Dispersal - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 203

decreased inbreeding or increased retention of unique alleles) increase the
average fi tness of individuals of a population, selection to increase more
distantly distributed microgametes may have different effects from selection
to distribute offspring more widely. Imagine two identical populations
or species with conditions similar to those given above for population N
(microgamete and offspring maximum dispersal distances are equal, being 5
units). Call them N1 versus N2. If evolutionary forces act on N1 to increase
the average distance of effective microgamete dispersal, then this should
be reinforced by natural selection: increasing dispersal of microgametes
should improve population growth and genetic diversity retention ( Figs.
12.11 and 12.12) , at least up to a point. However, if evolutionary forces act
on population N2 that promote greater average offspring dispersal, if there
is no commensurate evolutionary force acting to increase microgamete
average dispersal, then the resulting decreased population growth and
genetic diversity would militate against the evolution of greater offspring
dispersal. If true, then for species with the characteristics described in this
and the earlier section (and if these were the only life history characters
that mattered!), we should fi nd that species with microgamete dispersal
distances exceeding offspring dispersal distances predominate. Obviously,
microgametes are smaller than offspring, are produced in larger numbers,
and can often be dispersed to greater distances on average.
Thus, within a species, there is a continuing tension between the
evolution of pollen versus fruit dispersal distances. When one expands,
the other must adjust, especially for pollen distance in the examples above.
For example, as fruits evolve to disperse farther, individuals that have
features promoting more distant pollen dispersal may be favored. Longer-
range dispersal is in some ways benefi cial up to a point. Or, if the fruits
of a species evolve to disperse a shorter distance, then maintaining high
pollen dispersal characteristics may be costly (e.g., smaller, less fi t pollen,
greater amounts of pollen production previously needed to ensure long-
distance dispersal, pollen wastage). Thus, the evolution of pollen dispersal
distance can affect the evolution of fruit dispersal distance and vice versa
in establishing populations. NEWGARDEN can be used to explore how the
changing evolution of the dispersal of one dispersule affects or constrains
the evolution of the other.
These considerations put emphasis on the importance of obtaining as
much knowledge as possible about the relative microgamete and offspring
dispersal characteristics for a species considered for reintroduction.
Upon analyses of the known facts, or upon monitoring reproduction in
an establishing population, NEWGARDEN modeling may suggest that
dispersal of one or another dispersule should be manipulated to improve
population growth and/or genetic diversity retention. These issues take
on increased relevance where the site of reintroduction may have lost the