Evolution of low female fertility in plants: Pollen limitation, resource allocation and genetic load.

Some plant populations have low seed production, even when plenty of compatible pollen is supplied. Perennial species tend to have lower fertility than annuals, and outbreeders than inbreeders. The differences show up both in the proportion of flowers that produce fruits, and in the fraction of the ovules in those flowers that form seeds. Possible evolutionary explanations for low female fertility must be consistent with these observed patterns. One hypothesis is that low female fertility has evolved in long-lived organisms in which the costs of seed and fruit production are high - especially in outbreeders, in which there may be great costs to attracting enough pollinators to compete, in terms of male fertility, with other individuals in the population. Another possible explanation is mutational load, such that fitness is reduced by the mutations carried.