1. Department of Entomology, St Petersburg State University, Universitetskaya nab. 7/9, St Petersburg 199034, Russia
2. Department of Organismic and Evolutionary Biology, 26 Oxford Street, Harvard University, Cambridge, Massachusetts 02138, USA
3. Bauer Center for Genomics Research, 7 Divinity Avenue, Cambridge, Massachusetts 02138, USA
4. *These authors contributed equally to this work

Correspondence to: Naomi E. Pierce² Correspondence and requests for materials should be addressed to N.E.P. (Email: npierce@fas.harvard.edu).

The reinforcement model of evolution argues that natural selection enhances pre-zygotic isolation between divergent populations or species by selecting against unfit hybrids^{1, 2} or costly interspecific matings³. Reinforcement is distinguished from other models that consider the formation of reproductive isolation to be a by-product of divergent evolution^{4, 5}. Although theory has shown that reinforcement is a possible mechanism that can lead to speciation^{6, 7, 8}, empirical evidence has been sufficiently scarce to raise doubts about the importance of reinforcement in nature^{6, 9, 10}. Agrodiaetus butterflies (Lepidoptera: Lycaenidae) exhibit unusual variability in chromosome number. Whereas their genitalia and other morphological characteristics are largely uniform, different species vary considerably in male wing colour, and provide a model system to study the role of reinforcement in speciation. Using comparative phylogenetic methods, we show that the sympatric distribution of 15 relatively young sister taxa of Agrodiaetus strongly correlates with differences in male wing colour, and that this pattern is most likely the result of reinforcement. We find little evidence supporting sympatric speciation: rather, in Agrodiaetus, karyotypic changes accumulate gradually in allopatry, prompting reinforcement when karyotypically divergent races come into contact.

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