opposite to the prediction of the hypothesis, but
evidently more work is needed to confirm these
results.
A specific prediction derived from this hypothe-
sis is that an increase in community diversity
through invasion by a new species will lead to a
reduction of genetic variation in the native species
within that community. So far, changes in genetic
variation as a result of community invasion have
been assessed in only one study system. Hild and
co-workers studied genetic variability in three na-
tive grass species after invasion of Russian knap-
weed (Acroptilon repens) and found a significant
reduction in genetic variability after invasion in
one native species, but not in the two others (Mea-
loret al. 2004). Also, genetic similarity of both grass
species was smaller between invaded and non-in-
vaded communities than within community type,
indicating that there is natural selection in response
to community invasion (Mealor and Hild 2006).
Consistent with this finding, an increasing number
of studies shows that natives do show phenotypic
changes after exotics have been introduced (re-
viewed in Strausset al. 2006; Strayeret al. 2006)
and these studies may provide indirect evidence
on changes in genetic diversity of native species.
Adaptation to exotics occurs often in the form of
shifts in single traits due to directional selection,
potentially decreasing genetic variation in these
traits. On the other hand, in the case of plant inva-
sions, native phytophagous insects can even be ex-
posed to divergent selection if the induced
evolutionary change involves adaptation to a
novel host plant, which would enhance genetic
variation. At present, we lack sufficiently detailed
studies to draw any clear conclusions on this hy-
pothesis.
11.6 Future directions
The empirical evidence presented in this chapter
overwhelmingly supports the reciprocal relation-
ship between community ecology and evolutionary
biology. Genetic diversity and phenotypic plasticity
of component species can shape community struc-
ture and composition. Similarly, interspecific inter-
actions and the species composition of communities
can influence evolutionary change of species. How-
ever, we are only at the beginning of the difficult
process of integrating evolutionary principles with
the community level. There is an obvious need for a
greater number of field studies to show the gener-
ality of the results discussed above, especially from
the part of the evolutionary biologists. Also, the
range of taxa involved in these studies needs to be
expanded, because the majority of studies concern
plants. Because for some communities measuring
genetic and phenotypic diversity for all species will
be a prohibitively large undertaking, a careful
choice of study systems as well as inventive use of
molecular tools is necessary.
If an integrated approach succeeds in adding
significant explanatory and predictive power, it
may develop into a new discipline of evolutionary
community ecology. A true integrative approach to
evolutionary community ecology can address a
much wider range of issues than just the reciprocal
effects of genetic and species diversity that are cur-
rently the main focus. Looking from an evolution-
ary perspective, a crucial question is if, and to what
extent, genetically diverse species persist longer in
a community than genetically uniform species. This
is particularly relevant if one considers plant spe-
cies that can propagate clonally, or animals that
reproduce parthenogenetically. Another essential
question is the relationship between community
composition and evolutionary change of species
within communities. To what extent can community
composition drive selection on traits and can diver-
gent selection caused by differences in community
composition perhaps initiate ecological speciation?
On the other hand, multispecies interactions invol-
ved in diffuse coevolution within communities may
constrain the possibilities for evolutionary change of
trait values, but favour the development of plastic
phenotypes.
A multitude of key questions can also be ad-
dressed from a community ecological point of
view. One question I would regard as crucial is
whether species at particular trophic positions in a
community are more likely to be genetically di-
verse? If genetic diversity is maintained by fine-
scale environmental variation in interspecific inter-
actions, the degree of connectiveness of species may
govern their genetic diversity to a large extent. A
second major question is the relationship betweenEVOLUTIONARY PROCESSES IN COMMUNITY ECOLOGY 161