Along the speciation continuum: Quantifying intrinsic and extrinsic isolating barriers across five million years of evolutionary divergence in California jewelflowers

Abstract

Understanding the relative roles of intrinsic and extrinsic reproductive barriers, and their interplay within the geographic context of diverging taxa, remains an outstanding challenge in the study of speciation. We conducted a comparative analysis of reproductive isolation in California Jewelflowers (Streptanthus, s.l., Brassicaceae) by quantifying potential barriers to gene flow at multiple life history stages in 39 species pairs spanning five million years of evolutionary divergence. We quantified nine potential pre- and postzygotic barriers and explored patterns of reproductive isolation in relation to genetic distance. Intrinsic postzygotic isolation was initially weak, increased at intermediate genetic distances, and reached a threshold characterized by complete genetic incompatibility. Climatic niche differences were strong at shallow genetic distances, and species pairs with overlapping ranges showed slight but appreciable phenological isolation, highlighting the potential for ecological barriers to contribute to speciation. Geographic analyses suggest that speciation is not regionally allopatric in the California Jewelflowers, as recently diverged taxa occur in relatively close proximity and display substantial range overlap. Young pairs are characterized by incomplete intrinsic postzygotic isolation, suggesting that extrinsic barriers or fine-scale spatial segregation are more important early in the divergence process than genetic incompatibilities.

Reproductive isolation and the maintenance of species boundaries in two serpentine endemic Jewelflowers

“…). The present study combined with previous work across the Streptanthus clade (Christie and Strauss ) provides a look at the evolution of RI across multiple scales of biological organization. For example, at the clade scale in Streptanthus , ecogeographic isolation manifests as broad differences in climatic niche tolerance and range overlap, whereas at the level of the sympatric species pair S. breweri and S. hesperidis , it manifests as a lack of local fine‐scale co‐occurrence.…”

“…Here, other factors besides the evolution of soil endemism are likely important in explaining diversification. A previous clade‐wide survey of RI in Streptanthus revealed several important patterns—intrinsic RI was incomplete in relatively recently diverged species pairs, and such pairs also shared overlapping ranges, suggesting that extrinsic barriers or ecological divergence play a crucial role in speciation process (Christie and Strauss ). This result leads to the question: if close relatives occur sympatrically and are at least partially interfertile, what other mechanisms might act to mitigate gene flow and maintain species boundaries?…”

Relative importance of isolation‐by‐environment and other determinants of gene flow in an alpine amphibian

“…Specifically, a difference in habitat preference coupled with assortative mating by habitat seems to be especially frequent early in the process (Forbes et al. 2017; Lackey and Boughman 2017; Christie and Strauss 2018). All three mechanisms have been detected in other examples of IBE (Via 1999; Via et al.…”

Genomic differentiation across the speciation continuum in three hummingbird species pairs

“…To help alleviate this problem, some studies have used independent pairs of closely related species that have different divergence times as a proxy for the different stages of speciation (e.g. [15,18,26,28,31,[36][37][38][39][40]). This can provide valuable insight into the genomic process of speciation over time.…”