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Abstract
Conservation biologists are interested in maintaining genetic variation in small
populations, with a view to maintaining fitness and the ability of the species to adapt to
changing environmental conditions. The most important type of genetic variation is
therefore that which affects fitness and reproduction, and is therefore subject to natural
selection. Such fitness traits are often quantitative, i.e. are the result of a suite of loci, and
are continuously variable. Microsatellite markers are a popular method of determining the
level of variation present in a species’ genome. The assumption is made that
microsatellites, which are neutral markers, behave in the same manner as quantitative
traits. If this assumption were proved incorrect, then the use of neutral markers in
conservation monitoring would have to be re-evaluated. In this study, experiments have
been conducted using Drosophila melanogaster to test the assumption that variation in
quantitative traits under stabilising selection declines at the same rate as heterozygosity in
microsatellite markers, during a population bottleneck. Experimental population
bottlenecks were of two effective population sizes (Ne), Ne=2 for one generation and
Ne=60 for 35 generations. Based on the effective population size, we expected both types
of bottlenecks to lose 25% of neutral genetic variation. Ten replicates of each bottleneck
were maintained, along with four large control populations with Ne=320. In each
population, heterozygosity (He) for eight microsatellite loci was compared with the
heritability and additive genetic variance of two quantitative traits subject to balancing
selection: fecundity and sternopleural bristle number. Microsatellite heterozygosity
decreased in accordance with neutral predictions, whereas additive genetic variation in
quantitative traits altered more than expected in both large and in bottlenecked
populations relative to the initial sampling values, indicating that variation in quantitative
traits was not being lost at the same rate as predicted by neutral theory. For most traits,
the changes in additive genetic variance were congruent in all populations, large or
bottlenecked. This congruence suggests that a common process was affecting all
populations, such as adaptation. A mite infestation in early generations is a possible
source of selective pressure. When bottlenecked populations were compared to the
contemporaneous large populations (Ne = 320), the additive genetic variance of most
traits was seen to have been lost in accordance with predictions from the loss of
microsatellite heterozygosity. Loss of variation in microsatellites can thus be used to
predict the loss of variation in quantitative traits due to bottlenecks, but not to predict the
potentially much larger changes due to other processes such as adaptation.
The effects of concurrent environmental stress and reduced population size were also
evaluated. Endangered populations are often subject to environmental stress in addition to
reduced population size, but the effect of stress on the additive genetic variance of fitness
traits in organisms undergoing population bottlenecks is unknown. If the presence of
stress alters the level of additive genetic variance in fitness traits, the viability of such
populations could be substantially affected. The loss of microsatellite heterozygosity was
not affected by the presence of a stress agent during a bottleneck. I found some
significant effects of stress on the additive genetic variance of sternopleural bristles and
fecundity; there was also a significant interaction between stress and the response to
directional selection in sternopleural bristles. There was also an increase in the coefficient
of variation of VA for sternopleural bristles. Stress may therefore affect the manner in
which populations respond to selective pressures.