Alice A. Winn

Plant mating systems in a changing world

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.

Ecological and Evolutionary Consequences of Seed Size in Prunella Vulgaris

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.

Correlated evolution of mating system and floral display traits in flowering plants and its implications for the distribution of mating system variation

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.

Analysis of Inbreeding Depression in Mixed-Mating Plants Provides Evidence for Selective Interference and Stable Mixed Mating

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.

Regulation of seed yield within and among populations of Prunella vulgaris

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 ...

Correlations among Fertility Components Can Maintain Mixed Mating in Plants

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.

A comparison of plastic responses to competition by invasive and non-invasive congeners in the Commelinaceae

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.

Latitudinal variation in seed weight and flower number in Prunella vulgaris

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.

The scope of Baker's law

Bakers 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 Bakers 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 Bakers law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Bakers 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 Bakers 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.

The Functional Significance and Fitness Consequences of Heterophylly

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.