Abstract
Until complete reproductive isolation is achieved, the extent of differentiation between two diverging lineages is the result of a dynamic equilibrium between genetic isolation and mixing. This is especially true for hybrid taxa, for which the degree of isolation in regard to their parental species is decisive in their capacity to rise as a new and stable entity. In this work, we explored the past and current patterns of hybridization and divergence within a complex of closely related butterflies in the genus Coenonympha in which two alpine species, C. darwiniana and C. macromma, have been shown to result from hybridization between the also alpine C. gardetta and the lowland C. arcania. By testing alternative scenarios of divergence among species, we show that gene flow has been uninterrupted throughout the speciation process, although leading to different degrees of current genetic isolation between species in contact zones depending on the pair considered. Nonetheless, at broader geographic scale, analyses reveal a clear genetic differentiation between hybrid lineages and their parental species, pointing out to an advanced stage of the hybrid speciation process. Finally, the positive correlation observed between ecological divergence and genetic isolation among these butterflies suggests a potential role for ecological drivers during their speciation processes.
Highlights
Hybridization between diverging taxa is possible throughout the speciation process until complete reproductive isolation is achieved (Abbott et al, 2013; Butlin, Galindo, & Grahame, 2008; Descimon & Mallet, 2009; Nosil, 2012)
We studied a complex of butterfly species in the genus Coenonympha (Nymphalidae, Satyrinae), and found, using molecular markers and an Approximate Bayesian Computation (ABC) framework, that C. darwiniana (Staudinger 1871) and C. macromma (Turati & Verity 1910) are the product of hybrid‐ ization between the Pearly Heath C. arcania Linné 1761 and the Alpine Heath C. gardetta Prunner 1798 (Capblancq, Després, Rioux, & Mavárez, 2015)
All the retained past demographic scenarios involved a significant degree of gene flow during the divergence and speciation of the four taxa, which led to the pattern of genetic differentiation observed within the com‐ plex today (Figure 3)
Summary
Hybridization between diverging taxa is possible throughout the speciation process until complete reproductive isolation is achieved (Abbott et al, 2013; Butlin, Galindo, & Grahame, 2008; Descimon & Mallet, 2009; Nosil, 2012). Speciation is not a punctual phe‐ nomenon but a continuous process over time, at the beginning of which the hybrid lineage is not expected to be fully reproductively isolated from the parental species and the two types of popula‐ tions, hybrid and parental, likely remain interconnected to some degree This situation has sometimes been called a “hybrid swarm” (Jiggins & Mallet, 2000; Seehausen, 2004) to illustrate the tenuous. We modeled and tested several past scenarios of divergence and gene flow to evaluate the most likely direction, degree and tim‐ ing of hybridization during the evolution of the complex We confronted these past scenarios against the current reproductive isolation among the four taxa inferred from patterns of hybridiza‐ tion in contact zones. We evaluated the relation between gene flow and patterns of morphological, ge‐ netic, and ecologic differentiation among the species in the com‐ plex to understand the possible influence of these factors in the speciation of the hybrid taxa
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