Dioecy - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 210

The preserve is a square with lower left corner coordinates (-2560, -2560)
and upper right corner coordinates (2559, 2559), which includes 26,214,400
possible grid points to be occupied.
Most runs are for 15 rounds of reproduction (unless indicated otherwise),
and data points in graphs represent the mean value of 30 replicate runs of
the input conditions for a trial.
In different trials described below, the 20 founders can be placed in the
center of the preserve in one of fi ve different confi gurations:
1. Central hollow square with six individuals on a side, males alternate
with females, no grid points between adjacent founders.
2. Two lines of 10 each, no grid points between adjacent founders, for
any given individual, the closest individuals are of an opposite sex
(juxtaposed confi guration).
3. Two lines of 10 each, no grid points between adjacent founders, one
line is all male, the other all female (unisex confi guration).
4. Two lines of 10 each, four spaces between adjacent individuals, sexes
juxtaposed.
5. Two lines of 10 each, four spaces between adjacent individuals, lines
unisexual.
We begin with analyses comparing trials where everything is held
constant except that populations have either bisexual individuals or
dioecious individuals. In these fi rst sample trials ( Figs. 14.1 and 14.2) ,
the 20 founders are always placed in a central hollow square. Population
growth rates are approximately equal for matched bisexual versus dioecious
populations when dispersal distance is either 10 (populations a versus g)
or 20 grid units (populations b versus l). The reproduction rate was 1.825
for the bisexual populations versus 3.65 for the dioecious populations,
demonstrating the effect of the halving of the number of offspring-producing
individuals in dioecious species. To be competitive with similar bisexual
species, dioecious species thus must have an elevated rate of offspring
production in female individuals compared to the average individual in a
bisexual species. This is supposedly one of the trade-offs involved in the
evolution of dioecy: since male investment occurs in separate individuals,
females have more resources available to invest in offspring production
(e.g., see Heilbuth et al. 2001).
Despite these similarities in population growth rates, some measures
of population genetic diversity differ between otherwise equivalent
bisexual versus dioecious populations. Dioecy, under the given conditions
and after 13 generations, appears to lower observed heterozygosity by
3.8% when dispersule maximum dispersal distance is 10, and by 2.2%
when maximum dispersal is 20 grid units. These results are refl ected in
the increased F values for the comparable dioecious populations. More