Type of Document Dissertation Author Oakley, Christopher G. Author's Email Address firstname.lastname@example.org URN etd-06102011-113849 Title The effects of population size and spatial structure on genetic variation and response to a novel environment in a perennial plant Degree Doctor of Philosophy Department Biological Science, Department of Advisory Committee
Advisor Name Title Alice A. Winn Committee Chair David Houle Committee Member Joseph Travis Committee Member Peter Beerli Committee Member J. Anthony Stallins University Representative Keywords
- Spatial Population Structure
- Quantitative Genetic Variation
- Phenotypic Selection
- Small Population
- Novel Environment
- Inbreeding Depression
- Effective Population Size
Date of Defense 2011-06-01 Availability unrestricted AbstractWhile theory shows that the distribution of populations in space can have profound ecological and evolutionary consequences, the complexity of numerical and genetical dynamics in space has made evaluation of model assumptions and predictions in natural populations challenging. Evolutionary dynamics in a spatial context will involve complex interactions between natural selection, genetic drift, migration, and genetic architecture. These factors have seldom been evaluated for natural populations. I explored evolution in a spatial context in the perennial plant Hypericum cumulicola, exploiting natural variation in population size and degree of isolation. This species has recently colonized a novel roadside habitat, and populations in the novel habitat show dramatic differences in life history traits suggesting divergent selection.
In chapter 2, I quantified the pattern of population size and migration using molecular marker data and coalescent methods, and sought to determine the effects of population size on patterns of recessive or nearly recessive unconditionally deleterious mutations. I found that migration was overall very low but that it was difficult to reliably distinguish differences among populations based on estimates of effective size. Agreement of point estimates of effective size, Nei's gene diversity, and habitat patch area and census size indicate that in this system, differences in census size represent a reasonable proxy for difference in long term differences in relative population size; populations with more individuals have higher genetic diversity and occupy larger habitat patches than populations with fewer individuals. I performed controlled pollinations and quantified the fitness of the progeny in the greenhouse to examine the effects of population size on the distribution of recessive or nearly recessive deleterious mutations for 16 populations. I found strong (71%) heterosis in small populations only, indicating an accumulation of deleterious mutations fixed by genetic drift. Qualitatively lower inbreeding depression in small populations was also consistent the action of drift in small populations.
In chapter 3, I conducted a field transplant experiment to examine if population size was positively associated with mean individual fitness in either the native scrub of novel roadside habitat. I also examined if there was local adaptation between populations in the two habitats, and quantified the pattern of phenotypic selection in each habitat. I found that population size is not associated with mean individual fitness in the native scrub habitat, but is in the roadside habit. A 200% increase in mean individual fitness in the road habitat, suggests that population size might influence the ability to respond to more beneficial growing conditions. I found no evidence of local adaptation between the native and novel habitat types. There was significant phenotypic selection for traits associated with faster growth, earlier reproduction, and larger adult size in both environments, with selection being stronger in the native scrub environment for some traits.
In chapter 4, I asked if larger populations had greater quantitative genetic variation for potentially important traits. I quantified population differentiation and broad sense coefficients of genetic variation (CV) for phenotypic traits and examined the relationship of trait means and CV to population size for plants grown in their natural scrub habitat and in the greenhouse. I found limited evidence for reduced quantitative genetic variation in small populations, or depressed trait means for traits related to fitness. I did find consistently positive associations between estimates of Nei's gene diversity and CV for traits measured in their natural habitat. I suggest that reduced plasticity for relevant traits and increased frequency of deleterious mutations could limit the ability of small populations to respond to a novel environment.
In sum, I detected an extreme degree of spatial subdivision in which populations of a dozen to a few hundred individuals are completely isolated on scales of just a few hundred meters. Such isolation and small population size appears to result in substantial fixation of recessive to nearly recessive deleterious mutations. Small populations also appear to have a reduced ability to capitalize on more beneficial environmental conditions. I find no effect of population size on levels of quantitative genetic variation, but suggest that the few cases in which populations are exchanging migrants may obscure the expected pattern. My results suggests that interactions among selection, drift, migration, and non-additive genetic architecture are unlikely to facilitate adaptive evolution in this system, but that there is an important role of genetic drift in how evolution proceeds.
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