Baker's Law Revisited: Reproductive Assurance in a Metapopulation.

Baker's Law states that it is more likely for self-compatible than for self-incompatible individuals to establish sexually reproducing colonies after long-distance dispersal, because only the former can do so with a single individual. This hypothesis, proposed by H. G. Baker 40 years ago is based largely on the observation that self-compatibility is particularly frequent among colonists of oceanic islands. Here we argue that the principle of Baker's Law applies equally in the context of a metapopulation in which frequent local extinction is balanced by recolonization of sites by seed dispersal: metapopulation dynamics will select for an ability to self-fertilize. We review several studies that support this hypothesis and present a metapopulation model in which the seed productivity required by obligate outcrossers for their maintenance in a metapopulation is compared with that of selfers. Our model also estimates the reduction in the advantage of reproductive assurance to selfers as a result of perenniality and seed dormancy. In general, selection for reproductive assurance is greatest when the colony occupancy rate, p, is low and is much reduced when p approaches its maximum. This provides an explanation for the observation that many highly successful colonizers, in which p is often high, are self-incompatible. The basic model we present also lends itself to comparisons of metapopulation effects between unisexuality and cosexuality and between different modes of self-incompatibility.