Perennials - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 225

comes from nearer the maternal parent (43% of the time pollen comes from
within the 0-6 units frame for the O-o trials), for offspring that are dispersed
to great distances, no other pollen donor will frequently be available (recall
also that 50% of the seeds dispersed are females, and that age is a factor in
both fruit and pollen production). In a sense, these results refl ect Baker's
law (reproductive assurance hypothesis; Baker 1955), which suggests that
species that are commonly dispersed long distances whereby individuals
are frequently isolated should have mechanisms for self-fertilization, which,
of course, these dioecious individuals do not. In contrast, when 43% of the
pollen comes from the 23-301 grid units frame (P and p trials), distantly
dispersed individuals are more likely to have a potential pollinator. Thus,
P-p populations grow more rapidly, O-o populations more slowly.
Population growth can thus be greatly affected by placement of the
founders and differences in pollen versus offspring dispersal. For example,
on average, o populations (founders at corners) realize on average only
around 70% of the growth achieved by O populations (founders in central
square). When founders are placed in a central square, population growth
rates exhibit approximately a 50% increase when more pollen comes from
greater distances (P) compared to when offspring are dispersed more closely
to the maternal plant.
Levels of observed heterozygosity are also affected in various ways by
these different trial conditions ( Fig. 15.2) . First, note that heterozygosity
remains constant in the earlier generations. Since heterozygosity of the
founders is at 99%, there is little or no loss of heterozygosity as age-specifi c
mortality causes a decrease in the number of founders ( Fig. 15.1) . During this
period of decrease, there is no initial reproduction (age-specifi c reproduction
does not begin until year 6 (pollen) or 7 (offspring)), and then reproduction
increases in an age-specifi c manner (output increases with age until dropping
off in the last few years of a given individual's life). Once reproduction
increases, heterozygosity decreases because of inbreeding. Placing founders
in a central square (B) increases the maintenance of heterozygosity compared
to placing founders in smaller groups at each corner (A; a difference of
approximately 4.2% exists between treatments P and p). The latter founder
positioning appears to induce more inbreeding (and see discussion of the
next fi gure). For both types of positioning, relative to basic trials M and m
in which dispersal to or from each frame is 25%, under the given conditions
the following was observed: (1) dispersing offspring more closely to the
maternal plant with pollen dispersal remaining the same (N and n) has no
appreciable effect; (2) when more pollen arrives from nearer to the maternal
plant (O and o trials), heterozygosity decreases (more local inbreeding); and
(3) when more pollen is dispersed from greater distances from maternal
plants (trials P and p), heterozygosity increases. Note that these differences
are not pronounced, although there is approximately a 6% difference in the