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Inbreeding is a potent evolutionary force shaping the distribution of genetic variation within and among populations of plants and animals. Yet, our understanding of the forces shaping the expression and evolution of nonrandom mating in general, and inbreeding in particular, remains remarkably incomplete. Most research on plant mating systems focuses on self‐fertilization and its consequences for automatic selection, inbreeding depression, purging, and reproductive assurance, whereas studies of animal mating systems have often assumed that inbreeding is rare, and that natural selection favors traits that promote outbreeding. Given that many sessile and sedentary marine invertebrates and marine macroalgae share key life history features with seed plants (e.g., low mobility, modular construction, and the release of gametes into the environment), their mating systems may be similar. Here, we show that published estimates of inbreeding coefficients (F IS) for sessile and sedentary marine organisms are similar and at least as high as noted in terrestrial seed plants. We also found that variation in F IS within invertebrates is related to the potential to self‐fertilize, disperse, and choose mates. The similarity of F IS for these organismal groups suggests that inbreeding could play a larger role in the evolution of sessile and sedentary marine organisms than is currently recognized. Specifically, associations between traits of marine invertebrates and F IS suggest that inbreeding could drive evolutionary transitions between hermaphroditism and separate sexes, direct development and multiphasic life cycles, and external and internal fertilization.

Many benthic marine invertebrates resemble plants in being modular and either sessile or sedentary, and by relying on an external vector to disperse their gametes. These shared features, along with recent evidence of inbreeding in these taxa, suggest that theory and practice bearing on the evolutionary costs and benefits of inbreeding for plants could advance our understanding of the ecology and evolution of invertebrate animals. We describe how the theory for the evolution of inbreeding and outbreeding could apply to benthic invertebrates, identify and compare techniques used to quantify inbreeding in plants and animals, translate relevant botanical concepts and empirical patterns to their zoological equivalents, and articulate predictions for how inbreeding might be associated with major axes of variation in sessile and sedentary marine invertebrates. The theory of inbreeding and outbreeding provides critical insight into major patterns of life-history variation in plants and holds similar promise as a complementary perspective on the evolution of reproductive traits, lifespan, ecological strategies, and dispersal in marine invertebrates. Extending what we have learned from plants to marine invertebrates promises to broaden the general study of mating systems.

Population introduction is frequently attempted to promote rare plant conservation but there is little evidence to evaluate its effectiveness. We measured size structure, survival, growth, and recruitment in three 20-year-old introduced populations of the endangered shrub Conradina glabra, an endemic in upland longleaf pine habitat in the Florida Panhandle, and compared these with measurements from three natural populations. We also quantified inbreeding depression from hand pollinations in two populations and incorporated its consequences for seed production and germination into demographic projections. Individuals in introduced populations, which had been subjected to recent prescribed fire, grew nearly three times faster and produced more than twice as many recruits per individual, but suffered 3.7% greater mortality than individuals in wild populations growing in habitat where fire was suppressed. Demographic matrix projections based on our data predict that introduced populations would grow...

Because establishing a new population often depends critically on finding mates, individuals capable of uniparental reproduction may have a colonization advantage. Accordingly, there should be an over-representation of colonizing species in which individuals can reproduce without a mate, particularly in isolated locales such as oceanic islands. Despite the intuitive appeal of this colonization filter hypothesis (known as Baker's law), more than six decades of analyses have yielded mixed findings. We assembled a dataset of island and mainland plant breeding systems, focusing on the presence or absence of self-incompatibility. Because this trait enforces outcrossing and is unlikely to re-evolve on short timescales if it is lost, breeding system is especially likely to reflect the colonization filter. We found significantly more self-compatible species on islands than mainlands across a sample of > 1500 species from three widely distributed flowering plant families (Asteraceae, Brassicaceae and Solanaceae). Overall, 66% of island species were self-compatible, compared with 41% of mainland species. Our results demonstrate that the presence or absence of self-incompatibility has strong explanatory power for plant geographical patterns. Island floras around the world thus reflect the role of a key reproductive trait in filtering potential colonizing species in these three plant families.

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.

The production of both potentially outcrossed (chasmogamous) and obligately self-fertilized (cleistogamous) flowers presents a clear exception to the prediction that the only evolutionarily stable mating systems are complete selfing and complete outcrossing. Although cleistogamy has evolved repeatedly, the reason for its stability is not known for any species. We tested the hypothesis that the production of cleistogamous and chasmogamous flowers by a perennial violet constitutes adaptive phenotypic plasticity. We manipulated the season of flowering for each flower type and determined fruit set and the germination percentage of seeds produced by cleistogamous and chasmogamous flowers to test the hypothesis that adaptive plastic response to seasonal environmental variation makes mixed mating stable. Cleistogamous flowers had greater fruit set in all seasons and produced seeds with germination percentages as great as or greater than those from chasmogamous flowers. The consistent advantage of cleistogamous flowers is clearly not consistent with a role of adaptive plastic response to seasonal variation. The biomass cost of seed production by chasmogamous flowers was nearly three times that for cleistogamous flowers. Explaining why chasmogamous flower have not been eliminated by natural selection requires that this difference be balanced by an advantage to chasmogamous flowers that has not yet been identified.

Cleistogamous species constitute strong evidence for the stability of mixed selfing and outcrossing but have been largely overlooked as theoretical and empirical systems for studying the forces thought to influence matingsystem evolution. Inbreeding depression is expected to play a major role in the maintenance of outcrossing, yet reliable estimates of inbreeding depression for cleistogamous species are virtually nonexistent. We estimated both population- and family-level inbreeding depression for all life cycle stages for a population of the cleistogamous perennial Viola septemloba. Our estimate of cumulative inbreeding depression of 43% is substantially greater than two previous estimates for cleistogamous species, but it is not sufficient by itself to explain the maintenance of potentially outcrossed chasmogamous flowers.

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.

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.

Many invasive species can respond opportunistically to favorable growing conditions. In a previous work, we found that invasive species in the family Commelinaceae were more oppor- tunistic than their noninvasive congeners and could therefore outperform noninvasive relatives in an environment with abundant resources and no competition. Contrary to the expectation that superior performance under favorable conditions comes at the cost of reduced performance under stressful conditions, invasive species did not perform more poorly relative to noninvasive congeners under any conditions we examined. Here we expand our search for potential costs of opportunism in invasive species to additional environmental conditions in which invasive taxa have been shown or predicted to perform poorly. We grew four invasive and four noninvasive species in environments consisting of all possible combinations of high and low soil resources and presence and absence of clipping (removal of aboveground biomass). We also fed leaves of each species to a generalist herbivore to assess resistance to herbivory. We found that the advantage of invasive species is reduced but not eliminated by low soil resources and clipping. At low soil resources, invasive species produced softer leaves than noninvasive species and might therefore be less resistant to generalist herbivory than noninvasive species, although a direct com- parison of resistance in a no-choice bioassay revealed no difference. The invasive species out- performed noninvasive species only under the most favorable conditions, and the noninvasive species did not outperform the invasive species in any environment.

Evidence supporting an association between phenotypic plasticity and invasiveness across a range of plant taxa is based primarily on comparisons between invasive species and native species whose potential invasiveness is typically unknown. Comparison of invasive and non-invasive exotic species would provide a better test of whether plasticity promotes invasion. Such comparisons should distinguish between adaptive and non-adaptive plasticity because they have different consequences for invasiveness. Adaptive plasticity is expected to promote the invasion of multiple habitats, but non-adaptive plasticity may reflect specialization for invading more favorable habitats only. We grew four invasive and four non-invasive species of the Commelinaceae with and without competitors and compared their putatively adaptive plasticity of three traits related to competitive ability and non-adaptive plasticity in performance. The invasive species grew significantly more than the non-invasive species only in the non-competitive environment. The invasive species had greater plasticity of performance (total biomass) in response to competition than non-invasives, but there was no consistent difference in the plasticities of the traits related to competitive ability. These results are consistent with specialization of these invasive taxa for invading the more productive non-competitive environment rather than a superior ability to invade both competitive and non-competitive environments. A comprehensive understanding of the relationship between plasticity and invasiveness will require many more comparisons of the plasticity of invasive and non-invasive taxa in a range of traits in response to a variety of environments.

Estimates of the form and magnitude of natural selection based on phenotypic relationships between traits and fitness measures can be biased when environmental factors influence both relative fitness and phenotypic trait values. I quantified genetic variances and covariances, and estimated linear and quadratic selection coefficients, for seven traits of an annual plant grown in the field. For replicates of 50 paternal half-sib families, coefficients of selection were calculated both for individual phenotypic values of the traits and for half-sib family mean values. The potential for evolutionary response was supported by significant heritability and phenotypic directional selection for several traits but contradicted by the absence of significant genetic variation for fitness estimates and evidence of bias in phenotypic selection coefficients due to environmental covariance for at least two of the traits analysed. Only studies of a much wider range of organisms and traits will reveal the frequency and extent of such bias.

Asymmetry along the longitudinal axis of a plant can be created by heterophylly, the production of leaves of different size or shape by an individual. Heterophylly is a widespread phenomenon ranging in degree from subtle variation in leaf size to striking differences in the size and shape of leaves produced by a single plant. Because leaf size and shape influence critical leaf functions, heterophylly may have significant consequences for plant fitness. To date, very few studies have directly addressed whether differences in leaf phenotypes produced by a single plant do result in differences in their functional properties or how any such differences affect individual survival and reproduction. I suggest some general approaches to determining the ecological and evolutionary significance of heterophylly and illustrate these approaches with two case studies from my work. One study demonstrates a functional difference between alternate leaf shapes in a heterophyllic violet. The other illustrates the use of phenotypic selection analysis to determine whether the optimum leaf phenotype differs among environments encountered by individual plants. Following the case studies, I review other published work relevant to the ecological and evolutionary significance of heterophylly, much of which has not been conducted explicitly in that context. In sum, this work suggests that the significance of longitudinal asymmetry created by heterophylly is case specific. Some instances of heterophylly may constitute adaptation to fine-grained environmental variation, and others may simply reflect inevitable effects of environmental factors on leaf development that have ultimately neutral or negative effects on individual fitness. The study of heterophylly provides common ground for physiologists, developmental biologists, ecologists, and evolutionary biologists. Recognition of this common interest and increased interaction among groups studying heterophylly would promote a more systematic and synthetic picture of the biological bases and significance of this widespread phenomenon.

Variation among modules of a single genet could provide a means of adaptation to environmental heterogeneity. Two mechanisms that can give rise to such variation are programmed developmental change and phenotypic plasticity. I quantified the relative roles of these two mechanisms in causing within-individual variation in six leaf traits of an annual plant. Under controlled temperatures, morphological, anatomical, and physiological traits of leaves produced by the same individual differed as a function of both the node at which they were produced and the temperature they experienced during development. Temperature, node, and interactions between them all contributed significantly to the pattern of within-individual variation in leaf traits, although the relative contributions of programmed developmental change and phenotypic plasticity differed for different traits. I hypothesize that these two mechanisms for generating within-individual variation in module phenotype are favored by different patterns of environmental heterogeneity; when the sequence of environments encountered by modules of a single individual is predictable, programmed developmental change may be favored, and phenotypic plasticity may be favored when the sequence of environments is irregular with respect to individual ontogeny and therefore not predictable.

Recent studies of evolution in heterogeneous environments have concentrated on the role of coarse-grained environmental variation. Here I explore the potential for a modular organism to adapt to fine-grained environmental variation through within-individual variation among modules. I describe the pattern of variation among leaves of single individuals and report results of initial analyses of genetic variation for within-individual variability in leaf traits and of genetic correlations that could influence the rate of further evolution of within-individual variation of these traits. Plants from 24 paternal half-sib families were raised in growth chambers, and five traits were measured for two leaves produced by each plant. Four of the five traits differed significantly between sampling times. Genetic analyses revealed significant additive genetic variation for within-individual variation in several traits. Estimates of family mean correlations between traits expressed at different times suggest few relationships that would be expected to impede response to selection for changes in the pattern of within-individual variation in leaf traits. These results support the possibility that within-individual variation could evolve as an adaptive response to fine-grained environmental variation and suggest a need for further investigation to improve understanding of evolution in heterogeneous environments.

Although population density is believed to be an important factor influencing evolutionary processes, surprisingly few studies have documented the existence or nature of density-dependent selection. We quantified the effects of density on directional selection on seed mass (the mass of a sown seed) and emergence time in the greenhouse and field for the annual plant Plantago wrightiana. In the greenhouse, we quantified selection on seed mass and emergence time at each of five planting densities (1 m−2 to 10,000 m−2) using the relationship between final plant mass and each trait at each density. We observed no significant selection on either seed mass or emergence time when plants were grown alone. At all higher densities, there was significant selection favoring early emergence and large seed mass, but there were no significant differences among the selection gradients determined at densities greater than individually grown plants. In the field, we detected no relationship between the magnitude of selection for early emergence and density. Our results suggest that selection on seed mass and time of emergence is density dependent, but the relationship between density and the magnitude of directional selection on these traits is not continuously increasing. Over broad ranges of density in the greenhouse and in the field, there was no detectable relationship between density and the magnitude of directional selection.

• In small isolated populations, genetic drift is expected to increase chance fixation of partly recessive, mildly deleterious mutations, reducing mean fitness and inbreeding depression within populations and increasing heterosis in outcrosses between populations. • We estimated relative effective sizes and migration among populations and compared mean fitness, heterosis, and inbreeding depression for eight large and eight small populations of a perennial plant on the basis of fitness of progeny produced by hand pollinations within and between populations. • Migration was limited, and, consistent with expectations for drift, mean fitness was 68% lower in small populations; heterosis was significantly greater for small (mean = 70%, SE = 14) than for large populations (mean = 7%, SE = 27); and inbreeding depression was lower, although not significantly so, in small (mean = -0.29%, SE = 28) than in large (mean = 0.28%, SE = 23) populations. • Genetic drift promotes fixation of deleterious mutations in small populations, which could threaten their persistence. Limited migration will exacerbate drift, but data on migration and effective population sizes in natural populations are scarce. Theory incorporating realistic variation in population size and patterns of migration could better predict genetic threats to small population persistence.

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 (rgreater than0.8), mixed-mating (0.2 ≤r≥ 0.8), and highly outcrossing (rless than0.2) 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.

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.

International audience; Classical models studying the evolution of self-fertilization in plants conclude that only complete selfing and complete outcrossing are evolutionarily stable. In contrast with this prediction, 42% of seed-plant species are reported to have rates of self-fertilization between 0.2 and 0.8. We propose that many previous models fail to predict intermediate selfing rates because they do not allow for functional relationships among three components of reproductive fitness: self-fertilized ovules, outcrossed ovules, and ovules sired by successful pollen export. Because the optimal design for fertility components may differ, conflicts among the alternative pathways to fitness are possible, and the greatest fertility may be achieved with some self-fertilization. Here we develop and analyze a model to predict optimal selfing rates that includes a range of possible relationships among the three components of reproductive fitness, as well as the effects of evolving inbreeding depression caused by deleterious mutations and of selection on total seed number. We demonstrate that intermediate selfing is optimal for a wide variety of relationships among fitness components and that inbreeding depression is not a good predictor of selfing-rate evolution. Functional relationships subsume the myriad effects of individual plant traits and thus offer a more general and simpler perspective on mating system evolution.

Substantial variation in seed mass within individual seed parents is at odds with predictions of models for the evolution of optimum offspring size and with empirical observations of directional selection on seed mass. To elucidate the ultimate causes of this variation, I examined several proximal sources of within-individual variation in seed mass in the perennial herb Prunella vulgaris. Position of inflorescence, position of flower within an inflorescence, date of anthesis, and number of seeds produced per flower all explained some within-individual variation in seed mass. Hand pollination in the field failed to reveal any effect of pollen source (self pollen or outcross pollen) on seed mass. My results, in conjunction with those from studies of selection on seed mass in P. vulgaris, do not support hypotheses that within-individual variation in seed mass is favored by the pattern of natural selection on seed mass. Rather, the results suggest that seed parents are not capable of producing a uniform seed crop in the face of changes in resource availability in the course of a season. The inability to produce a uniform seed crop may persist because of selection for variability in traits correlated with seed mass or because of a true constraint on the evolution of uniform offspring size.

There is increasing evidence that human disturbance can negatively impact plant–pollinator interactions such as outcross pollination. We present a meta-analysis of 22 studies involving 27 plant species showing a significant reduction in the proportion of seeds outcrossed in response to anthropogenic habitat modifications. We discuss the evolutionary consequences of disturbance on plant mating systems, and in particular whether reproductive assurance through selfing effectively compensates for reduced outcrossing. The extent to which disturbance reduces pollinator versus mate availability could generate diverse selective forces on reproductive traits. Investigating how anthropogenic change influences plant mating will lead to new opportunities for better understanding of how mating systems evolve, as well as of the ecological and evolutionary consequences of human activities and how to mitigate them.

Summary • Local adaptation is common, but tests for adaptive differentiation frequently compare populations from strongly divergent environments, making it unlikely that any influence of stochastic processes such as drift or mutation on local adaptation will be detected. Here, the hypothesis that local adaptation is more likely to develop when the native environments of populations are more distinct than when they are similar was tested. • A reciprocal transplant experiment including two populations from each of three habitats was conducted to determine the pattern of local adaptation. In addition to testing for local adaptation at the population level, the hypothesis was tested that local adaptation is more common between populations from different habitats than between populations from the same habitat. • Local adaptation was not common, but more evidence was found of local adaptation between populations from different habitats than between populations from the same habitat. Two instances of foreign genotype fitness advantage confirm that stochastic processes such as drift can limit local adaptation. • These results are consistent with the hypothesis that stochastic processes can inhibit local adaptation but are more likely to be overwhelmed by natural selection when populations occur in divergent environments.

The size and age structures for four assemblages of North American tyrannosaurs— Albertosaurus, Tyrannosaurus, Gorgosaurus , and Daspletosaurus —reveal a pronounced, bootstrap-supported pattern of age-specific mortality characterized by relatively high juvenile survivorship and increased mortality at midlife and near the maximum life span. Such patterns are common today in wild populations of long-lived birds and mammals. Factors such as predation and entrance into the breeding population may have influenced tyrannosaur survivorship. This survivorship pattern can explain the rarity of juvenile specimens in museum collections.

Summary • Phenotypic traits differ between plants in different environments and within individuals as they grow and develop. Comparing plants in different environments at a common age can obscure the developmental basis for differences in phenotype means in different environments. Here, we compared trait means and patterns of trait ontogeny for perennial (Viola septemloba) plants growing in environments that differed in quality either naturally or due to experimental manipulation. • Consistent with predictions for adaptive stress resistance, plants grown in lower-quality environments allocated proportionately more biomass to roots and rhizomes, and produced smaller, thicker and longer-lived leaves. The developmental trajectory of almost all traits differed between environments, and these differences contributed to observed differences in trait means. • Plants were able to alter their initial developmental trajectory in response to an increase in resources after 8 wk of growth. This result contrasts with previous findings, and may reflect a difference in the way that annual and perennial species respond to stress. • Our results demonstrate the complexity of interactions between the environment and the development of the phenotype that underlie putatively adaptive plastic responses to environment quality.

Studies of seed-weight variation across altitudinal and latitudinal gradients have led to conflicting hypotheses regarding the selective value of this traint in relation to the length of the growing season. Growing-season length may also influence the evolution of seed number, and population differentiation in seed weight may be constrained by a negative genetic correlation between seed weight and seed number within populations. We examined variation in seed weight and an estimate of seed number (flower number) and the covariance of these traits among populations of Prunella vulgaris at five latitudes between northern Michigan and South Carolina. We measured seed weight and flower number in native habitats and in a common environment to determine the extent to which patterns observed in the field reflect genetic differentiation. We observed no genetically based variation in seed weight across the latitudinal gradient, although genetic variation among populations within a latitude was observed. In contrast to the lack of variation in seed weight, flower number increased clinally from northern Michigan to Tennessee in a common environment. Population mean flowering date in a common environment was successively later from north to south. Later-flowering individuals appear to achieve a larger size before flowering and consequently possess more resources for seed production. This difference may account for the greater flower production of late-flowering, southern populations. Independence of population mean seed weight and flower number across the latitudinal gradient suggests that population differentiation in seed weight has not been constrained by a trade-off between seed size and number within populations.

1 A reciprocal-transplant experiment tested the hypothesis that two populations of a perennial grass were locally adapted. Because population differentiation might be due to differential adaptation to the presence of surrounding vegetation, plants were transplanted into undisturbed areas and areas from which surrounding vegetation had been removed. As an indirect investigation of the effects of temporal environmental heterogeneity on the detection of local adaptation, three age-classes of plants were included in the experiment. 2 Seeds collected from the two populations were sown into soil from both sites in the greenhouse. Seedlings of three age-classes were transplanted reciprocally between the two field sites into both undisturbed quadrats and quadrats from which vegetation had been removed. Seedling survival was recorded for six months, and dry weight was determined for all seedlings that survived to the end of this period. 3 Comparisons of seedling survival and biomass revealed evidence for local adaptation, and the expression of local adaptation was affected very little by the presence of surrounding vegetation. Results for vegetation-removal quadrats closely paralleled those for unmanipulated quadrats. This result does not support the hypothesis that the two populations are differentiated with respect to performance in the presence of vegetation. 4 The detection of local adaptation depended strongly on the age of transplants. Home-site survival and final biomass advantages were greater for older age-classes. The effect of transplant age on the detection of population differentiation suggests that evidence of population differentiation may not always be apparent because of temporal variation in environmental conditions.

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 emer- gence, 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 seeds 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 phe- notypic plasticity in seed size suggests that there may be little opportunity for an evolu- tionary response despite strong selection favoring large seeds.

Knowledge of how seed production is regulated by components of seed yield can provide insight into what determines fitness in natural populations. This study examines patterns and sources of variation and interaction among components of seed yield within and among 10 populations of a perennial weed. Much of the variation in total yield among individuals within populations could be explained by differences in plant size. In general, relationships between components of yield were weak, indicating that total yield was not tightly buffered by trade—offs among yield components. The pattern of relationships between yield components differed among populations, demonstrating that these relationships are not a constant characteristic of a species. The patterns of relationship between each component and an estimate of the number of successful seedlings produced also differed among populations, but seed number was generally more important than biomass per seed in determining plant fitness. Yield component means and ...

(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.