Abstract
Incorporating species' eco‐evolutionary responses to human‐caused disturbances remains a challenge in marine management efforts. A prerequisite is knowledge of geographic structure and scale of genetic diversity and connectivity—the so‐called seascape genetic patterns. The Baltic Sea is an excellent model system for studies linking seascape genetics with effects of anthropogenic stress. However, seascape genetic patterns in this area are only described for a few species and are completely unknown for invertebrate herbivores, which constitute a critical part of the ecosystem. This information is crucial for sustainable management, particularly under future scenarios of rapid environmental change. Here, we investigate the population genetic structure among 31 locations throughout the Baltic Sea, of which 45% were located in marine protected areas, in one of the most important herbivores of this region, the isopod crustacean Idotea balthica, using an array of 33,774 genome‐wide SNP markers derived from 2b‐RAD sequencing. In addition, we generate a biophysical connectivity matrix for I. balthica from a combination of oceanographic current models and estimated life history traits. We find population structure on scales of hundreds of kilometers across the Baltic Sea, where genomic patterns in most cases closely match biophysical connectivity, indicating passive transport with oceanographic currents as an important mean of dispersal in this species. We also find a reduced genetic diversity in terms of heterozygosity along the main salinity gradient of the Baltic Sea, suggesting periods of low population size. Our results provide crucial information for the management of a key ecosystem species under expected changes in temperature and salinity following global climate change in a marine coastal area.
Highlights
How species manage to colonize a novel environment remains somewhat of a conundrum in evolutionary biology (Bock et al 2015)
We find strong population structure on small scales across the Baltic Sea, and that genomic patterns in most cases closely match biophysical connectivity, suggesting that current patterns are important for dispersal of this species
The asymmetrical distance values "d" (Jost's D), "gst" (Nei's GST) and "Nm" (Alcala et al 2014) were calculated among all population pairs using the divMigrate package in R (Sundqvist et al 2016), within which relative migration networks were generated, in order to examine the main directions of gene flow in well-connected areas
Summary
How species manage to colonize a novel environment remains somewhat of a conundrum in evolutionary biology (Bock et al 2015). Low population sizes at colonization fronts and multiple sequential founder effects should increase the strength of genetic drift, and prevent populations from adapting to novel environmental conditions (Brandvain & Wright 2016). One pattern that might arise from this process of multiple founder effects along a recently colonized environmental gradient would be a rapid evolution of population structure (Excoffier et al 2009). This would be visible in loci under divergent environmental selection along the gradient, and in neutral loci due to rapid drift during bottleneck events and reduced gene flow across locally adapted populations (Ibrahim et al 1996). Once the range expansion is concluded, the population structure might be diffused again due to population growth and increased gene flow (Hagen et al 2015) – unless there are mechanisms to prevent this from happening, such as assortative mating combined with local adaptation, or if there are strong dispersal barriers in the newly colonized area (Lee 2011)
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