In English

Clines resulting from selection-dispersal balance in the presence of multiple environmental changes

Oskar Fridell
Göteborg : Chalmers tekniska högskola, 2017. 57 s.
[Examensarbete på avancerad nivå]

A hybrid zone is a spatial area in which two genetically and phenotypically different populations of the same species (so called divergent ecotypes) meet and produce viable and fertile offspring. Hybrid zones are commonly observed in nature. Examples include hybrid zones found in the sea snail Littorina saxatilis on the Swedish West coast. This species has formed two divergent ecotypes (a crab-exposed and a wave-exposed one) as a response to the adaptation to the local environmental conditions, primarily the presence and absence of crabs, respectively. However, other environmental conditions, such as the presence of additional species’ interactions in a given ecosystem, may have contributed to the formation and maintenance of the hybrid zones observed. The spatial positions of the environmental transitions corresponding to different environmental factors may or may not coincide with each other. Yet, most theoretical studies on hybrid zones assume only a single environmental transition in the habitat in question (Gay et al. 2008; but see Slatkin, 1975). These studies have shown that allele frequencies at the locus under selection exhibit a sigmoid-like function of the spatial position in the habitat (so called genetic clines). However, when analyzing empirical data, it is of crucial interest to answer: Is the hybrid zone in question subject to multiple, spatially separated, environmental transitions? To answer this question, it is necessary to understand how the presence of multiple environmental transitions is reflected in spatial patterns of genetic variation at the loci under selection. Slatkin (1975) considered a model with two environmental transitions, assuming that two loci are under the selection pressures imposed by these environmental changes. His main finding concerns the relationship between the average cline centres at the two loci under selection in dependence of the recombination rate between the loci, and the distance between the two environmental transitions. However, these results are insufficient to answer under which conditions it becomes possible to detect multiple environmental transitions in the habitat based on empirical genetic data on allele frequencies. To answer this question it is necessary to estimate the probability distributions of the cline centers at the two loci, and the corresponding mixture distribution. Notably, in the case of a single environmental transition the distribution of cline centers is unimodal. By contrast, and as shown in this thesis, in the case of two environmental transitions the resulting mixture distribution may be bimodal. A bimodal mixture distribution is a direct indicator of the existence of two environmental changes. Using a test of bimodality, I estimate the critical value of the distance between the two environmental transitions above which a bimodal pattern of the mixture distribution of cline centres emerges. These results can be further improved by assessing the patterns of neutral loci linked to the loci under selection, and the bias these may introduce when inferring the presence of multiple environmental transitions in the hybrid zone in question.

Nyckelord: cline, Littorina saxatilis, hybrid zone, genetic drift, bimodality.