On the Relative Advantages of Cross-and Self-Fertilization

An optimality model based on the tradeoffs between seed set efficiency and outbreeding is presented that predicts under what conditions selfing should be favored over outcrossing.The model predicts that local density and distributional pattern, degree of environmental predictability, and adult and seed longevity are the independent variables that determine the shape of the marginal benefit curves for seed set and offspring heterogeneity.Some data supporting the model are presented derived from a study of species of the genus Leavemcorthia (Cruciferae).Flowering plants can reproduce in two basic ways.All species have the capacity to produce new individuals by vegetative means, such as runners, stolons, bulbs, conns, etc.Nearly all species in addition reproduce by seeds.The embryo contained in the seed is normally the result of the union of the egg cell with a gamete produced by a pollen grain from another plant and transported to the style by some pollinating agent.However, in a number of species, pollen from the same plant occasionally, or habitually, fertilizes the egg.Finally, seeds can also be produced apomictically without recourse to fertilization.From an evolutionary standpoint, the latter is better viewed not as a form of reproduction, but as a means of enlarging the parental genotype.The term vegetative propagation is therefore preferable, reserving the term reproduction for the formation of seed.The diversity of modes of reproduction encountered in flowering plants presents a challenging problem to the evolutionist.In the present paper I review the most accepted theory and indicate what I consider to be inconsistencies with the requirements of individual Darwinian selection.I then present an alternative model and some arguments in its support.Finally I apply the model to explain the different breeding systems encountered in the mustard genus Leavemcorthia.Reproduction produces a number of effects.It results in the production of new individuals possessing a fraction of the parental genes.The same effect results from vegetative propagation.However, reproduction by seeds also very effectively disperses the progeny in space and/or time because seeds have the potential of being transported for long distances and going dormant for variable lengths of time to avoid unfavorable periods in the environment.The selective advantages of dispersal and dormacy have been discussed elsewhere (Crocker,