Pollination ecotypes and the origin of plant species.

Ecological niche shifts are a key driver of phenotypic divergence and contribute to isolating barriers among lineages. For many groups of organisms, the history of these shifts and associated trait-environment correlations are well-documented at the macroevolutionary level. However, the processes that generate these patterns are initiated below the species level, often by the formation of ecotypes in contrasting environments. Here, I review the evidence in plants for 'pollination ecotypes' as microevolutionary responses to environmental gradients in pollinator availability. Pollinators are critical for population establishment and persistence in most species, thereby forming part of their fundamental niche. Novel floral trait combinations allow species to exploit particular pollination opportunities in local habitats and evolve primarily through sexual selection due to their effects on mating success. I examine selected case studies on the evolution of pollination ecotypes, including self-pollinating forms, and use these to illustrate challenging practical and conceptual issues. These issues include the paucity of reliable natural history data, the problem of implementing and interpreting reciprocal translocation experiments, and establishing criteria for when allopatric ecotypes should be considered species.