Current Research: Susan Mazer

Evolutionary processes in wild flowering plants

The theme of my research is the examination of evolutionary processes and their outcome in wild species of flowering plants. Evolutionary biologists can detect the signature of natural selection at a variety of ecological and temporal scales; accordingly, my research has included evolutionary studies of populations, communities, and higher taxa.

My research focuses on traits whose function or effects on individual fitness have been demonstrated and whose evolution has captivated evolutionary ecologists and theoreticians for several decades. These include flower size, seed mass, age of reproduction, investment in eggs and sperm, components of floral and fruit display, and sex allocation. In a variety of projects, I am pursuing three main goals:

1. To predict the evolutionary trajectories of life history and reproductive components within wild plant species. Evolutionary theory predicts that, as natural selection occurs over many generations, genetic variation in traits that affect individual fitness should decline to very low levels. My work on quantitative trait variation in wild species, however, has found that many fitness-related traits continue to exhibit substantial genetic variation in natural populations. This apparent paradox motivates my research to detect the processes that contribute to the maintenance of genetic variation in wild populations.

   The first step in such studies is to determine the causes and consequences of variation in traits that affect survival and reproductive success in wild and laboratory populations. I have focused on quantitative traits (as opposed to discrete characters) for two reasons. First, most life history and morphological traits are quantitative, and second, these traits often have a strong effect on plant performance in wild populations. This in turn means that these traits are exactly the ones expected to be open to ongoing evolution by natural selection.

   In my research, I use the experimental protocols and statistical tools from the fields of quantitative genetics and plant breeding. I've used these methods in several species to measure the portion of phenotypic variation that is "visible" to natural selection (i.e., the heritable, or genetic, component) and to predict or to measure the strength of selection under natural conditions. To achieve these goals, it is necessary to design large experiments that control for a variety of phenomena that can either mask or inflate the genetic component of variation in fitness-related traits. Consequently, much of my work includes detailed analyses of ontogenetic (developmental) and environmental causes of phenotypic variation, of maternally inherited effects on progeny phenotype, and of interactions between genetic and environmental effects on phenotype (e.g., Mazer and Delesalle, 1996a,b,c; Mazer and Wolfe, 1998). This research has shown that, while genetic variation in fitness-related traits is often obscured by ontogenetic and environmental sources of variation, substantial genetic variation remains in many fitness-related traits in wild populations.

2. To detect the mechanisms by which natural selection may be prevented from resulting in evolutionary change in traits that influence fitness. Several phenomena have been proposed to be important "constraints" on the rate of evolutionary change. These include: negative genetic correlations between traits that enhance (or diminish) fitness; Genotype x Environment interactions (where the differences in fitness among genotypes depend on the environment they experience); environment-specific heritabilities and genetic covariances; phenotypic plasticity of fitness-related traits; maternal environmental effects on progeny phenotype; and strong environmental (including ontogenetic) components of variation. All of these phenomena may delay the rate of evolutionary change in quantitative traits even when the strength of selection on these traits is strong.

   In several experiments recently completed on Spergularia marina, Raphanus sativus, Clarkia unguiculata and Clarkia exilis, I am measuring the potential roles of negative genetic correlations, Genotype x Environment interactions, environment-specific heritabilities, and phenotypic plasticity as alternative (but not mutually-exclusive) phenomena that may slow down or prevent the expected decline in genetic variation of fitness-related traits as natural selection progresses (e.g., Mazer, Delesalle and Neal, 1999)

3. To examine the associations among life history traits, reproductive traits, and ecological preferences among species, in order to detect patterns that appear to result from long-term ecological and evolutionary processes. In contrast to short-term genetic and demographic studies within species, studies of variation in life history and reproductive characters among species rely on the statistical analysis of broad-scale comparative surveys and phylogenetic patterns. Interspecific studies can be used to test the "null" hypotheses that life history and/or reproductive characters of wild species are independent of the kinds of habitats they occupy or of their ecological role. A variety of quantitative methods are now at hand (although hotly debated) to evaluate inter-specific data (briefly reviewed in Mazer, 1998). Most of these include broad-scale comparative surveys that evaluate the joint distribution of reproductive characters and ecological attributes across a taxonomically diverse array of species or within monophyletic groups of which the phylogeny is well established.

My current research projects that address these goals include the following

1. Testing the assumptions of sex allocation theory: quantitative genetic variation and covariation among floral traits in the Sand-Spurrey, Spergularia marina (Caryophyllaceae)
2. The evolution of gender-related traits in species with different mating systems: a quantitative genetic comparison of selfing and outcrossing species of Clarkia (farewell-to-spring: Onagraceae)
3. The Evolutionary Significance of Variation in Traits Subject to Ontogenetic Change and Maternal Environmental Effects
4. Genetic and environmental influences on life history, floral traits, and sex allocation in Raphanus sativus (Brassicaceae; wild radish): the stability of genetic parameters across environments.
5. The Detection of the Long-term Outcome of Natural Selection and the Ecological Sorting of Species Among Habitats: Comparative Studies of Plant Reproductive Characters
6. Ecological adaptation, gene flow, and the potential for hybrid breakdown in restoration projects.

Susan Mazer | Research | Publications | Curriculum Vitae

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