Environmental Stress Can Constrain Evolutionary Rates
Erin K. Espeland PhD
Department of Plant Sciences
The ability of native plant populations to persist in the face of environmental changes such as global warming and invasion by non-native species may depend on their adaptive potential. When plant populations are small and isolated, genes may be lost from populations by chance alone, genes that could be useful in adapting to environmental change. The primary cause of loss of genes in populations is some sort of death. When death is linked to lack of adaptation to an environment, death becomes part of the process of natural selection. Often, however, death can be a random event, and survival is not dependent on the genetic makeup of the survivor.
Characterizing a soil-induced stress gradient.
Erin Espeland is a conservation and restoration biologist who wants to know how to minimize the random loss of genes in native plant populations and how to establish new plant populations that can maintain large amounts of genetic diversity and adaptive potential. For her PhD research Dr. Espeland, along with her advisor and collaborator Kevin Rice in the Department of Plant Sciences, asked whether there are some environments in which there is more random death than others? If so, we might actually be able to predict when plant populations may be less able to adapt to changing conditions.
Drs. Espeland and Rice used the diminutive plant Plantago erecta for their work. Plantago occurs at the McLaughlin Reserve at high densities both on and off serpentine soils. They designed their experiments to examine how plant neighbors (competing for resources) and soil type (stressful serpentine vs. non-stressful non-serpentine) affect an index of genetic diversity known as “effective population size.” While serpentine soils are renowned for having evolved highly specialized floras, Drs. Espeland and Rice found that a large amount of death on serpentine meant that the populations growing on these soils had a greater challenge to local adaptation compared to plants growing on non-serpentine soils. The result was counterintuitive: adaptation in non-stressful environments actually can occur more quickly and easily than adaptation in stressful environments.
Measuring survival of Pantago erecta in an experimental plot
The experiment produced another surprising result, namely that there were positive interactions among plants in survivorship. Normally we suppose that interactions among plants are competitive as resources are contested. While Drs. Espeland and Rice did find that plants growing with other plants were smaller, they also found that plants were more likely to survive and produce a few seeds when they grew with other plants, compared to growing alone. This indicates that a dense population of fifty seeds that grow into small plants actually may lose fewer genes by chance than a sparse population of fifty seeds. The resulting plants from the sparse population will be larger and produce more seeds but more plants will die before reproducing.
The results from the experiments Dr. Espeland conducted as a graduate student at McLaughlin are informing her work as a plant ecologist for the USDA as she continues to link ecological and evolutionary processes in conserving native plant populations and preventing the spread of noxious weeds into new environments.