Geographical Parthenogenesis: General Purpose Genotypes and Frozen Niche Variation

Abstract

Clonally reproducing all-female lineages of plants and animals are often more frequent at higher latitudes and altitudes, on islands, and in disturbed habitats. Attempts to explain this pattern, known as geographical parthenogenesis, generally treat the parthenogens as fugitive species that occupy marginal environments to escape competition with their sexual relatives. These ideas often fail to consider the early competitive interactions with immediate sexual ancestors, which shape alternative paths that newly formed clonal lineages might follow. Here we review the history and evidence for two hypotheses concerning the evolution of niche breadth in asexual species – the “general-purpose genotype” (GPG) and “frozen niche-variation” (FNV) models. The two models are often portrayed as mutually exclusive, respectively viewing clonal lineages as generalists versus specialists. Nonetheless, they are complex syllogisms that share common assumptions regarding the likely origins of clonal diversity and the strength of interclonal selection in shaping the ecological breadth of asexual populations. Both models find support in ecological and phylogeographic studies of a wide range of organisms, and sometimes generalist and specialist traits (e.g., physiological tolerance, microspatial preference, seasonal abundance, food habits, etc.) are found together in an asexual organism. Ultimately, persistent natural clones should be viewed as microspecies in ecological models that consider spatial and temporal heterogeneity rather than multi-locus genotypes in simplistic population models.

“It is not entirely clear, however, how forms whose genetic system must be very inflexible manage to become adapted to new environments when they do get transported to them: the apparent ecological versatility in space seems to be at variance with their lack of ecological versatility in time” (Original italics; MJD White, 1973, p. 748).