Analyzing Genetic Diversity in Small, Isolated, and Developing Populations - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 15

generation, so population growth rate = 0), it will be seen that, because of
the random sampling error, allele frequencies and the unique alleles that
become lost or fi xed vary randomly across trials, thus demonstrating the
principle of genetic drift. Founder effects and population bottlenecks can
contribute to random drift since they both involve small populations.
Standard explanations of random drift (e.g., see texts above) begin with
examples in which succeeding generations remain small (usually constant
from generation to generation, e.g., n always = 10). NEWGARDEN allows
the user to explore what happens to random drift as populations grow at
different rates, and as particular population life history characteristics are
varied.
Inbreeding
Inbreeding generally refers to matings between individuals that are more
closely related than on average for a population. The degree of inbreeding
increases from matings between very distantly related individuals to the
most extreme form, selfi ng.
One commonly used measure of genetic diversity in a population is
gene diversity (h), which is the probability that two alleles at a locus chosen
randomly from a population differ. Often, h is calculated as the level,
based on allele frequencies, of expected heterozygosity for a population
according to Hardy-Weinberg equilibrium (e.g., see Frankham et al. 2002;
Freeland 2005). Inbreeding is often detected because as inbreeding increases,
homozygosity increases (heterozygosity decreases), while allele frequencies
remain the same. Thus, observed heterozygosity deviates increasingly from
expected heterozygosity with increased inbreeding. With obligate and
continued selfi ng in a population, heterozygosity decreases each generation
by 50%. Thus, after numerous rounds of complete selfi ng, different lines
approach approximately 100% homozygosity. In general, inbreeding
changes genotype frequencies, but not gene frequencies, as it continues
generation after generation in a population. Thus, inbreeding tends to
reduce genetic variation in populations because of its aforementioned
reduction of heterozygosity.
In using NEWGARDEN, changing the conditions for different
population growth trials can alter the average heterozygosity calculated
for populations as they develop through generations. Consider two
populations derived from the same number of founders taken from the
same source population, but developing under different input conditions:
the derived population with higher homozygosity is generally considered
more inbred.
When a species is being introduced to a community, inbreeding should,
in most cases, be avoided to prevent potential inbreeding depression