Jeffrey K. Conner

Patterns of phenotypic and genetic correlations among morphological and life-history traits in wild radish, Raphanus raphanistrum

Genetic correlations can have profound effects on evolutionary change (Lande and Arnold, 1983; Mitchell-Olds and Rutledge, 1986). Present patterns of genetic correlations in an organism may be caused by pre-existing pleiotropic and developmental relationships among traits and may produce constraints on evolution by natural selection (Cheverud, 1984; Maynard Smith et aI., 1985; Via and Lande, 1985; Futuyma, 1986; Mitchell-Olds and Rutledge, 1986; Barker and Thomas, 1987; Clark, 1987a; Zeng, 1988). Alternatively, selection may directly alter the patterns of genetic correlations, especially in cases in which two or more traits interact to perform a given function (Cheverud, 1984; Lande, 1984; Clark, 1987a. 1987b). In this paper we examine the patterns of phenotypic and additive genetic correlations among 10 morphological and life-history traits in wild radish plants. We hypothesized that some ofthese correlations have been influenced by selection. We predicted that floral and vegetative traits would be uncorrelated and that correlations among the lengths of the corolla tube, pistil and stamens would be higher than the rest of the floral correlations. The results were consistent with most of these predictions. Wild radish, Raphanus raphanistrum (Brassicaceae), is an annual weed of disturbed areas. The hermaphroditic flowers of wild radish are almost entirely selfincompatible (Sampson, 1964; Stanton et aI., 1989) and this species does not propagate vegetatively, so virtually all reproduction depends on successful insect pollination. Wild radish is pollinated by a variety of insects, mainly bees, butterflies, and flies (Kay, 1976;

ECOLOGICAL GENETICS OF AN INDUCED PLANT DEFENSE AGAINST HERBIVORES: ADDITIVE GENETIC VARIANCE AND COSTS OF PHENOTYPIC PLASTICITY

Abstract.— Adaptive phenotypic plasticity in chemical defense is thought to play a major role in plant‐herbivore interactions. We investigated genetic variation for inducibility of defensive traits in wild radish plants and asked if the evolution of induction is constrained by costs of phenotypic plasticity. In a greenhouse experiment using paternal half‐sibling families, we show additive genetic variation for plasticity in glucosinolate concentration. Genetic variation for glucosinolates was not detected in undamaged plants, but was significant following herbivory by a specialist herbivore, Pieris rapae. On average, damaged plants had 55% higher concentrations of glucosinolates compared to controls. In addition, we found significant narrow‐sense heritabilities for leaf size, trichome number, flowering phenology, and lifetime fruit production. In a second experiment, we found evidence of genetic variation in induced plant resistance to P. rapae. Although overall there was little evidence for genetic correlations between the defensive and life‐history traits we measured, we show that more plastic families had lower fitness than less plastic families in the absence of herbivory (i.e., evidence for genetic costs of plasticity). Thus, there is genetic variation for induction of defense in wild radish, and the evolution of inducibility may be constrained by costs of plasticity.

Genetic mechanisms of floral trait correlations in a natural population

Genetic correlations among traits are important in evolution, as they can constrain evolutionary change or reflect past selection for combinations of traits. Constraints and integration depend on whether the correlations are caused by pleiotropy or linkage disequilibrium, but these genetic mechanisms underlying correlations remain largely unknown in natural populations. Quantitative trait locus (QTL) mapping studies do not adequately address the mechanisms of within-population genetic correlations because they rely on crosses between distinct species, inbred lines or selected lines (see ref. 5), and they cannot distinguish moderate linkage disequilibrium from pleiotropy because they commonly rely on only one or two episodes of recombination. Here I report that after nine generations of enforced random mating (nine episodes of recombination), correlations between six floral traits in wild radish plants are unchanged, showing that pleiotropy generates the correlations. There is no evidence for linkage disequilibrium despite previous correlational selection acting on one functionally integrated pair of traits. This study provides direct evidence of the genetic mechanisms underlying correlations between quantitative traits in a natural population and suggests that there may be constraints on the independent evolution of pairs of highly correlated traits.

Visitation, effectiveness, and efficiency of 15 genera of visitors to wild radish, Raphanus raphanistrum (Brassicaceae)

Plant-pollinator interactions are one of the most important and variable mutualisms in nature. Multiple pollinators often visit plants and can vary in visitation rates, pollen removal and deposition, and spatial and temporal distribution, altering plant reproduction and patterns of pollinator-mediated selection. Although some visitors may not be effective pollinators, pollinator effectiveness is rarely estimated directly as seed set resulting from a single visit by each taxon visiting generalist plants. For two years, effectiveness of visitors to wild radish, Raphanus raphanistrum, was quantified by counting seeds set and pollen grains removed as a result of a single visit. We calculated a pollinators importance to plant reproduction as the product of visitation rate and single-visit seed set, and regressed pollinator body size on pollen-removal and on seed set effectiveness. Although pollinators differed in effectiveness and visitation rates, pollinator importance was primarily determined by visitation rates. In contrast to similar 2-yr studies, pollinator assemblage composition varied little, suggesting pollinator-mediated selection can be consistent across years for this generalist. Larger pollinators were more effective than smaller at effecting seed set, but body size was a poor predictor of pollen removal ability. Instead, pollen-removal effectiveness may be more influenced by foraging behavior than size.

ARTIFICIAL SELECTION: A POWERFUL TOOL FOR ECOLOGISTS

Artificial selection has been practiced by humans since the dawn of agriculture, but only recently have evolutionary ecologists turned to this tool to understand nature. To perform artificial selection, the phenotypic trait of interest is measured on a population, and the individuals with the most extreme phenotypic values are bred to produce the next generation. The change in the mean of the selected trait across each generation is the response to selection, and other traits can also evolve due to genetic correlations with the selected trait. Artificial selection can directly answer the question of how quickly a trait will evolve under a given strength of selection. This kind of result can help ecologists determine whether range or niche boundaries are determined by a lack of variation for a key phenotypic trait or trade-offs due to genetic correlations with other fitness-related traits. In a related approach, controlled natural selection, the organisms are not selected according to their values for a given trait, but rather are allowed to evolve for one to several generations under experimentally imposed environmental treatments such as temperature, light, nutrients, presence or absence of predators or competitors, etc. The results of this kind of study can tell us how quickly a population can adapt to a given environmental change, either natural or anthropogenic. Finally, artificial selection can create more variation for measurements of natural selection or can be coupled with QTL mapping; both these combinations provide new insights into adaptation. I discuss advantages and disadvantages of these approaches relative to other kinds of studies and highlight case studies showing how these tools can answer a wide range of basic and applied questions in ecology, ranging from niche and range boundaries and character displacement to climate change and invasive species.

EXPRESSION OF ADDITIVE GENETIC VARIANCES AND COVARIANCES FOR WILD RADISH FLORAL TRAITS: COMPARISON BETWEEN FIELD AND GREENHOUSE ENVIRONMENTS

Abstract Measurements of the genetic variation and covariation underlying quantitative traits are crucial to our understanding of current evolutionary change and the mechanisms causing this evolution. This fact has spurred a large number of studies estimating heritabilities and genetic correlations in a variety of organisms. Most of these studies have been done in laboratory or greenhouse settings, but it is not well known how accurately these measurements estimate genetic variance and covariance expressed in the field. We conducted a quantitative genetic half‐sibling analysis on six floral traits in wild radish. Plants were grown from seed in the field and were exposed to natural environmental variation throughout their lives, including herbivory and intra‐ and interspecific competition. The estimates of heritabilities and the additive genetic variance‐covariance matrix (G) obtained from this analysis were then compared to previous greenhouse estimates of the same floral traits from the same natural population. Heritabilities were much lower in the field for all traits, and this was due to both large increases in environmental variance and decreases in additive genetic variance. Additive genetic covariance expressed was also much lower in the field. These differences resulted in highly significant differences in the G matrix between the greenhouse and field environments using two complementary testing methods. Although the G matrices shared some principal components in common, they were not simply proportional to each other. Therefore, the greenhouse results did not accurately depict how the floral traits would respond to natural selection in the field.

Field measurements of natural and sexual selection in the fungus beetle, Bolitotherus cornutus

Selection on three phenotypic traits was estimated in a natural population of a fungus beetle, Bolitotherus cornutus. Lifetime fitness of a group of males in this population was estimated, and partitioned into five components: lifespan, attendance at the mating area, number of females courted, number of copulations attempted, and number of females inseminated. Three phenotypic characters were measured—elytral length, horn length, and weight; there were strong positive correlations among the three characters. Selection was estimated by regressing each component of fitness on the phenotypic traits.

Density-dependent sexual selection in the fungus beetle, Bolitotherus cornutus

The hypothesis that population density can affect sexual selection on male horn size was tested in a three‐year study of a fungus beetle, Bolitotherus cornutus. Males of this species have horns that vary greatly in length. These horns are used in fights over females; longer‐horned males win the majority of fights, regardless of population density. However, density does affect the relationship between horn length and access to females. In six populations of naturally and experimentally varying densities, longer‐horned males gained a greater advantage in access to females in low‐density populations than at high density. This increase in access to females causes an increase in the number of females inseminated by longer‐horned males; thus, sexual selection for longer horns is stronger at lower densities.

Characterizing ecological generalization in plant-pollination systems

Despite the development of diversity indices in community ecology that incorporate both richness and evenness, pollination biologists commonly use only pollinator richness to estimate generalization. Similarly, while pollination biologists have stressed the utility of pollinator importance, incorporating both pollinator abundance and effectiveness, importance values have not been included in estimates of generalization in pollination systems. In this study, we estimated pollinator generalization for 17 plant species using Simpson’s diversity index, which includes richness and evenness. We compared these estimates with estimates based on only pollinator richness, and compared diversity estimates calculated using importance data with those using only visitation data. We found that pollinator richness explains only 57–65% of the variation in diversity, and that, for most plant species, pollinator importance was determined primarily by differences in visitation rather than by differences in effectiveness. While simple richness may suffice for broad comparisons of pollinator generalization, measures that incorporate evenness will provide a much more accurate understanding of generalization. Although incorporating labor-intensive measurements of pollinator effectiveness are less necessary for broad surveys, effectiveness estimates will be important for detailed studies of some plant species. Unfortunately, at this point it is impossible to predict a priori which species these are.

The Resurrection Initiative: Storing Ancestral Genotypes to Capture Evolution in Action

ABSTRACT In rare circumstances, scientists have been able to revive dormant propagules from ancestral populations and rear them with their descendants to make inferences about evolutionary responses to environmental change. Although this is a powerful approach to directly assess microevolution, it has previously depended entirely upon fortuitous conditions to preserve ancestral material. We propose a coordinated effort to collect, preserve, and archive genetic materials today for future studies of evolutionary change—a “resurrection paradigm.” The availability of ancestral material that is systematically collected and intentionally stored using best practices will greatly expand our ability to illuminate microevolutionary patterns and processes and to predict ongoing responses of species to global change. In the workshop “Project Baseline,” evolutionary biologists and seed storage experts met to discuss establishing a coordinated effort to implement the resurrection paradigm.