Corridors - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 296

However, there are other considerations for perennials. In trials
1 through 4, the results shown above ( Fig. 17.12) are only across 13
generations, and thus few of the original founders have died. Therefore,
almost all of the original unique alleles brought by the founders are still
present in the total population. But NEWGARDEN allows the user to also
examine the data by new-cohort-individuals-only by next-new-cohort-
individuals-only to discover patterns of population growth, unique allele
retention, heterozygosity and F values for members of each individual
cohort only. For example, while most unique alleles are present in the
total population because the founders are still present, if all the alleles are
not being effi ciently transferred to each, or at least all, of the consecutive
cohorts, then when all the founders have died, the data may refl ect a sudden
decrease in unique alleles present in the total population. This is especially
important if a population is reduced by some environmental factor (e.g.,
drought, pathogens, beetle outbreak) that practically eliminates older or
larger classes.
Figure 17.13 shows data on cohort-by-cohort population growth (graph
A) and unique alleles retained (graph B) for the four trials just examined
in Fig. 17.12. For population growth, from the founding cohort 0 (172
individuals) to the next cohort, there is a large increase in the size of the latter.
Cohorts remain approximately the same size early on, but then decline:
mortality rates designated in the input gradually remove some founders,
which are the only reproductive individuals. Also, density-dependent
effects may decrease the survival of offspring in each successive cohort as
the number of total individuals in the population increases with increasing
generations. Only when the fi rst new cohort of individuals produced
by the founders reaches reproductive age is there an upswing in rate of
population growth. From that point, as successive cohorts also become
newly reproductive individuals, new cohort sizes, and the entire population,
begin to increase more rapidly. Trials 2 and 4 (founders in one preserve;
upper “curve”) outpace trials 1 and 3 (founders split into two preserves) in
successive cohort sizes, a trend that refl ects the pattern seen for the entire
population (Fig. 17.12). As for unique alleles retained by cohorts, all trials
decrease in alleles in the early cohorts (in the most extreme case, trial 1, less
than 1% of the alleles in the founding generation are lost), possibly because
of loss of unique alleles due to increasing density-dependent selection
against offspring. However, the unique alleles in individual cohorts begin
to climb once cohort 1 reaches reproductive age, and increase thereafter
as more individuals become reproductive in successive cohorts, and thus
reproduction including the original founders increases. These cohort-by-
cohort results emphasize the importance of the survival of individuals of
the earliest cohorts to the growth rate of, and the unique allele retention
by, the later generations (including all individuals).