Preferring the fittest mates: An analytically tractable model Susanne Schindler a,b,n , Olaf Breidbach c ,J¨ urgen Jost b,d,e a Imperial College London, Buckhurst Road, Ascot, SL5 7PY, United Kingdom b Max-Planck-Institute for Mathematics in the Sciences, Inselstr. 22, 04103 Leipzig, Germany c Friedrich-Schiller-Universit¨ at Jena, Ernst-Haeckel-Haus, Berggasse 7, 07745 Jena, Germany d Fakult¨ at f¨ ur Mathematik und Informatik, Universit¨ at Leipzig, 04081 Leipzig, Germany e Santa Fe Institute, Santa Fe, NM 87501, USA HIGHLIGHTS c We present a population-genetic model of a preference for fit mating partners. c The alleles coding the mating preference go rapidly to fixation. c The selection differentials between different niches are potentiated. c The polymorphism exceeds the selection–migration equilibrium by several magnitudes. c Fitness-based mating can be seen as a trigger of prezygotic isolation and divergence. article info Article history: Received 25 April 2012 Received in revised form 17 September 2012 Accepted 18 September 2012 Available online 2 October 2012 Keywords: Non-allopatric speciation Parapatric speciation Sexual selection Female choice Indicator model abstract We develop an analytically tractable model of female preference for fit mates. Our population-genetic model allows to trace the dynamics at both the individual and the population level. The preference for fit mates links ecological adaptation and mating success is individually advantageous and causes polymorphic subpopulations. This polymorphism is a strong and stable clustering in genotype and phenotype space. The alleles coding for the mating preference spread rapidly through the population, thereby increasing the selection pressure between different habitats. The resulting polymorphism exceeds the expected selection–migration equilibrium by several orders of magnitude. The evolution of preference for fit mates can, thus, act as the trigger for parapatric speciation because it initiates prezygotic isolation and divergence. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction How new species arise is a central question for the theory of evolution. In the case of gene flow between subpopulations, i.e., in non-allopatric situations, the question is how the homogeniz- ing effect of gene flow can be overcome such that genetic and phenotypic differences can accumulate and lead to speciation (parapatric or sympatric). Several mechanisms of parapatric and sympatric speciation have been proposed and explored (see review Bolnick and Fitzpatrick, 2007), ranging from mechanisms based on mate preferences such as sexual conflict (Gavrilets and Waxman, 2002) and reinforcement (Servedio and Kirkpatrick, 1997; Servedio, 2000, 2004) over frequency dependent disruptive selection (e.g. via resource competi- tion) to chromosome rearrangements and other genetic mechanisms (Wolfe, 2003). An example of a mate preference that can hinder gene flow is assortative mating (originally proposed by Maynard Smith, 1966). Assortative mating means that individuals prefer to mate with phenotypically similar individuals and avoid matings with dissimilar members of their population. Doebeli and Dieckmann demonstrated that in the presence of environmental heterogene- ity, assortative mating can lead to genetic clustering with two (or more) distinct phenotypes within the population, and that it can sufficiently reduce gene flow between these types so that speciation can set in Dieckmann and Doebeli (1999, 2004) and Doebeli and Dieckmann (2003). A different mating strategy is to try to mate with the fittest partner available (e.g. Proulx, 2001; Proulx and Servedio, 2009; van Doorn et al., 2009). The preference for high-quality males has been named ‘‘LAMMA’’ for locally adapted male mating advantage Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/yjtbi Journal of Theoretical Biology 0022-5193/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jtbi.2012.09.018 n Corresponding author at: Imperial College London, Buckhurst Road, Ascot, SL5 7PY, United Kingdom. Tel.: þ44 20 759 42223. E-mail addresses: s.schindler@imperial.ac.uk (S. Schindler), olaf.breidbach@uni-jena.de (O. Breidbach), jjost@mis.mpg.de (J. Jost). Journal of Theoretical Biology 317 (2013) 30–38