The influence of genetic variation on invasion success has captivated researchers since the start of the field of invasion genetics 50 years ago. We review the history of work on this question and conclude that genetic variation-as surveyed with molecular markers-appears to shape invasion rarely. Instead, there is a significant disconnect between marker assays and ecologically relevant genetic variation in introductions. We argue that the potential for adaptation to facilitate invasion will be shaped by the details of genotypes affecting phenotypes, and we highlight three areas in which we see opportunities to make powerful new insights. (i) The genetic architecture of adaptive variation. Traits shaped by large-effect alleles may be strongly impacted by founder events yet more likely to respond to selection when genetic drift is strong. Large-effect loci may be especially relevant for traits involved in biotic interactions. (ii) Cryptic genetic variation exposed during invasion. Introductions have strong potential to uncover masked variation due to alterations in genetic and ecological environments. (iii) Genetic interactions during admixture of multiple source populations. As divergence among sources increases, positive followed by increasingly negative effects of admixture should be expected. Although generally hypothesized to be beneficial during invasion, admixture is most often reported among sources of intermediate divergence, supporting the possibility that incompatibilities among divergent source populations might be limiting their introgression. Finally, we note that these details of invasion genetics can be coupled with comparative demographic analyses to link genetic changes to the evolution of invasiveness itself.

Species introductions often bring together genetically divergent source populations, resulting in genetic admixture. This geographic reshuffling of diversity has the potential to generate favorable new genetic combinations, facilitating the establishment and invasive spread of introduced populations. Observational support for the superior performance of admixed introductions has been mixed, however, and the broad importance of admixture to invasion questioned. Under most underlying mechanisms, admixture’s benefits should be expected to increase with greater divergence among and lower genetic diversity within source populations, though these effects have not been quantified in invaders. We experimentally crossed source populations differing in divergence in the invasive plant Centaurea solstitialis. Crosses resulted in many positive (heterotic) interactions, but fitness benefits declined and were ultimately negative at high source divergence, with patterns suggesting cyto-nuclear epistasis. We explored the literature to assess whether such negative epistatic interactions might be impeding admixture at high source population divergence. Admixed introductions reported for plants came from sources with a wide range of genetic variation, but were disproportionately absent where there was high genetic divergence among native populations. We conclude that while admixture is common in species introductions and often happens under conditions expected to be beneficial to invaders, these conditions may be constrained by predictable negative genetic interactions, potentially explaining conflicting evidence for admixture’s benefits to invasion.

Climate change may soon threaten much of global biodiversity, especially if species cannot adapt to changing climatic conditions quickly enough. A critical question is how quickly climatic niches change, and if this speed is sufficient to prevent extinction as climates warm. Here, we address this question in the grass family (Poaceae). Grasses are fundamental to one of Earth's most widespread biomes (grasslands), and provide roughly half of all calories consumed by humans (including wheat, rice, corn and sorghum). We estimate rates of climatic niche change in 236 species and compare these with rates of projected climate change by 2070. Our results show that projected climate change is consistently faster than rates of niche change in grasses, typically by more than 5000-fold for temperature-related variables. Although these results do not show directly what will happen under global warming, they have troubling implications for a major biome and for human food resources.

Invasive species have become a pervasive threat on every continent and across a broad array of environments. Several traits predicted to promote invasion success, such as small seed size, rapid vegetative growth and short time to reproduction, are correlated with smaller genome sizes in a number of systems. To understand the influence of genome size on plant invasion dynamics, we compared genome sizes and traits in Centaurea solstitialis (YST) genotypes from the Californian invasion to those from their native source region in Spain. We conducted a common garden experiment and genome size survey to ask: (1) Is the invasion associated with genome size reduction? (2) To what extent can differences in genome size explain previously observed increases in plant size and reproduction in YST invasions? (3) Finally, we tested for expected evolutionary patterns in genome size across populations, including evidence of selection favouring reduced genome sizes at higher elevations, and evidence of stochastic processes leading to increases in genome sizes where effective population sizes are smaller. We found a reduction in corrected genome size in the invaded range, as well as significant interaction effects of range x elevation on genome size, and range x genome size on flowering time variation. Specifically, larger genomes tended to flower later and genome size decreased with increasing elevation in the invasion only. These emergent relationships in invading YST suggest potential selection for smaller genomes following introduction of YST to its invaded range. We also found a significant effect of measurement date on genome size estimation by flow cytometry, and this effect was more pronounced among native range genotypes.

Yellow starthistle (Centaurea solstitialis) is a highly invasive species that has been a model system for the potential contribution of evolution to invader traits. Here, we report the construction of a chromosome-scale reference genome for C. solstitialis using a combination of PacBio HiFi and Dovetail Omni-C technologies, and functional gene annotation with RNAseq. We validate the reference genome using a restriction site-associated DNA (RAD)-based genetic map from an F2 mapping population. We find that syntenic comparisons to other taxa in the Asteraceae reveal a chromosomal fusion in the lineage of C. solstitialis, and widespread fission in globe artichoke (Cynara cardunculus). Using a QTL analysis from the mapping population (derived from a cross between native and invading parents) we identify 13 QTL underpinning size traits that are associated with adaptation in the invaded range, including a putative large-scale chromosomal inversion that has a pleiotropic and overdominant effect on key invader traits.