My main interests are in speciation, adaptive radiation, hybridization and evolutionary genomics. I would like to understand what factors constrain or facilitate speciation and what factors maintain biodiversity. These factors may include different ecological or geographical conditions, patterns of gene flow, mate preferences, genetic incompatibilities or demographic events. I am also very interested in the role of hybridization in the generation and loss of species diversity. Much of the biodiversity is thought to have evolved in adaptive radiations, i.e. bursts of speciation events, associated with adaptation to different ecological niches. Therefore, understanding the evolution of adaptive radiation is key to understand how biodiversity evolves.
Lake Victoria cichlid fishes
African cichlids are an ideal system as they have undergone rapid adaptive radiations in several lakes and show an incredibly high species diversity. There are many replicate cases of adaptation and speciation that can be used for comparisons to draw more general conclusions. We mainly work on cichlids in Lake Victoria and nearby lakes. In Lake Victoria, 500 species evolved in only ~15’000 years, implying an incredibly high speciation rate. The same cichlid lineage has also formed adaptive radiations in all other major lakes in the Lake Victoria Region. In each radiation, the species are adapted to different niches e.g. they may utilize different depth ranges or food sources such as algae, insect larvae, zooplankton, fish eggs, detritus, snails, or smaller fish. The young age of these species allows us to better disentangle processes involved in speciation from those acting after speciation is complete. The study of these processes is further aided by the availability of species pairs covering the entire continuum from weakly divergent populations to species with complete reproductive isolation.
During my PhD, I uncovered an important role of hybridization in the diversification of these cichlids. The entire radiation of cichlids in Lake Victoria and nearby lakes has arisen from a hybrid swarm between cichlid lineages from the Congo and the Nile. The hybridization event of these highly divergent lineages likely facilitated the adaptive radiations by generating genetic polymorphisms that subsequently became recombined and sorted into many new species. As an example, it provided the high genetic variation at the long-wave sensitive (LWS) opsin gene which is known to play an important role both in divergent ecological adaptation and reproductive isolation among species. Together with the Hawaiian silversword alliance, the Lake Victoria Region cichlids are thus a prime example of how hybridization my fuel adaptive radiation in the presence of ecological opportunity.
Hybridization also played a role at later stages of the adaptive radiation of Lake Victoria cichlids. Interspecific hybridization among members of the adaptive radiation facilitated further speciation events by introducing functional genetic variation. In the Mwanza Gulf in Lake Victoria, a new species pair of red Pundamilia cichlids adapted to deep water and blue cichlids adapted to shallow water evolved after hybridization between P. nyererei (red) and P. pundamilia (blue). The new species pair in the Mwanza Gulf shows signatures of divergent adaptation or reduced gene flow in genomic regions that are also divergent between P. nyererei and P. pundamilia. However, a large proportion of the genomic regions that are strongly differentiated between the new species seem to be novel targets of divergent selection.
In my postdoc project with Ole Seehausen, I used whole-genome resequencing of a large number of Lake Victoria cichlids to shed more light on their exceptionally fast species diversification. I study their demographic history, genome-wide signatures of selection and introgression, the evolutionary history of traits involved in speciation, and the role of hybridization.
Heliconius and ithomiine butterflies
Since October 2018, I am now based in Cambridge and work on butterfly radiations. Heliconius and ithomiine butterflies are two groups of toxic butterflies that live in South and Central America. They show their toxicity with vibrant warning colours. Predators learn to avoid them by memorizing their warning colours. Multiple toxic butterfly species (and some non-toxic imposters) look alike wherever they co-exist which means they share the cost of teaching predators. Some of the butterfly species look so similar that it is very difficult to distinguish them even though they may not be closely related. However, in different regions in the Americas, the butterflies look different. In some species, the same species can look very different at different places to mimic the locally abundant warning colour patterns. This means that butterflies that look very similar can belong to distantly related species, and butterflies from different geographic regions that look very different, can belong to the same species. I study how these butterflies adapt to look alike and if hybridisation played a role in providing the genetic basis for the locally common warning colours. Two groups of ithomiine butterflies have speciated exceptionally fast. I study why they have diversified so much faster than their slowly speciating relatives.