Conclusions and Discussion - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 321

signifi cantly different from one another, but they do differ from trials V
and v (p < 0.0001). Fig. 18.1B shows the percentage of the original founding
unique alleles retained as the populations develop. While the populations
with greater allelic diversity (v and w) lose unique alleles at higher rates
in early generations, both of the populations with lower allelic diversity
(V and W) also show large continuing losses of allelic diversity. Observed
heterozygosity ( Fig. 18.2A) declines at higher rates when founders are split
into two reserves (W and w), and heterozygosity losses occur when there
is lower allelic diversity (V and W; note these two trials begin with lower
heterozygosity due to their lower founding allelic diversity). F values (Fig.
18.2B) for trials sharing the same founder placement patterns follow similar
trends through generations whether there are 100 different alleles (v and
w) or 21 different alleles (V and W) at each locus. After approximately
generation 15, F values for the W and w trials become erratic most likely
because of the small population sizes that develop.
Taken together, Figs. 18.1 and 18.2 demonstrate that populations with
lower allelic diversities can lose appreciable genetic diversity at rates that
approximate trends exhibited by populations that are identical in every
way except that they have higher allelic diversity.
How Common Are Low-frequency Alleles in Founding
Populations?
As noted above, the bulk of our NEWGARDEN analyses have involved
loci with relatively high numbers of alleles, each with low-frequency, to the
degree that each different allele might be considered rare in the founding
generation. However, we still lack accurate determinations of how often
functionally important, rare alleles occur in any population or species. Still,
rare alleles have been demonstrated repeatedly across surveys of widely
divergent organisms (see below), and there are a number of ways by which
low-frequency alleles can arise in newly founded populations.
For example, perhaps the simplest way for low-frequency founding
alleles to arise is that numerous alleles are rare in the source population, and
founders arrive with a proportional representation of all source population
alleles. Founder effects, drift, local selection, random culling, and other
factors can also produce rare alleles in source populations. As noted in
the conclusions presented at the beginning of this chapter, several of the
NEWGARDEN trials demonstrated that, despite large early losses of some
unique alleles, numerous low-frequency alleles are likely to be preserved
in moderate-size populations that developed from even a low number of
founders.