Analyzing Genetic Diversity in Small, Isolated, and Developing Populations - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 16

(reduced fi tness due to the increased expression of deleterious alleles that
become homozygous as general homozygosity increases in a population).
One use of NEWGARDEN is to explore which types of species are most
likely to suffer inbreeding and under what conditions, and therefore to
determine best introduction practices to facilitate its avoidance in different
situations.
F values
F values quantify the departure of a population from Hardy-Weinberg
equilibrium. The F statistical formula used by NEWGARDEN is:
F = (He - Ho)/ He
where He is the expected heterozygosity (under the Hardy-Weinberg
equilibrium) and Ho is the observed heterozygosity.
F typically ranges from 0 (no inbreeding and/or subdivision) to 1
(complete inbreeding and/or subdivision). F values may increase because of
inbreeding and/or increased population subdivision (Wahlund effect). The
latter occurs when a subdivided population is sampled as though it is one
population, resulting in an observed excess of homozygosity compared to
Hardy-Weinberg expectations. In many of the examples below, F values are
seen to increase when the founders are subdivided into groups compared to
when the founders are situated in one group, suggesting that the Wahlund
effect is involved. F values can also drop below 0 when there is an excess
of heterozygosity, which can be an artifact of very low population sizes.
As will be seen, the latter can occur early in trials when populations are
very small.
Population Subdivision
Another assumption of the Hardy-Weinberg model is that population
matings proceed completely at random (panmixia). Under panmixia,
matings occur randomly without regard to phenotype, genotype, or
degree of relatedness. As will be seen in NEWGARDEN results presented
below, this defi nition implies that distance does not matter: under Hardy-
Weinberg expectations, any two individuals of the infi nite population are
equally likely to mate without regard to the distance between them. This
should act to homogenize allele frequencies across different portions of a
population.
However, if alleles or allele frequencies vary for different regions of
a population, that population is said to be subdivided. A recognizable
subpopulation differs statistically in its genetic makeup from other portions
of the population.