Biology Reference
In-Depth Information
of 5 grid units; c, g, and k dispersed up to 12 grid units, and d, h, and l
dispersed to 30 units throughout population development. Populations
grew more rapidly with greater spacing between founders and with
increasing offspring and pollen dispersal distance ( Fig. 12.2) because of
reduced competition among offspring for viable sites.
When founders are most tightly packed, observed heterozygosity
declines most rapidly with short dispersal distance, this decline greatly
reduced when dispersal is 12 grid units or more ( Fig. 12.3A) . This more
rapid decline is due to a combination of more inbreeding (F) and greater
loss of unique alleles when dispersal is more localized (see population a,
Fig. 12.4A, and Fig. 12.5A) .
When there are 4 grid units between founders (graphs marked B in
Figs 12.2 through 12.5), observed heterozygosity declines and inbreeding
increases even more dramatically with short dispersal distances (trial e).
Recall that in all of these trials, Random Mating is set to true, meaning
that the relative amount of selfi ng increases when the number of eligible
microgamete donors is lower. When dispersal is only 2 grid units,
founders are separated by 4 grid units, and selfi ng can occur, the number
of self-matings will increase, driving observed heterozygosity down and
inbreeding up. This effect can be seen to an even greater degree when
founders were separated by 10 grid units (graphs labeled C in Figs. 12.3 and
12.4). With a maximum fruit and microgamete dispersal of 2 or 5, selfi ngs
will be more common in early phases of population development when
founders are more widely spaced. These heterozygosity and inbreeding
effects are greatly reduced when dispersal is 12 units or greater, promoting
greater gene fl ow (trials c and d, g and h, and k and l). Note that population
growth in trials e versus i (both have dispersal up to 2 grid units but i has
more spaces between founders), or in f versus j (dispersal up to 5 grid
units, j founders more widely spaced) shown in Fig. 12.2, graphs B and
C respectively, is greater when founders are more widely spaced. Even
though populations i and j are growing more rapidly than their respective
counterpart populations e and f with closer founder spacing (compare
graphs B and C of Fig. 12.2), loss of observed heterozygosity and increase
in inbreeding are greater in the more rapidly growing populations. In other
words, increased population growth will not always bring about relatively
decreased inbreeding or levels of homozygosity due to such spatial founder
effects. For some species restoration projects, it may be necessary to make
decisions over which is more important: avoiding inbreeding or rapidly
increasing population size. The former may be less important for species or
populations that are thought to have already purged deleterious alleles.
As for unique allele retention (Fig. 12.5), in general, more alleles are lost
when there is less space between founders (e.g., compare graphs A versus
C) and when dispersal is more limited. Again, just because a population
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