Aim

Past glacial climate cycles have generated lineage diversity in alpine habitats, acting as a climate-driven species pump. It is not clear how much this process contributes to ecological diversification of alpine species. To examine this problem, we test patterns of genetic and phenotypic divergence in two co-distributed species complexes of flightless alpine ground beetles. Greater differentiation in ecologically-important functional traits would indicate that ecological selection is an outcome of oscillating climate change, whereas greater differentiation in non-ecological traits would indicate niche conservatism.

Location

The Cascades Range and Trinity Mountains of western North America.

Taxon

Members of the Nebria paradisi and N. vandykei species complexes (Insecta: Coleoptera: Carabidae: Nebriinae)

Methods

We generated genome-wide single nucleotide polymorphism data and mitochondrial sequence data, as well as morphological and physiological data, to compare populations spanning the range of both species. Phylogenetic and population genetic analyses were used to infer the relationships among taxa and populations within each species complex, as well as historical population demography. Support vector machines were used to test for classification of taxa and populations based on ecomorphological, ecophysiological, and male reproductive traits. Mantel tests were then used to assess statistical associations between phenotypic and genetic divergence among populations.

Results

The N. vandykei and N. paradisi species complexes are each comprised of genetically distinctive populations exhibiting long-term demographic declines. Each phylogeny supports multiple monophyletic groups with geographical cohesion. By examining phenotypic traits among populations in both species’ complexes, we show that reproductive trait divergence can discriminate species and population status more effectively than ecomorphological or ecophysiological traits. Reproductive and genetic divergence are significantly correlated in the N. vandykei species complex.

Main Conclusions

We found limited evidence of ecological selection acting on functional traits. Instead, reproductive and genetic divergence evolved among isolated populations in both species complexes, suggesting niche conservatism may be a common outcome in alpine species diversification.