Detection of barriers to dispersal is masked by long lifespans and large population sizes

Abstract

Population genetic analyses of species inhabiting fragmented landscapes are essential tools for conservation. Occasionally, analyses of fragmented populations find no evidence of isolation, even though a barrier to dispersal is apparent. In some cases, not enough time may have passed to observe divergence due to genetic drift, a problem particularly relevant for long‐lived species with overlapping generations. Failing to consider this quality during population structure analyses could result in incorrect conclusions about the impact of fragmentation on the species. We designed a model to explore how lifespan and population size influence perceived population structure of isolated populations over time. This iterative model tracked how simulated populations of variable lifespan and population size were affected by drift alone, using a freshwater mussel, Quadrula quadrula (mapleleaf), as a model system. In addition to exhibiting dramatic lifespan variability among species, mussels are also highly imperiled and exhibit fragmentation by dams throughout the range of many species. Results indicated that, unless population size was small (<50 individuals) or lifespan short (<22 years), observing genetic divergence among populations was unlikely. Even if wild populations are isolated, observing population structure in long‐lived mussels from modern damming practices is unlikely because it takes longer for population structure to develop in these species than most North American dams have existed. Larger population sizes and longer lifespans increase the time needed for significant divergence to occur. This study helps illuminate the factors that influence genetic responses by populations to isolation and provides a useful model for conservation‐oriented research.

Population structure, genetic diversity, and colonization history of the eastern pondmussel, Sagittunio nasutus, in the Great Lakes drainage

“…We believe this more detailed view of geographical structure in the Great Lakes region will be more informative when developing conservation and restoration plans for S. nasutus . It is possible that the genetic structure found in the microsatellite analyses, but not in the mtDNA analyses performed by Scott et al (), is the result of recent anthropogenically induced population declines (Hoffman, Willoughby, Swanson, Pangle, & Zanatta, ). We also recommend that in larger genetically diverse populations (i.e.…”

High genetic diversity and low differentiation in North American Margaritifera margaritifera (Bivalvia: Unionida: Margaritiferidae)

“…The long life spans (> 100 years; Bauer, 1992) and generation times (at least 20 years; Bauer, 1987) of M. margaritifera mean that rates of evolutionary change (i.e. genetic drift and natural selection) will be very slow and there will be a long lag period before detectable changes in genetic diversity and structure can be observed even if populations become isolated (Hoffman et al, 2017). It is possible that many of the collection locations sampled in this study are isolated, but do not yet show genetic isolation because the amount of time they have been isolated (c. < 10 000 years since post-glacial colonization) is not sufficient for genetic effects (Hoffman et al, 2017) to be detected using microsatellite markers.…”

“…However, some populations (especially in the southern part of the North American distribution) are showing signs of declines (Strayer & Jirka, 1997;Nedeau et al, 2000;Gouvernement du Québec, 2010;Connecticut DEP, 2013). Due to the longevity of the species, long lagtimes between habitat degradation such as loss of connectivity and visible consequences on a population genetic level are to be expected (Hoffman et al, 2017).…”

Effects of dams and their environmental impacts on the genetic diversity and connectivity of freshwater mussel populations in Poyang Lake Basin, China

“…Freshwater mussel populations have been devastated by dam construction in rivers (Vaughn & Taylor, ). Long lag‐times between habitat degradation (leading to loss of connectivity) and evidence of impacts on markers of population genetic diversity are to be expected in long‐lived species (Hoffman, Willoughby, Swanson, Pangle, & Zanatta, ; Zanatta et al, ). Unionoid dispersal is inhibited by habitat fragmentation caused by dams, which can alter mussel assemblages by preventing longitudinal migration of host fishes and associated glochidia (Baxter, ; Watters, ).…”