No abstract

Hermaphroditic individuals can produce both selfed and outcrossed progeny, termed mixed mating. General theory predicts that mixed-mating populations should evolve quickly toward high rates of selfing, driven by rapid purging of genetic load and loss of inbreeding depression (ID), but the substantial number of mixed-mating species observed in nature calls this prediction into question. Lower average ID reported for selfing than for outcrossing populations is consistent with purging and suggests that mixed-mating taxa in evolutionary transition will have intermediate ID. We compared the magnitude of ID from published estimates for highly selfing (r > 0.8), mixed-mating (0.2 ≤ r ≥ 0.8), and highly outcrossing (r < 0.2) plant populations across 58 species. We found that mixed-mating and outcrossing taxa have equally high average lifetime ID (δ = 0.58 and 0.54, respectively) and similar ID at each of four life-cycle stages. These results are not consistent with evolution toward selfing in most mixed-mating taxa. We suggest that prevention of purging by selective interference could explain stable mixed mating in many natural populations. We identify critical gaps in the empirical data on ID and outline key approaches to filling them.

Summary• Reduced allocation to structures for pollinator attraction is predicted in selfing species. We explored the association between outcrossing and floral display in a broad sample of angiosperms. We used the demonstrated relationship to test for bias against selfing species in the outcrossing rate distribution, the shape of which has relevance for the stability of mixed mating.• Relationships between outcrossing rate, flower size, flower number and floral display, measured as the product of flower size and number, were examined using phylogenetically independent contrasts. The distribution of floral displays among species in the outcrossing rate database was compared with that of a random sample of the same flora.• The outcrossing rate was positively associated with the product of flower size and number; individually, components of display were less strongly related to outcrossing. Compared with a random sample, species in the outcrossing rate database showed a deficit of small floral display sizes.• We found broad support for reduced allocation to attraction in selfing species. We suggest that covariation between mating systems and total allocation to attraction can explain the deviation from expected trade-offs between flower size and number. Our results suggest a bias against estimating outcrossing rates in the lower half of the distribution, but not specifically against highly selfing species.

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.

Cleistogamous species present strong evidence for the stability of mixed mating, but are generally not considered in this context. Individuals of cleistogamous species produce both obligately selfing cleistogamous flowers (CL) and potentially outcrossed chasmogamous flowers (CH) with distinct morphologies. Greater energetic economy and reliability of CL relative to CH suggest that forces that maintain selection for outcrossing may be stronger in these species than in mixed maters with monomorphic flowers. We reviewed data from 60 studies of cleistogamous species to evaluate proposed explanations for the evolutionary stability of mixed cleistogamous and chasmogamous reproduction and to quantify the magnitude of selection necessary to account for the maintenance of CH. We found circumstantial support for existing hypotheses for the stability of cleistogamy, and that forces that maintain CH must account for a 15-342% advantage of reproduction via CL. We suggest that heterosis and the effects of mass action pollination should be considered.

Latitudinal gradients in biotic interactions have been suggested as causes of global patterns of biodiversity and phenotypic variation. Plant biologists have long speculated that outcrossing mating systems are more common at low than high latitudes owing to a greater predictability of plant-pollinator interactions in the tropics; however, these ideas have not previously been tested. Here, we present the first global biogeographic analysis of plant mating systems based on 624 published studies from 492 taxa. We found a weak decline in outcrossing rate towards higher latitudes and among some biomes, but no biogeographic patterns in the frequency of self-incompatibility. Incorporating life history and growth form into biogeographic analyses reduced or eliminated the importance of latitude and biome in predicting outcrossing or self-incompatibility. Our results suggest that biogeographic patterns in mating system are more likely a reflection of the frequency of life forms across latitudes rather than the strength of plant-pollinator interactions.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology. SUMMARY(1) The effects of seed size and microsite characteristics on seedling emergence were examined in two woodland and two old-field habitats of Prunella vulgaris. Seeds of known size were sown in the field and subsequent seedling emergence was monitored. Characteristics of the microsites in which seedlings emerged were also noted.(2) Both seed size and microsite characteristics affected seedling emergence. In general, large seeds had a higher percentage emergence. Litter and herbaceous cover inhibited seedling emergence in woodland habitats and herbaceous cover inhibited emergence in old-field habitats. In all habitats, larger seeds had less stringent requirements.(3) Within a habitat, the magnitude of the effect of seed size on seedling emergence may be influenced by the mean size of seeds produced and the abundance of microsites favourable for germination.(4) Seed size is a phenotypically plastic characteristic in P. vulgaris. A reciprocaltransplant experiment demonstrated that, in a habitat with a high herbaceous cover, plants produced small seeds regardless of their origin. Larger seeds were produced by both transplanted and native individuals in a habitat with a low herbaceous cover.

Seed size has significant demographic consequences in Prunella vulgaris. Measurements of the effects of seed size were obtained by sowing seeds of known size at four field sites and recording seedling emergence, survival, and size. The intensity of selection on seed size was calculated from these data. Large seeds had a significantly greater probability of emergence at most study sites in each of 2 yr. The effects of seed size were expressed most strongly during the early part of the life cycle, between sowing and emergence, and much less so in later phases of the life cycle. In contrast to a prediction based on the greater relative frequency of large—seeded species in later successional habitats, the effect of seed size on percent seedling emergence did not differ significantly between an old—field and a woodland habitat. However, it is likely that the intensity of natural selection on seed size is greater in an old—field than in a woodland population because the natural distributions of seed size in old—field populations include more small seeds than do those in woodland populations. An analysis of the costs and benefits of producing large and small seeds revealed that in addition to selection favoring large seeds, there was selection favoring individuals that produced large seedlings at three of the four study sites. At the fourth site, there was no selection favoring larger seeds or parents that produced larger seeds. Substantial capacity for phenotypic plasticity in seed size suggests that there may be little opportunity for an evolutionary response despite strong selection favoring large seeds.