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only one locus is under consideration, so each founder has two alleles
randomly selected from the source population, which has two alleles for
that locus, both at frequency 0.5. NEWGARDEN selects and records the
number of
unique
alleles (only 1 or 2 are possible per founder) and resulting
observed heterozygosity (this can assume only a value of 0 (individual is
homozygous = 0% heterozygosity) or 1 (individual is heterozygous = 100%
heterozygosity)) for that founder. Then, for the next founder, one of the two
alleles for each parental chromosome of the solitary locus is likewise selected
randomly from the source population and unique alleles and heterozygosity
are recorded, and so on for 25 replicates (runs) of this drawing process. In
other words, one trial has 25 runs, each of which begins anew with the initial
input conditions. Then, when one particular trial is fi nished, NEWGARDEN
calculates an average value (with standard deviation) across the 25 runs
of the trial, for various population genetic parameters (e.g., unique alleles
retained in, or observed heterozygosity of, the founding population).
To gain a sense of how variation in the number of unique alleles drawn
from the source population into the founding population decreases as the
number of founders drawn increases, each trial (25 runs per trial) in this
example was repeated fi ve times. Figure 7.1 shows how the degree of variation
of the mean of the unique alleles retained in replicate trials decreases as the
2.1
2.0
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1.9
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1.8
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1.7
1.6
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1.5
1.4
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1.3
1
10 100
FOUNDERS
Fig. 7.1
As the number of founders chosen from a source population with two alleles of equal
frequency increases, the chance that both alleles will be retained in at least one of the founders
increases. Each letter represents the average alleles retained for one trial where the indicated
number of founders was drawn 25 times. Five separate replicate trials (each with 25 runs per
trial) were conducted for each number of founders (1, 2, 3, 10, 20, 40, 100, 400, or 900 founders)
to accentuate levels of variation, and where fewer than fi ve letters are apparent, some or all of
those letters overlap (e.g., for 20 founders, all fi ve letters overlap). Across 125 replicate runs
of drawing the specifi ed number of founders, the two alleles were always passed into the
founding population when founder number was 10 or more (at least 20 alleles drawn).
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