Genetic diversity and genetic structuring at multiple spatial scales across the range of the Northern Leopard Frog, Rana pipiens

An understanding of the factors driving genetic diversity in populations is increasingly important in many areas of biology, including epidemiology, biological consequences of climate change, and the spread of invasive species. However, despite hundreds of papers studying genetic diversity across species’ ranges, we lack a cohesive view of the factors driving that diversity. Few papers have empirically addressed alternative explanations for genetic diversity, and even fewer have incorporated information on the history of populations or include assessments of diversity continuously, rather than categorically. Genetic diversity can be reduced in edge populations by position on the edge per se (i.e. by factors inherent to range edges), or by founder effects in recent colonization of newly available habitat, but these two confounding explanations are rarely addressed. We studied genetic diversity across the range of the Northern Leopard Frog, first to test whether genetic diversity is predicted by position at range center or edge, and second to test whether diversity is predicted by the history of the population (position above or below the glacial maximum in the last ice age). A second shortcoming of many studies available on the distribution across species’ ranges is that they use only a categorical approach, as we did in the first part, and thus 2 This chapter is co-authored by Ryan P. O’Donnell and Karen E. Mock. 12 little is known about how diversity changes from range center to edge. Accordingly, we examined the pattern of genetic diversity continuously from range center to edge in two transects. In the first part, we found that genetic diversity was reduced in edge populations relative to central populations, but was not reduced in populations in glaciated areas versus unglaciated ones, providing evidence that position at range edge has a stronger effect in reducing diversity than recent colonization in this species. In the second part, we found that genetic diversity declined linearly towards the range edge in one transect, but did not decline in the other. These transects differed in the nature of the range edges they included: a transect with increasingly isolated populations showed a decline in diversity, and a transect through continuous habitat did not. Taken together, these results provide evidence that genetic diversity in this species is reduced by position at the range edge, but that these effects may not be evident in all edges. Introduction An often cited “rule” of population genetics is that genetic diversity tends to be higher in the interior of species’ ranges and lower at the periphery (e.g. Hutchison 2003; Gapare and Aitken 2005; Arnaud-Haond et al. 2006; Lind et al. 2007). This may be directly related to the abundant center distribution (reviewed in Sagarin and Gaines 2002): if populations are smaller at the periphery, they may be more susceptible to loss of alleles through genetic drift (Vucetich and Waite 2003) and to stochastic extinction and recolonization (Caughley 1994), reducing genetic diversity through repeated founder effects (Holt and Keitt 2000). Edge populations may further have less gene flow to counteract the stochastic loss of alleles because they can only receive immigrants from 13 one direction (García-Ramos and Kirkpatrick 1997; Rowe and Beebee 2001). Populations at the edge of the range may also lose diversity through selection if they encounter unique selective pressures, such as competitors, parasites, or predators not present in the central part of the range, or climatic conditions at the limit of the species’ tolerance (Gorodkov 1986; Holt and Keitt 2000; Gaston 2003). All of these varied processes may combine to reduce genetic diversity symmetrically within all peripheral populations around the edge of a species’ range (Fig. 2-1A). However, an additional explanation for reduced diversity at range edges may be at work, which would not usually affect all range edges equally (Fig. 2-1B). If ranges shift in space, populations in the expanding part of the range may harbor reduced genetic diversity due to founder effects, a special case of reduced population size (e.g. Green et al. 1996; Austin et al. 2004a). Founder effects could reduce genetic diversity in peripheral populations as well as populations that are currently geographically central, if both were part of a recent range expansion. If founder effects are more prevalent than processes affecting peripheries per se, it may explain why more than one third of range peripheries do not have reduced genetic diversity relative to central parts of the range (Eckert et al. 2008) if those peripheries were not part of a recent range expansion. Most of the studies that have attempted to investigate causes of reduced diversity at the periphery of species’ ranges have been concerned with distinguishing between anthropogenic and prehistoric reductions of diversity (e.g. Matocq and Villablanca 2001; Hoffman and Blouin 2004b). When reduced diversity in peripheral populations is found to be a result of prehistoric processes, either through comparison with reference populations or with museum specimens, there is often little further investigation into the 14 causes of reduced genetic diversity at the periphery. In two exceptions, genetic diversity was found to follow a gradient declining from a presumed glacial refugium to the most recently established populations, supporting the idea that range expansion, not periphery per se, was responsible for reducing genetic diversity in peripheral populations (Garner et al. 2004; Gapare and Aitken 2005). In two other cases, a similar pattern of reduced diversity in edge populations was explained by reduced population sizes in the marginal habitats inhabited by peripheral populations (Johansson et al. 2006; Lind et al. 2007). More studies are needed that test alternative hypotheses explaining diversity in edge populations (Gaston 2003). A further limit on many previous studies of the factors driving population genetic diversity is that most studies use only a categorical approach. Over two thirds of studies in a recent review have considered only discrete categories of population types, “edge” vs. “center” (Eckert et al. 2008). This approach reveals little about the manner in which diversity declines towards a range edge. The gradual breakup of populations towards the range edge as depicted, for example, by Gorodkov (1986) suggests that a decline in diversity should occur gradually. However, reduced genetic diversity can be part of a series of positive feedback loops which may result in an abrupt decline in diversity at a threshold of isolation among subpopulations (Gilpin and Soulé 1986). For example, if populations are smaller and more isolated at the periphery, they may be more susceptible to genetic drift (Vucetich and Waite 2003) and to stochastic extinction and recolonization (Caughley 1994), reducing genetic diversity through repeated founder effects. Reduced genetic diversity may in turn make a population more susceptible to extinction. This positive feedback between extinction rate of isolated populations and reduced genetic 15 diversity is one way in which genetic diversity could decline abruptly at a threshold of population isolation. However, more studies using a continuous sampling strategy (rather than categorical) are needed to test whether genetic diversity declines linearly or with a threshold towards range peripheries. An additional pattern in genetic diversity from range edge to range center has been described, where diversity peaks at an intermediate optimum level of isolation. This pattern was found in one of very few other studies of genetic diversity along a transect (Kark et al. 2008). Kark et al. speculated that the intermediate maximum in diversity they found might result from a balance between bottlenecks that reduce genetic diversity in small edge populations, and large effective population sizes in the range center that make the establishment of new, rare alleles unlikely. This hypothesis of an intermediate optimum “goldilocks zone,” where populations are small and isolated enough to be likely to retain new mutations, but also connected enough to gain diversity through gene flow, likewise requires additional testing. We investigated patterns of genetic diversity around the range of the Northern Leopard Frog, Rana pipiens, to test two hypotheses for predicting levels of genetic diversity across species’ ranges. First, we tested the hypothesis that historic shifts in range have affected the current distribution of genetic diversity across the range of the species (Fig. 2-1B), against the alternative hypothesis that position at the edge of the range affects genetic diversity directly, without consideration of shifts in the species range through time (Fig. 2-1A). Second we tested the hypothesis that genetic diversity declines linearly towards range edges, against the alternatives that it declines abruptly at a threshold or that it has an intermediate optimum. We tested our first hypothesis by 16 sampling populations from around the range of the species, selecting populations that were at range edge or range center, and that were covered by ice at the most recent glacial maximum or colonized since that glacial maximum. We tested our second hypothesis by sampling population genetic diversity across two transects in the range of the Northern Leopard Frog, to test whether genetic diversity declines linearly, as predicted by the gradually increasing isolation between populations, or in a threshold, as predicted by the positive feedback mechanisms of Gilpin and Soulé (1986), or an intermediate optimum (Kark et al. 2008). The Northern Leopard Frog is particularly well suited for a study of the factors affecting population genetic diversity. Its range covers much of North America (Rorabaugh 2005), so differences in diversity between interior and peripheral populations are ex