1 Strong spatial heterogeneity in male frequencies and allocation to male and female function was found within and between androdioecious populations of the wind-pollinated ruderal Mercurialis annua in south-western Spain.
2 Sex determination is largely genetic and the constancy from year to year of the frequency of males in small quadrats suggests that seed and pollen dispersal are very limited.
3 Male frequency correlated significantly with stand density in two populations in the field and varied nonrandomly between densities in a glasshouse density experiment, strongly suggesting an environmental component to sex determination.
4 In the density experiment, stand density had a strong negative effect on relative allocation to reproduction by both males and cosexes (monoecious individuals). At higher densities, cosexes were quantitatively more female than at lower densities. Also, larger males and cosexes produced more pollen per vegetative biomass than did smaller ones, while allocation to seed production by cosexes increased only proportionally with plant biomass, so that cosexual gender shifted consistently towards greater maleness in larger plants.
5 Males produced between about five and 10 times as much pollen as cosexes, indicating that cosex sex allocation is strongly female-biased. This, and inflorescence architecture, suggest that female-biased cosexuality in M. annua has evolved as a response to selection for facultative self-fertilization in a wind-pollinated species.
6 In M. annua, androdioecy appears to be maintained by a balance between selection for reproductive assurance during colonization, which favours self-fertile cosexuality with female-biased sex allocation, and frequency-dependent selection within populations, favouring increased male allocation and thus allowing the invasion and spread of males.