A hypothesis for the evolution of androdioecy: the joint influence of reproductive assurance and local mate competition in a metapopulation

In a subdivided population with recurrent local extinction and re-colonisation, competition amongst related pollen or sperm to fertilise ovules or eggs ('local mate competition') is expected to select for female-biased sex allocation. Population turnover should also select against unisexuality in favour of self-fertile cosexuality, because males and females are unable to establish new populations on their own ('Baker's Law'). Here I argue that androdioecy, a rare breeding system in which males co-occur with hermaphrodites, may evolve in a metapopulation under the joint action of local mate competition and Baker's Law if rates of self-fertilisation decrease with increasing population size. The hypothesis makes several predictions regarding patterns of life-history and sex allocation that are borne out by recent observations of androdioecious species in several unrelated lineages of plants and animals.