▪ Abstract  Examples of androdioecy, the coexistence of males and hermaphrodites, was unknown when the subject was last reviewed about two decades ago. Since then, several examples have been discovered in both plants and animals, and we are now in a position to reappraise theoretical work on the subject. Whereas early ideas were framed largely in terms of the invasion of males into hermaphroditic populations, all of the clearest examples of androdioecy now known appear to have evolved from dioecy. There are strong indications that this has occurred repeatedly as a result of the selection of self-fertile hermaphroditism for reproductive assurance during colonization. Male frequencies in these species are highly variable, self-fertilization in hermaphrodites is delayed, and mating opportunities appear to depend strongly on population density. Results from theoretical work on the evolution and maintenance of androdioecy in single populations and in metapopulations are summarized, and several case studies of androdioecious plants and animals are reviewed.

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.

Many species persist as a metapopulation under a balance between the local extinction of subpopulations or demes and their recolonization through dispersal from occupied patches. Here we review the growing body of literature dealing with the genetic consequences of such population turnover. We focus our attention principally on theoretical studies of a classical metapopulation with a 'finite-island' model of population structure, rather than on 'continent-island' models or 'source-sink' models. In particular, we concern ourselves with the subset of geographically subdivided population models in which it is assumed that all demes are liable to extinction from time to time and that all demes receive immigrants. Early studies of the genetic effects of population turnover focused on population differentiation, such as measured by F(ST). A key advantage of F(ST) over absolute measures of diversity is its relative independence of the mutation process, so that different genes in the same species may be compared. Another advantage is that F(ST) will usually equilibrate more quickly following perturbations than will absolute levels of diversity. However, because F(ST) is a ratio of between-population differentiation to total diversity, the genetic effects of metapopulation processes may be difficult to interpret in terms of F(ST) on its own, so that the analysis of absolute measures of diversity in addition is likely to be informative. While population turnover may either increase or decrease F(ST), depending on the mode of colonization, recurrent extinction and recolonization is expected always to reduce levels of both within-population and species-wide diversity (piS and piT, respectively). One corollary of this is that piS cannot be used as an unbiased estimate of the scaled mutation rate, theta, as it can, with some assumptions about the migration process, in species whose demes do not fluctuate in size. The reduction of piT in response to population turnover reflects shortened mean coalescent times, although the distribution of coalescence times under extinction colonization equilibrium is not yet known. Finally, we review current understanding of the effect of metapopulation dynamics on the effective population size.

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656I. 657II. 658III. 660IV. 661V. 663VI. 663VII. 664VIII. 664 665 References 665 Summary Baker's law refers to the tendency for species that establish on islands by long‐distance dispersal to show an increased capacity for self‐fertilization because of the advantage of self‐compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Baker's law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Baker's law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Baker's law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Baker's law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions.