Evolutionary diversification and the genetics of speciation
Speciation is frequent and ongoing, and has resulted in myriads of extinct and extant species. Nevertheless, single speciation events have proven difficult to disentangle, because several evolutionary processes (genetic drift, selection and admixture) co-occur during divergence and leave similar genomic footprints. Conceptually, the study of speciation has made major advancements and we have reached quite clear expectations of how new species may arise. Adaptation to different selective regimes can trigger population differentiation and lead to the evolution of reproductive isolation directly, when the traits under differential selection also play a role in reproductive isolation (‘ecological speciation’), or indirectly when the evolution of pre- and/or post-mating reproductive barriers comes about as a by-product of the accumulated differentiation. The importance of epistatic interactions between genes in general, and between autosomal and sex-linked genes in particular, in causing reproductive incompatibilities of diverging lineages is well-founded theoretically, but the support is often circumstantial. Likewise, the relative importance of natural and sexual selection in driving evolutionary diversification remains understudied.
Our research explores different aspects of evolutionary diversification, including the roles of ecology and natural selection, hybridization and introgression, and sexual selection. We study
- ecological speciation in endemic birds on isolated Atlantic islands
- reproductive isolation in range expanding and hybridizing damselflies
- sexual selection in diversifying birds with newly evolved sex chromosomes
Our work on the Nesospiza buntings in the Tristan da Cunha archipelago and the Crithagra finches on the Gulf of Guinea islands shows that selection has been a major force during the speciation process and that speciation can occur even in the face of gene flow. We are gathering genome-wide molecular evidence for genetic regions that are under selection, in order to determine the genetic bases of adaptation. In particular, we are interested in identifying genes underlying the evolution of bill size/shape. Bill constitutes a classic trait associated with resource-driven bird diversification, and therefore establishing the links between phenotype–fitness–genotype will significantly advance our overall understanding of speciation in birds.
The aim of our work on damselflies is to gaining insights into the process of reproductive isolation between recently diverged but still hybridizing species. Understanding the evolutionary consequences of hybridisation is a pressing issue in the context of global warming, because several taxa currently undergo temperature-induced range expansions. Range expansions can cause overlap between formerly allopatric species and lead to extensive hybridisation in the new sympatric areas. Our data of the two sister species Ischnura elegans and I. graellsii show extensive population overlap in southern Europe with strong and ongoing hybridisation in several regions of sympatry. We are currently exploring genome and transcriptome datasets of these species to understand several hypotheses related to range expansions and hybridisation, including whether adaptive genes cross species boundaries more easily than non-adaptive ones and the relative importance of sex-linked genes for maintaining species integrity.
Finally, we are looking into the role of sexual selection and sex chromosomes during diversification and speciation in Sylvioidea passerines. This group of birds has a newly evolved enlarged sex chromosome (formed by a fusion with an autosome) which is interesting because the number of sex-linked genes has increased and because many genes located on the female-specific W chromosome are still functional. Several species in the group are sexually dimorphic either in plumage or reproductive traits. We are currently using phylogenomics approaches to understand the speciation process in this clade.