New PhD-thesis from the lab: Anna Runemark and evolution on islands

Posted by Erik Svensson

Some of the proudest moments in the careers and life of advisors come when their students successfully finish their PhD-theses. Now it is time again, this time for Anna Runemark, who will defend her PhD-thesis on Friday May 25 in the "Blue Hall" (Ecology Building, Department of Biology) at 09.30. The thesis defence is open to all interested, as with all PhD-thesis defences at our university.

The Science Faculty's external opponent is Professor Scott Edwards from Harvard University. The examination committé consists of Drs. Alexandre Antonelli (Gothenburg University, Sweden), Folmer Bokma (Umeå University, Sweden) and Jessica Ware (Rutgers University, USA). On May 24 (Thursday, the day before Anna's thesis defence), we will have a small and informal research symposium with the opponent and the members of the thesis-committé entitled "Island Biology and Evolution", which will start at 13.00 in the "Blue Hall" (Ecology Building).

The title of the thesis is "Island biogeography and population divergence in the Skyros wall lizard". You can find a Swedish summary of the thesis here, and here is the link to the entire thesis, for those of you work at Lund University.

On a more personal note, Anna Runemark is my fifth PhD-student, preceeded by Jessica Abbott (2006), Thomas Gosden (2008), Fabrice Eroukhmanoff (2009) and Kristina ("Tina") Karlsson-Green (2010). I am proud of all these students, and I have learnt tremendously much from being an advisor of these five PhD-theses. Above all, I have learnt that there is not one way of being a good advisor or a good student - but many. 

There is thus no single "magic formula" to do an excellent PhD-thesis, but many different avenues. In a way, one could compare the writing of a thesis with the hill-climbing process on Sewall Wright's Adaptive Landscape: there are multiple adaptive peaks (excellent theses) and many different combinations of skills that make a good thesis. There are also many different kinds of students, each with different social and intellectual backgrounds, personalities, skills and interests. 

Equally, there are many different ways of a being a good advisor, and being a good advisor or a good student is, in itself, no guarantee for success. It is rather the combination of advisor and student that is crucial. One could perhaps compare this with the concept of genetic compatability in sexual selection: some combinations are less fit than others, even if the component parts (genes, advisors, students) do not differ and are excellent in isolation. 


The ideal student-advisor relationship is one of complementarity and mutual respect for each other's skills: The advisor provides the broader overview, theoretical and conceptual background and helps the student to see "the Big Picture" of the work, and forces the student to keep looking at the horizon, rather than dwelling in to methodological details. The student, on the other hand, should develop his/her methodological and technical skills and become "good" at it, and work to build his/her understanding of scientific concepts and how the thesis work can contribute to increased understanding of the natural world. I feel that Anna's thesis about a classical topic in evolutionary biology (evolution on islands) exemplifies this type of advisor-student collaboration that characterizes an excellent PhD-thesis.   

New journal cover in Molecular Ecology: Vicariance divergence and gene flow among islet populations of an endemic lizard

I study genetic, morphological and behavioural divergence in islet populations of the Skyros wall lizard, Podarcis gaigeae. This species shows strong morphological divergence, including island gigantism (see the cover image, with adult male lizards from mainland populations to the left and islet populations to the right ). In this paper (found here) we have used isolation with migration models to investigate divergence times and levels of gene flow between islet populations and their closest mainland populations. Such background information is valuable for example for inferring rates of morphological and genetical divergence. Our results support that the studied islet populations have been sequentially separated by rising sea levels in the Aegean.

Abstract:

Allopatry and allopatric speciation can arise through two different mechanisms: vicariance or colonization through dispersal. Distinguishing between these different allopatric mechanisms is difficult and one of the major challenges in biogeographical research. Here, we address whether allopatric isolation in an endemic island lizard is the result of vicariance or dispersal. We estimated the amount and direction of gene flow during the divergence of isolated islet populations and subspecies of the endemic Skyros wall lizard Podarcis gaigeae, a phenotypically variable species that inhabits a major island and small islets in the Greek archipelago. We applied isolation-with-migration models to estimate population divergence times, population sizes and gene flow between islet–mainland population pairs. Divergence times were significantly correlated with independently estimated geological divergence times. This correlation strongly supports a vicariance scenario where islet populations have sequentially become isolated from the major island. We did not find evidence for significant gene flow within P. g. gaigeae. However, gene-flow estimates from the islet to the mainland populations were positively affected by islet area and negatively by distance between the islet and mainland. We also found evidence for gene flow from one subspecies (P. g. weigandi) into another (P. g. gaigeae), but not in the other direction. Ongoing gene flow between the subspecies suggests that even in this geographically allopatric scenario with the sea posing a strong barrier to dispersal, divergence with some gene flow is still feasible.

Island biology and morphological divergence of the Skyros wall lizard Podarcis gaigeae: a combined role for local selection and genetic drift on color morph frequency divergence?

 

















We have recently published a paper on evolutionary processes in isolated islet populations of the Skyros wall lizard, Podarcis gaigeae in BMC Evolutionary Biology. The paper investigates the relationship between neutral genetic divergence and morphological divergence in islet- and mainland populations. The morphological trait we use in the comparisons is throat colour morph, and islet populations show pronounced frequency differences with different morphs being common on different islets. Our data suggests that stochastic forces such as genetic drift and/or founder effects can interact with selection and have an effect even at a morphological level in islet populations with low effective sample sizes. BMC Evolutionary Biology is an open access journal and a link to the paper is found here. The abstract is as follows:

Island biology and morphological divergence of the Skyros wall lizard Podarcis gaigeae: a combined role for local selection and genetic drift on color morph frequency divergence?

Anna Runemark, Bengt Hansson, Panagiotis Pafilis, Efstratios D. Valakos & Erik I. Svensson

Background

Patterns of spatial variation in discrete phenotypic traits can be used to draw inferences about the adaptive significance of traits and evolutionary processes, especially when compared to patterns of neutral genetic variation. Population divergence in adaptive traits such as color morphs can be influenced by both local ecology and stochastic factors such as genetic drift or founder events. Here, we use quantitative color measurements of males and females of Skyros wall lizard, Podarcis gaigeae, to demonstrate that this species is polymorphic with respect to throat color, and the morphs form discrete phenotypic clusters with limited overlap between categories. We use divergence in throat color morph frequencies and compare that to neutral genetic variation to infer the evolutionary processes acting on islet- and mainland populations.

Results

Geographically close islet- and mainland populations of the Skyros wall lizard exhibit strong divergence in throat color morph frequencies. Population variation in throat color morph frequencies between islets was higher than that between mainland populations, and the effective population sizes on the islets were small (N_e:s <>ST) for throat color morph frequencies fell within the neutral F_ST-distribution estimated from microsatellite markers, and genetic drift could thus not be rejected as an explanation for the pattern. Moreover, for both comparisons among mainland-mainland population pairs and between mainland-islet population pairs, morph frequency divergence was significantly correlated with neutral divergence, further pointing to some role for genetic drift in divergence also at the phenotypic level of throat color morphs.

Conclusions

Genetic drift could not be rejected as an explanation for the pattern of population divergence in morph frequencies. In spite of an expected stabilising selection, throat color frequencies diverged in the islet populations. These results suggest that there is an interaction between selection and genetic drift causing divergence even at a phenotypic level in these small, subdivided populations.