Abstract
The Pleistocene glacial cycles had a profound impact on the ranges and genetic make-up of organisms. While it is clear that the contact zones that have been described for many sister taxa are secondary and have formed in the current interglacial, it is unclear when the taxa involved began to diverge. Previous estimates based on small numbers of loci are unreliable given the stochasticity of genetic drift and the contrasting effects of incomplete lineage sorting and gene flow on gene divergence. Here, we use genome-wide transcriptome data to estimate divergence for 18sister species pairs of European butterflies showing either sympatric or contact zone distributions. We find that in most cases, species divergence predates the mid-Pleistocene transition or even the entire Pleistocene period. We also show that although post-divergence gene flow is restricted to contact zone pairs, they are not systematically younger than sympatric pairs. This suggests that contact zones are not limited to the initial stages of the speciation process, but can involve notably old taxa. Finally, we show that mitochondrial divergence and nuclear divergence are only weakly correlated and mitochondrial divergence is higher for contact zone pairs.
Highlights
Divergence in allopatry provides a simple null model of speciation (Mayr, 1947)
We find that estimates of sister species divergence based on the distribution of pairwise differences are highly correlated (Pearson's correlation coefficient = 0.79) with the estimates based on mean da (Figure S5)
The IM model fit significantly better than a strict divergence model for two species pairs (Pontia and Colias), and, as expected, T estimates under the IM model for these two species pairs are older (3.30 vs 0.919 and 5.08 vs 2.33 million years ago (MYA), respectively) than estimates based on da (Table S3)
Summary
Divergence in allopatry provides a simple null model of speciation (Mayr, 1947). Following geographic isolation and given enough time, reproductive isolation is inevitable as incompatibilities will eventually become fixed as a result of genetic drift and/or selection (Bateson, 1909; Dobzhansky, 1937; Muller, 1942). Hewitt (2011) summarizes age estimates for European hybrid-zones taxa including mammals, insects, amphibians and reptiles, which range from hundreds of thousands to several million years ago Given that these estimates are based on different markers and calibrations, the extent to which glacial cycles have initiated speciation events remains unknown. In the absence of gene flow, divergence at a single locus may substantially predate the onset of species divergence, while it may be much more recent in the presence of gene flow (Knowles & Carstens, 2007; Wang & Hey, 2010) Mitonuclear discordance in both directions has been found in a large number of animal systems (Toews & Brelsford, 2012) including several closely related species of European butterflies (Dincӑ et al, 2019; Hinojosa et al, 2019; Wiemers et al, 2010). (iii) Is there evidence for gene flow between contact zone species? (iv) How strongly correlated are mitochondrial and nuclear divergence and do contact zone pairs show increased mitonuclear discordance?
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