Partial clonality and the speed of range expansion along an environmental gradient

Reproductive mode interacts in important ways with patterns of spatial adaptation, such as expansion of species’ ranges along environmental gradients. Partial clonality (both asexual/clonal and sexual reproduction) characterizes a broad range of taxa, including important species that structure many terrestrial and marine ecosystems as well as many pathogens, pests, and invasives. We explore the interplay of invasion and local adaptation under partial clonality using an individual-based model, with space modeled as a unidimensional landscape of grid cells. An environmental gradient exists along the direction of expansion, such that the optimal phenotype changes in this direction. We investigate how the fraction of clonal reproduction (rc) and the association between random components of parent and clonal offspring phenotype (ρ) affect the speed of range expansion. We see slower expansion as amount of clonality (rc or ρ) increases, with a sharp drop at complete clonality (rc, ρ = 1), indicating that only a small amount of sexual reproduction suffices for partially clonal populations to show very similar behavior to purely sexual populations—in clear contrast to what has been observed for spatial adaptation across two distinct demes with differing phenotypic optima. Expansion lag load (fitness differences between core and leading-edge sub-populations) is greatest at intermediate environmental gradient steepness, with both gradual and very steep change decreasing this lag load. Thus, clonality may hinder invasion by partially clonal species across an environmental gradient; the reproductive assurance benefits of asexual reproduction may be a better explanation for clonal species expansion in such cases.