Gene flow dynamics in Baboons - The influence of social systems

Abstract

The relationship between genes and behaviour has been of longstanding interest to evolutionary biologists. Certain behaviours can shape the genetic structure of natural populations, thereby altering their genetic diversity and influencing their evolutionary fate. Dispersal is the behaviour that mediates gene flow, the extent of which determines population genetic structure. Because both historic and contemporary gene flow are considered to have greatly impacted their evolutionary history, baboons (genus Papio) are especially intriguing to study the relationship between behaviour and population genetic structure. Both species-specific male- and female-biased dispersal can be observed in this genus, their current distribution was shaped by range expansion and contraction, and interspecific gene flow is prevalent. In this thesis, I investigated how different dispersal patterns influence gene flow in baboons to contribute to a better understanding of the interrelation between behavioural ecology and genetic makeup of natural populations. I specifically addressed how differences in the social system of baboon species impact their genetic structure and also used the observed patterns to draw inferences about sex-biased dispersal in Guinea baboons, one of the least known members of the genus. I examined in detail how both historic and contemporary gene flow shape the genetic structure of Guinea baboons and whether we can draw inferences about human evolution from the analysis of range expansions in baboons. To answer these questions, I used a population genetic approach based on distribution-wide, geo-referenced faecal samples of baboons for which I analysed both autosomal microsatellites and part of the mitochondrial hypervariable region I. I could show that the genetic structure of Guinea baboons is best explained by female-biased dispersal, both on a local and a distribution-wide scale. Female gene flow results in high intrapopulation diversity and a lack of genetic-geographic structuring in mitochondrial DNA. In contrast, there is significant structuring of nuclear markers on a global scale and males exhibit higher population structuring than females on a local scale, as expected if males are the more philopatric sex. Over the whole distribution, locally restricted dispersal appears to limit effective gene flow to a distance of below 200 km, resulting in a strong isolation-by-distance effect and genetically divergent populations. Signatures of population expansion, the clinal structure of genetic variation, and potential traces of allele surfing, point to an historic west-ward expansion of Guinea baboons. Introgressive hybridization with olive baboons can be invoked to explain genetic patterns in the contact zone, but warrant further investigation. Additionally I could show the ‘southern route’ from Africa to Arabia could have been used by hamadryas baboons during the same time period in the Late Pleistocene as proposed for modern humans. My study is the first comprehensive analysis of the genetic population structure in Guinea baboons and provides evidence for female-biased dispersal in this species. It corroborates the notion that the Guinea baboons’ social system shares some important features with that of hamadryas baboons, suggesting similar evolutionary forces have acted to distinguish them from all other baboons. In conjunction with the importance of range expansions in shaping their distribution and genetic diversity, this strengthens baboons as an intriguing model to elucidate the processes that also influenced the evolution of our own species.