Why this project?
The evolution of biological diversity is strongly facilitated by natural selection, both in space and time. Spatial environmental differences induce divergent natural selection, driving adaptive divergence and, ultimately, the evolution of reproductive isolation (i.e. ecological speciation). However, the propensity and degree of adaptive diversification may strongly depend on organismal dispersal and on geographic isolation (i.e. gene flow may counteract adaptive divergence and introduce novel genetic variation). Likewise, environments commonly fluctuate over time, giving rise to temporal shifts in selection.
Natural selection acts on the composite phenotype, which is the product of genetic variation and phenotypic plasticity. Hence, the determinants of phenotypic variation – and their mechanisms of inheritance (i.e. direct genetic and plastic effects, and transgenerational effects) – are of key importance for the evolution of biological diversity.
Our work here investigates the evolution of natural populations in response to interacting forces of selection and gene flow (in space and time), and the interactions between genetic variation, phenotypic plasticity (developmental plasticity) and transgenerational effects (in particular maternal investment in egg size) in diversification (an ECO-EVO-DEVO approach).
- Threespine stickleback (Gasterosteus aculeatus) – in and around lake Myvatn, Iceland. Past work (by Räsänen) includes studies on lake-stream stickleback in the Misty lake, Canada.
- Arctic charr (Salvelinus alpinus) – Dwarfed charr in complex of lava caves near Myvatn and resource polymorphic populations in multiple lakes across Iceland.
Approaches: Spatially and temporally replicated field surveys, population genetics, transcriptomics and laboratory rearing experiments.
Stickleback – Iceland:
- Stickleback within lake Myvatn show phenotypic divergence in feeding morphology, body size, brain size and thermal performance across major habitat types within the lake. This divergence occurs in face of extensive gene flow (as reflected by microsatellite markers and lack of geographic barriers to dispersal).
- At the same time, the stickleback population shows strong temporal fluctuations (across years) in population densities and phenotypic variation (e.g. age at maturation, body size and feeding morphology).
- Recent rearing experiments indicate that this intra-lacustrine phenotypic divergence is due to a combination of direct genetic and plastic responses, as well as parental effects.
- Stickleback across a pond complex (19 ponds) in the vicinity of lake Myvatn, show subtle phenotypic divergence and strong population genetic structure. Based on population genetic work, this divergence is likely strongly influenced by genetic drift in small populations, as well as different degrees of connectivity among ponds – mediated by seasonal flooding events.
Stickleback – Canada:
- In the Misty lake system, the lake and inlet stream stickleback show extensive phenotypic and genetic divergence, whereas the lake and outlet stream stickleback show subtle divergence. Lake-stream divergence is facilitated by both genetic and plastic effects on phenotypes.
- Intriguingly, there is an apparent lack of mating isolation between the lake and stream stickleback in the Misty system. This suggests that other mechanisms (e.g. temporal isolation or selection against migrants) maintain phenotypic and genetic divergence in this system.
Arctic charr – lava caves:
The cave charr present small (dwarfed) benthic phenotypes. The populations have small population sizes and show extensive population genetic structure across the caves and repeated parallel phenotypic divergence from the ancestral type of lake charr. Ongoing work on this system focuses on understanding evolutionary processes in small populations.
Arctic charr – lakes:
This work is ongoing, but first results (PhD thesis work of Samantha Beck) indicate that both egg size and variation in gene expression during juvenile development are key components of the evolution of resource polymorphism.