The Coalescent in Boundary-Limited Range Expansions
by Jens Nullmeier
Date of Examination:2014-01-15
Date of issue:2014-10-14
Advisor:Dr. Oskar Hallatschek
Referee:Dr. Oskar Hallatschek
Referee:Prof. Dr. Annette Zippelius
Persistent Address:
http://dx.doi.org/10.53846/goediss-4734
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Abstract
English
Habitat ranges of most species shift over time, for instance due to climate change, human intervention, or adaptation. These demographic changes often have drastic effects on the genetic composition of the population, such as a stochastic resampling of the gene pool through the “surfing” phenomenon. Most models assume that the speed of range expansions is only limited by the dispersal ability of the colonizing species and its reproductive potential. While such models of “phenotype-limited” expansions apply for instance to species invasions, it is clear that many range expansions are limited rather by the slow motion of habitat boundaries, as driven for instance by global warming. Here, we develop a coalescent model to study the genetic impact of such “boundary-limited” range expansions. Our simulations and analytical calculations show that the resulting loss of genetic diversity is markedly lower than in species invasions if large carrying capacities can be maintained up to the habitat frontier. Counterintuitively, we find that the total loss of diversity does not depend on the speed of the range expansion: Slower expansions have a smaller rate of loss, but also last longer. Based on our results, we conclude that boundary-limited range expansions have a characteristic genetic footprint and should be distinguished from range expansions limited only by intrinsic characteristics of the species. In the second part of the thesis we pursue two different approaches in the context of expanding populations. Inspired by observations in microbial experiments, we first ask whether the detailed colonization paths have a major impact on the evolution of an expanding population. If so, under what conditions has this effect to be acknowledged for in population models? The second approach is much closer to experimental work: we aim at developing tools that can actually be used in experiments to characterize range expansions in two–dimensional habitats based on genetic data. We will address both approaches on the basis of observables that describe the shape of mutation patches, that is spatially grouped individuals with the same mutation at a specific locus. Our results confirm drastic differences between the predictions of models with and without attention to the colonization paths. Furthermore, we develop a flexible method for the analysis of neutral genetic data.
Keywords: range expansion; population genetics; boundary-limited; phenotype-limited; continuous founder effect; Eden model; sectoring