Draining the Swamping Hypothesis: Little Evidence that Gene Flow Reduces Fitness at Range Edges

Kottler, Ezra J.; Dickman, Erin E.; Sexton, Jason P.; Emery, Nancy C.; Franks, Steven J.

doi:10.1016/j.tree.2021.02.004

Cited by 54 publications

(53 citation statements)

References 88 publications

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“…2008), making “swamping gene flow” scenarios unlikely to be a cause of variation in QGV near range limits or of range limits themselves (Kottler et al. 2021). Finally, moderate or high QGV for certain traits may be maintained, even in edge populations, if there is not strong directional selection on those traits.…”

Section: Discussionmentioning

confidence: 99%

How is adaptive potential distributed within species ranges?

et al. 2021

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Quantitative genetic variation (QGV) represents a major component of adaptive potential and, if reduced toward range‐edge populations, could prevent a species’ expansion or adaptive response to rapid ecological change. It has been hypothesized that QGV will be lower at the range edge due to small populations—often the result of poor habitat quality—and potentially decreased gene flow. However, whether central populations are higher in QGV is unknown. We used a meta‐analytic approach to test for a general QGV‐range position relationship, including geographic and climatic distance from range centers. We identified 35 studies meeting our criteria, yielding nearly 1000 estimates of QGV (including broad‐sense heritability, narrow‐sense heritability, and evolvability) from 34 species. The relationship between QGV and distance from the geographic range or climatic niche center depended on the focal trait and how QGV was estimated. We found some evidence that QGV declines from geographic centers but that it increases toward niche edges; niche and geographic distances were uncorrelated. Nevertheless, few studies have compared QGV in both central and marginal regions or environments within the same species. We call for more research in this area and discuss potential research avenues related to adaptive potential in the context of global change.

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Section: Discussionmentioning

confidence: 99%

How is adaptive potential distributed within species ranges?

et al. 2021

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“…On a global scale, the S. aucuparia populations in the Hyrcanian forest are considered as a range-edge population of this species in the Northern Hemisphere. These range-edge populations are isolated from the populations in Europe and are vulnerable to genetic drift [ 57 ]. Inbreeding depression, genetic drift, and differentiation of peripheral populations are all exacerbated by persistent reductions in gene flow among small isolated and less dense populations [ 57 , 58 ].…”

Section: Discussionmentioning

confidence: 99%

“…These range-edge populations are isolated from the populations in Europe and are vulnerable to genetic drift [ 57 ]. Inbreeding depression, genetic drift, and differentiation of peripheral populations are all exacerbated by persistent reductions in gene flow among small isolated and less dense populations [ 57 , 58 ]. These interpretations contrast strongly with the high levels of individual heterozygosity, suggesting a heavy selection against selfed offspring [ 59 ].…”

Section: Discussionmentioning

confidence: 99%

Genetic Diversity and Structure of Rear Edge Populations of Sorbus aucuparia (Rosaceae) in the Hyrcanian Forest

Yousefzadeh

Raeisi

Esmailzadeh

et al. 2021

Plants

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Sorbus aucuparia (Rosaceae) is a small tree species widely distributed in Eurasia. The Hyrcanian forest is the southernmost distribution limit of this species. Severe habitat degradation and inadequate human interventions have endangered the long-term survival of this species in this region, and it is necessary to develop and apply appropriate management methods to prevent the loss of its genetic diversity. In this study, we used 10 SSR markers in order to evaluate the genetic diversity of this taxon. Leaf samples were collected from five known populations of S. aucuparia throughout its distribution area in the Hyrcanian forest. Expected heterozygosity ranged from 0.61 (ASH) to 0.73, and according to the M-ratio, all populations showed a significant reduction in effective population size, indicating a genetic bottleneck. Global FST was not statistically significant and attained the same values with and without excluding null alleles (ENA) correction (FST = 0.12). Bayesian analysis performed with STRUCTURE defined two genetic clusters among the five known populations, while the results of discriminant analysis of principal components (DAPC) identified three distinct groups. The average proportion of migrants was 22. In general, the gene flow was asymmetrical, with the biggest differences between immigration and emigration in Barzekoh and Asbehriseh. The Mantel test showed that there was no significant correlation between genetic distance (FST) and geographic distance in S. aucuparia. The best pathway for theoretical gene flow is located across the coast of the Caspian Sea and significant spatial autocorrelation was observed in only one population. In order to reduce the extinction risk of very small and scattered populations of S. aucuparia in the Hyrcanian forest, it is very important to establish and/or enhance the connectivity through habitat restoration or genetic exchange.

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“…This could be due to simple numeric effects -range shifts are easier when there are more individuals at the range edge to start with -or to genetic effects. That is, low abundances (and hence lower genetic diversity) could reduce local rates of adaptation at range edges, with gene flow from the abundant range center swamping any effect of local adaptation at the range edge that would facilitate range expansion when conditions change (Haldane, 1956; but see Kottler et al, 2021). Studies on invasive species have the potential to improve our understanding of the mechanisms that promote expansion from original range boundaries (e.g., the tolerance niche of the Asian Openbill Bird - Lei & Liu, 2021; and climate matching in the European Bee-eater - Stiels et al, 2021).…”

Section: Are Species Particularly Rare At Their Range Limits?mentioning

confidence: 99%

An introduction to predictive distribution modelling for conservation to encourage novel perspectives

MacPherson

Burgio

DeSaix

et al. 2021

Preprint

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Global change creates an urgent need to predict spatial responses of biota to support the conservation of sufficient habitat to maintain biodiversity. We present species distribution model theory and a synthesis of avian literature on approaches to collecting occurrence data, selecting explanatory variables and analytical processes currently in use to predict future distributions. We find that interpreting the validity of current predictive distributions is hindered by variation in spatio-temporal resolution of data sets that force hypothesis testing under the Grinnellian niche concept. Broadly, the capacity of species to shift their geographic ranges under land use and climate change is expected to be limited by both large scale (i.e., the physiological or fundamental niche) and small scale (i.e., the realized or tolerance niche) factors. We highlight the strengths and weaknesses of widely used explanatory variables and analytical approaches tailored to macrohabitat characteristics and the Grinnellian niche concept. This synthesis addresses if and how current approaches align with theory and makes recommendations for future directions to improve the accuracy of predictive distribution modelling.

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Draining the Swamping Hypothesis: Little Evidence that Gene Flow Reduces Fitness at Range Edges

Cited by 54 publications

References 88 publications

How is adaptive potential distributed within species ranges?

How is adaptive potential distributed within species ranges?

Genetic Diversity and Structure of Rear Edge Populations of Sorbus aucuparia (Rosaceae) in the Hyrcanian Forest

An introduction to predictive distribution modelling for conservation to encourage novel perspectives

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