Population Genetics Edge Effects - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 112

However, given the high frequency of the allele among the founders,
it is highly probable that identical copies of that allele are present in
other individuals that will be successful in transmitting them to future
generations (lowercase symbols in Fig. 8.17) . Alternatively, if only one
copy of a unique allele exists in only one of the 20 heterozygous founders
(recall that heterozygosity is higher and approaches 100% when there are
100 different alleles available per source population locus: see Fig. 8.14) ,
then that allele is at much higher risk of randomly being lost, especially in
the early generations, because of events such as those listed above.
As outlined earlier, the maximum number of unique alleles that could
be drawn for the 20 founders across 30 loci, each with 100 unique alleles,
is 1,200. In Fig. 8.17A, it can be seen that in all trials using the more diverse
loci panel (uppercase letters), approximately 1,000 alleles were drawn. Thus,
even though heterozygosity approaches 100% in these populations (Fig.
8.16), there must be cases where a unique allele is represented by more than
one copy. However, each set of founders for the different trials may have
different numbers of unique alleles, and tracking change in these alleles
may often be better compared across trials by using percent retention in
each generation relative to the total number of unique alleles among the
founders (Fig. 8.17), especially where the effects of different loci panels or
numbers of founders are being compared.
Loss of unique alleles in Fig. 8.17 for populations with high allelic
diversity (uppercase letters) is not always strictly due to random genetic
drift. As noted above, while distance from the edge has no obvious effects
on allele retention when only two unique alleles of equal frequency are
available for loci, when more alleles per locus, and thus alleles with lower
frequencies, are available, genetic edge effects become obvious: unique
alleles are lost more rapidly and to a greater extent when the 20 founders
are positioned at the corner of the preserve (Fig. 8.17, trial L), with less
than 40% of the founding unique alleles ultimately retained. This loss is
rapidly reduced if founders are inset even a small distance from the corner
(5 average density grid units: trial M); if founders are placed 10 (N) or more
units from the border, such trials are practically indistinguishable from trial
V, in which founders are placed approximately 2,560 grid units from any
border. Still, in the latter case under the given conditions, losses of unique
alleles are about 30%. One difference in unique allele loss from trial V
(founders central) to trial L (founders at a corner) is loss due to population
genetic edge effects, which includes heightened genetic drift due not only
to lower population growth rate (Fig. 8.13) , but also to increased losses to
dispersal outside the preserve when founders are situated nearer preserve
borders.