-B48F-1D18A9856A870000-0002-8548-5240 NiBa department If distinct biological species are to coexist in sympatry, they must be reproductively isolated and must exploit different limiting resources. A two-niche Levene model is analysed, in which habitat preference and survival depend on underlying additive traits. The population genetics of preference and viability are equivalent. However, there is a linear trade-off between the chances of settling in either niche, whereas viabilities may be constrained arbitrarily. With a convex trade-off, a sexual population evolves a single generalist genotype, whereas with a concave trade-off, disruptive selection favours maximal variance. A pure habitat preference evolves to global linkage equilibrium if mating occurs in a single pool, but remarkably, evolves to pairwise linkage equilibrium within niches if mating is within those niches--independent of the genetics. With a concave trade-off, the population shifts sharply between a unimodal distribution with high gene flow and a bimodal distribution with strong isolation, as the underlying genetic variance increases. However, these alternative states are only simultaneously stable for a narrow parameter range. A sharp threshold is only seen if survival in the 'wrong' niche is low; otherwise, strong isolation is impossible. Gene flow from divergent demes makes speciation much easier in parapatry than in sympatry. Royal Society2010 eng 20439284 00027720860000910.1098/rstb.2010.0001 36515471825 - 1840 Barton, N. H. (2010). What role does natural selection play in speciation? <i>Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences</i>. Royal Society. <a href="https://doi.org/10.1098/rstb.2010.0001">https://doi.org/10.1098/rstb.2010.0001</a> Barton, Nicholas H. “What Role Does Natural Selection Play in Speciation?” <i>Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences</i>. Royal Society, 2010. <a href="https://doi.org/10.1098/rstb.2010.0001">https://doi.org/10.1098/rstb.2010.0001</a>. Barton, Nicholas H. “What Role Does Natural Selection Play in Speciation?” <i>Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences</i>, vol. 365, no. 1547, Royal Society, 2010, pp. 1825–40, doi:<a href="https://doi.org/10.1098/rstb.2010.0001">10.1098/rstb.2010.0001</a>. N. H. Barton, “What role does natural selection play in speciation?,” <i>Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences</i>, vol. 365, no. 1547. Royal Society, pp. 1825–1840, 2010. Barton NH. What role does natural selection play in speciation? <i>Philosophical Transactions of the Royal Society of London Series B, Biological Sciences</i>. 2010;365(1547):1825-1840. doi:<a href="https://doi.org/10.1098/rstb.2010.0001">10.1098/rstb.2010.0001</a> Barton NH. 2010. What role does natural selection play in speciation? Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 365(1547), 1825–1840. N.H. Barton, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 365 (2010) 1825–1840. 37732018-12-11T12:05:05Z2025-09-30T09:45:54Z