Ecological and spatial patterns associated with diversification of South American Physaria (Brassicaceae) through the general concept of species

Abstract

Analyzing the roles of ecology and geography on speciation and lineage diversification can shed light on the processes that generate and maintain biodiversity. Additionally, lineages rapidly diversifying across unstable habitats provide substantial challenges for resolving evolutionary histories and delimiting species. Physaria is represented in South American by six species distributed from southern Bolivia to northern central Argentina and growing in highlands of the southern-central Andes, but also along the hills and lowlands of central-eastern Argentina. This biogeographical variability, not common among other South American crucifers (Brassicaceae), prompted us to conduct different climatic niche and geographical range comparisons to study the potential roles of ecology and geography through the diversification of the group. However, the remarkable similarity between these species, coupled with the continuous variability of the diagnostic morphological characters, blurs the species boundaries. Therefore, in order to identify independent evolving lineages, we first employed species delimitation methods together with the general lineage concept of species, and used molecular sequences from nuclear ribosomal and chloroplast loci. Secondly, and in the light of the lineages obtained, we explored the roles of geography and ecology on the diversification of South American Physaria and tested for presence of phylogenetic niche-conservatism or niche-divergence patterns, as well as potential ecological speciation. Lineages identified by these delimitation methods were highly congruent with described species; nevertheless, some morphospecies were included under the same independent evolutionary lineage. Our results suggest that the climatic niche divergence along the heterogeneous landscape apparently was a major factor promoting diversification of the South American Physaria. Divergence was registered mainly on the temperature dimension, which promoted shifts between cold-temperate habitats associated with the highlands of the central-southern Andes and warm lowlands from central-eastern Argentina, i.e., the Monte and Dry Chaco ecoregions. In addition, some degree of niche divergence along the precipitation gradient was also secondarily recovered. Allopatry and dispersal capabilities also seem to be associated with the diversification of the group, presumably through the Late Pliocene-Pleistocene, and promoted by glacial cycles and climatic oscillations during the Quaternary. Results of these analyses are also discussed in a general context, which will contribute to the understanding of the evolutionary and ecological patterns of South American Brassicaceae.