Biology Reference
In-Depth Information
For trials a, b, and f, enough rounds of reproduction must pass so that
cohorts achieve a size of approximately 80 to 90 offspring before, on average,
all unique alleles present in the founders are found in an individual cohort
under the given conditions. In these populations, by the time the entire
population achieves a size of 4,000, several cohorts each have included all
of the founding unique alleles. Trial e, the lower “distinct” curve, differs
from the other three trials. Since, after 100 rounds of mating, trial e with
its reduced reproduction was still not producing cohorts with more than
450 of the founding unique alleles (
Fig. 16.3A)
,
trial e was run out to 138
generations in Fig. 16.3B. Even then, the highest average value for unique
alleles present in a new cohort for trial e (approximately 490) is still less
than the total number of unique alleles in the founders. At West Salem, no
rare alleles were lost because of drift (or selection, which is not a factor in
NEWGARDEN trials). Trials a, b, and f refl ect this fi nding in that numerous
successive cohorts carry all of the unique alleles present in the founders.
Even though trial e does not generate cohorts that bear all founding unique
alleles even after 138 bouts of mating, note that there are later successive
cohorts that hold a large fraction of those alleles; it is likely that, taking all of
these cohorts together, all of the founding unique alleles would be present
in the entire descendant population well before a cohort is produced that
carries all of those alleles. Such NEWGARDEN studies are useful not only
in modeling unique allele retention across generations, but also in designing
optimal seed cohort collection strategies that provide greater likelihood that
all of the founding unique alleles will be harvested at least once in a seed
(latest single cohort) sample.
Relative to the issues of concern in the West Salem study outlined above,
from the simulation trials we can conclude that (1) none of the low-frequency
unique alleles found in only one of the parental trees was fi xed or lost in
the descendants in these virtual populations growing in a fashion similar
to what occurred at West Salem, or even when growing at a much slower
rate (trial e), and (2) no evidence was found that inbreeding, subdivision,
and/or drift caused major changes in heterozygosity or F values. Thus, the
NEWGARDEN simulations are consistent with actual genetic analyses on
the American chestnut population at West Salem.
More Founders, Subdivided Founders
The previous results suggest that even when starting with only 9 to 10
founders, and even if the population expands rather slowly as in trial e,
loss of genetic diversity is not always inevitable when there is suffi cient,
constant population growth, outcrossing is prevalent, offspring and pollen
dispersal distances are within the ranges used, the original founders remain
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