Mercurialis annua is an ugly little European weed of disturbed habitats and roadside ditches. It is wind-pollinated and thus has inconspicuous, unattractive flowers. Yet it displays remarkable variation in its sexual system and ploidy across its distributional range. Particularly unusual for an annual plant is its expression of separate sexes throughout most of its European range.

Accordingly, there is a long history of studying M. annua to address questions concerning the evolution of combined versus separate sexes, mating systems, sex determination, sex chromosomes and sexual dimorphism. The species has also been a valuable model for testing hypotheses on the evolution of colonizing species, both during range expansions and in the context of metapopulations with recurrent population extinction and re-colonization.

In this seminar, I will outline the development of M. annua as a model for evolutionary and ecological genetic analysis, drawing on several research highlights relevant to the above topics. I will also summarise what we have been learning from a long-term experimental evolution study in which natural selection has driven transitions between dioecy and hermaphroditism in real time. This experiment has also generated new variation that now allows us to test long-standing hypotheses concerning sex-allocation and life-history evolution that have resisted convincing tests in the past.