Expansion history and environmental suitability shape effective population size in a plant invasion

Abstract

The margins of an expanding range are predicted to be challenging environments for adaptation. Marginal populations should often experience low effective population sizes (Ne) where genetic drift is high due to demographic expansion and/or census population size is low due to unfavourable environmental conditions. Nevertheless, invasive species demonstrate increasing evidence of rapid evolution and potential adaptation to novel environments encountered during colonization, calling into question whether significant reductions in Ne are realized during range expansions in nature. Here we report one of the first empirical tests of the joint effects of expansion dynamics and environment on effective population size variation during invasive range expansion. We estimate contemporary values of Ne using rates of linkage disequilibrium among genome‐wide markers within introduced populations of the highly invasive plant Centaurea solstitialis (yellow starthistle) in North America (California, USA), and within native Eurasian populations. As predicted, we find that Ne within the invaded range is positively correlated with both expansion history (time since founding) and habitat quality (abiotic climate). History and climate had independent additive effects with similar effect sizes, indicating an important role for both factors in this invasion. These results support theoretical expectations for the population genetics of range expansion, though whether these processes can ultimately arrest the spread of an invasive species remains an unanswered question.

Deep-Time Demographic Inference Suggests Ecological Release as Driver of Neoavian Adaptive Radiation

“…It is worth recalling that Ne is a measure that is based on genetic diversity that may differ significantly from census population size. Measurements of Ne may also be affected by population structure [144][145][146][147][148], selection [149,150], range expansion [151,152], introgressive hybridization, and intralocus recombination [131], even in the context of the same meaning of Ne and when measured by the same method. The scale at which these effects are measured is also relevant.…”

“…However, Ne is expected to increase with range expansion, as has been observed for many invasive species and as would be expected in a scenario of range expansion following the Chicxulub meteor impact. However, there may be a significant difference between the reduced Ne of marginal populations that are not well adapted to local conditions (which have been the conservation focus of most studies of existing small populations) and the increased Ne of the metapopulation, depending on the level of population structure and gene flow [152]. Similarly, the ecological theory of adaptive radiation predicts that selection will vary through time, beginning with a reduction in normalizing selection and flattening of the selection landscape, followed by local increases in selection driving speciation either in response to local conditions or due to intraspecific competition [56].…”

“…Similarly, the ecological theory of adaptive radiation predicts that selection will vary through time, beginning with a reduction in normalizing selection and flattening of the selection landscape, followed by local increases in selection driving speciation either in response to local conditions or due to intraspecific competition [56]. Selection may vary considerably among loci used in any particular genetic analysis of Ne [152], and again among sub-populations (whereas indels are assumed to be neutral markers). Most contemporary studies amplify these disparities, since they have overwhelmingly focused on demic populations of conservation concern.…”

Climate‐induced range shifts shaped the present and threaten the future genetic variability of a marine brown alga in the Northwest Pacific

“…Despite the short evolutionary time since the post‐LGM expansion, potential adaptation to novel environments (habitat quality and stability) in the centre‐northern range limit of S . thunbergii (Braasch et al, 2019 ) could drive rapid evolutionary sweeps, but the genetic markers used in this study are not expected to be affected by selection. The most plausible explanation to the observed patterns of distinct genetic lineages is supported by the unique alleles in the southern group coupled with the potential genetic loss in such source population, a hypothesis that is in agreement with our modelling results of past distributional shifts.…”

The different fates of two Asian horseshoe crab species with different dispersal abilities

“…Understanding an organism's evolutionary responses to such range shifts is a critical prerequisite for the conservation of coastal marine biodiversity in this era of anthropogenic climate change (Chuang & Peterson, 2016 ; Miller et al, 2020 ; Nadeau & Urban, 2019 ). To evaluate species viability during environmental changes, most previous studies have estimated the evolutionary potential (e.g., genetic variation) of expanding populations by comparing them with populations in the core range (Berthouly‐Salazar et al, 2013 ; Braasch et al, 2019 ; Chen et al, 2018 ; Pierce et al, 2000 ; Robalo et al, 2020 ; Yang et al, 2016 ). However, many environmental fluctuations result in original habitats becoming uninhabitable, rendering core habitats difficult to identify, or individuals in core habitats undergo major demographic events (e.g., bottlenecks) which hamper a meaningful estimation of the evolutionary potential of expanding and receding populations.…”