Speciation with gene flow in marine systems

Over the last century, a large body of literature emerged on mechanisms driving speciation. Most of the research into these questions focussed on terrestrial systems, while research in marine systems lagged behind. Here, we review the population genetic mechanisms and geographic context of 33 potential cases of speciation with gene flow in the marine realm, using six criteria inferred from theoretical models of speciation. Speciation with gene flow occurs in a wide range of marine taxa. Single traits, which induce assortative mating and are subjected to disruptive selection, such as differences in host-associations in invertebrates or colour pattern in tropical fish, are potentially responsible for a decrease in gene flow and may be driving divergence in the majority of cases. However, much remains unknown, and with the current knowledge, the frequency of ecological speciation with gene flow in marine systems remains difficult to estimate. Standardized, generally applicable statistical methods, explicitly testing different hypotheses of speciation, are, going forward, required to confidently infer speciation with gene flow

An eye for change:Evolutionary consequences of visual plasticity

For a long time, biologists have been trying to understand the patterns of biodiversity. Darwin showed that the environment is an important factor in speciation, as it exerts the force of natural selection on properties of organisms. If this selection is aimed at heritable properties, then, gradually, a genetic difference will evolve and the property under selection will change. However, some properties are plastic and can change in response to the environment, without genetic change. This phenomenon, known as phenotypic plasticity, might affect speciation, but it is unclear if plasticity promotes or impedes speciation, if it plays a role at all. In this thesis, I studied the association between speciation and phenotypic plasticity of the visual system in African cichlid fish. Cichlids constitute one of the most species-rich vertebrate families and are found in a wide range of aquatic habitats, differing in light conditions. I explored the association between plasticity and speciation at multiple levels, combining gene expression data, data on the behaviour of fish and data on the habitats in which the cichlids occur. My results show that visual plasticity varies considerably between species, at all studied levels, but does not play a consistent role in the diversification of African cichlids, neither positively nor negatively. The results in my thesis also underline the importance of studying evolutionary questions at multiple levels, combining for example gene expression data with data on the behaviour. Only then will we be able to understand the evolutionary consequences of phenotypic plasticity

The importance of being plastic: Evolutionary and ecological correlates of phenotypic plasticity

Phenotypic plasticity allows individuals to cope with different environmental conditions without the need for genetic adaptation. Consequently, plasticity affects the nature and strength of selection on traits. Plasticity enables individuals to experience novel selection pressures, for example through migration, possibly leading to genetic adaptation. At the same time, plasticity may shield the genotype from selection, impeding genetic adaptation. Plasticity, therefore, can have opposing effects on evolutionary change and population divergence.The visual system of cichlid fish is a suitable model trait to study the relationship between plasticity and evolutionary change, because of the high species richness and the variation among species in visual habitat. Furthermore, plasticity in the expression of genes involved in the cichlid’ visual system is well established. Here, we test whether visual plasticity is correlated to the degree of ecological specialisation and clade species richness. We quantify plasticity in the expression of these genes in species from species-poor and species-rich clades, inhabiting different visual habitats in nature, by exposing individuals to different light conditions and determining light-induced variation in gene expression. Preliminary results indicate that expression indeed differs among light treatments, in multiple species. We expect our study to provide a better understanding of the impact of phenotypic plasticity on divergent evolution

The importance of being plastic:Evolutionary and ecological correlates of phenotypic plasticity

Phenotypic plasticity allows individuals to cope with different environmental conditions without the need for genetic adaptation. Consequently, plasticity affects the nature and strength of selection on traits. Plasticity enables individuals to experience novel selection pressures, for example through migration, possibly leading to genetic adaptation. At the same time, plasticity may shield the genotype from selection, impeding genetic adaptation. Plasticity, therefore, can have opposing effects on evolutionary change and population divergence.The visual system of cichlid fish is a suitable model trait to study the relationship between plasticity and evolutionary change, because of the high species richness and the variation among species in visual habitat. Furthermore, plasticity in the expression of genes involved in the cichlid’ visual system is well established. Here, we test whether visual plasticity is correlated to the degree of ecological specialisation and clade species richness. We quantify plasticity in the expression of these genes in species from species-poor and species-rich clades, inhabiting different visual habitats in nature, by exposing individuals to different light conditions and determining light-induced variation in gene expression. Preliminary results indicate that expression indeed differs among light treatments, in multiple species. We expect our study to provide a better understanding of the impact of phenotypic plasticity on divergent evolution