Population Genetics Edge Effects - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 98

0.51

0.48

5

0.45

0.42

2

0.39

0 5 10 15 20 25

GENERATION

Fig. 8.7 A comparison of loss of expected heterozygosity (indicated with an x) versus observed
heterozygosity (indicated with a +) through generations for two populations differing in the
distance of founders from a preserve border. The difference in these values for trial 2 versus
trial 5 is indicated by the vertical arrows. For trial 2, the 20 founders were placed at the reserve
origin (point 0,0 at lower left corner), while for trial 5 the 20 founders were placed 2,560 units
internally away from any border. All other parameters for the two trials were identical.

After 20 generations, the difference between the expected and observed
values for heterozygosity differ less when the founders are placed at the
origin (the difference is indicated by the arrow labeled “2” in Fig. 8.7) than
when the founders are placed approximately 2,560 units away from any
border (arrow labeled 5). Although observed heterozygosity drops more
rapidly with founders at the origin, expected heterozygosity drops more or
less apace compared to when the founders are placed in the center of the
preserve, for which observed heterozygosity drops more rapidly through
the generations. Thus, in the latter case, the expression

F = (He - Ho)/ He

is larger, yielding a larger F value.

The more or less commensurate drop in estimated and observed
heterozygosity when the founders are placed at the origin (trial 2) is likely
due not to inbreeding, but rather to increased drift that develops when the
founders are at the borders and population growth rate is lower ( Fig. 8.3 ).
Drift would randomly increase the shift away from even frequencies for both
alleles (but in opposite directions), driving expected heterozygosity lower
(Fig. 8.8) and driving observed heterozygosity lower at a similar rate. Levels
of inbreeding do not affect allele frequencies, so expected heterozygosity
is a more sensitive indicator in change of numbers of alleles via drift.
Observed heterozygosity will change because of both levels of inbreeding