Analyzing Genetic Diversity in Small, Isolated, and Developing Populations - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 14

founder effects and/or bottlenecks will often accompany restoration efforts.
NEWGARDEN allows one to explore such effects by keeping population
growth rate low. After severe bottlenecks, the resulting populations will
have relatively low genetic variation, which can then only be increased
by incoming gene fl ow (immigration) or mutation, both of which can be
modeled by NEWGARDEN. Further, comparative NEWGARDEN analyses
can be used to study how founder effects, continuing bottleneck effects,
spacing of founding or immigrating individuals, and several other life
history features (described below) can interact to affect the genetic diversity
of developing populations.
Genetic Drift
Populations tend to lose genetic diversity through generations at increased
rates as the size of a population decreases. There are two main causes that
drive this loss: random genetic drift (hereafter called “genetic drift”) and
inbreeding (discussed in the next section).
Genetic drift is due to random sampling errors that occur when low
numbers of individuals are involved in low numbers of matings. For
example, in the transmission to their offspring through random mating of
one particular sample of gametes drawn from a small number of founders,
the resulting genetic sample in the next generation will most likely deviate in
allelic proportions from the exact proportions expected based on an infi nite
population size, the representation of the alleles in the parental pool, and
the process by which the gametes are drawn and combined (e.g., sex ratios
may differ, rate of selfi ng may vary). If this process is repeated several times,
there will be variation in deviation from the expected transmission to the
offspring from trial to trial. As long as there is genetic variation among the
parents, the variation of the deviation of transmission to the offspring will
generally increase across trials as the offspring population decreases in
size. In such continuing small populations, allele frequencies change from
generation to generation, not only because of selection or immigration,
but also because of this random sampling error (in small populations,
genetic drift can often greatly outweigh selection as the driving force of
evolution). If populations remain small generation after generation, as this
process is repeated not only do allele frequencies change at random, but
some unique alleles will be lost randomly from the population, the rate of
loss being greater in smaller populations. Similarly, the frequency of some
unique alleles will randomly increase in some continuing small populations
until they become fi xed. Conducting replicate trials, always starting from
a genetically and numerically identical, small founder population and
allowing generations to grow at the same low rate (when demonstrating
this principle, successive generations are often equal in size to the founding