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Phil. Trans. R. Soc. B (2011) 366, 1401–1409
doi:10.1098/rstb.2010.0290
Review
Evolution in metacommunities
Charles J. Goodnight*
Department of Biology, University of Vermont, 120 MLS, Burlington, VT 05401, USA
A metacommunity can be defined as a set of communities that are linked by migration, and extinction and recolonization. In metacommunities, evolution can occur not only by processes that occur
within communities such as drift and individual selection, but also by among-community processes,
such as divergent selection owing to random differences among communities in species composition, and group and community-level selection. The effect of these among-community-level
processes depends on the pattern of migration among communities. Migrating units may be individuals (migrant pool model), groups of individuals (single-species propagule pool model) or
multi-species associations (multi-species propagule pool model). The most interesting case is the
multi-species propagule pool model. Although this pattern of migration may a priori seem rare, it
becomes more plausible in small well-defined ‘communities’ such as symbiotic associations between
two or a few species. Theoretical models and experimental studies show that community selection is
potentially an effective evolutionary force. Such evolution can occur either through genetic changes
within species or through changes in the species composition of the communities. Although laboratory studies show that community selection can be important, little is known about how important it
is in natural populations.
Keywords: metacommunity; indirect genetic effects; epistasis; multi-level selection;
community evolution; coevolution
1. INTRODUCTION
The evolution of communities is generally considered
to occur by processes of individual selection acting
within the individual community. From this perspective, community evolution can occur by the gain and
loss of species, and by genetic changes occurring
within species. Such evolutionary changes are limited
to changes occurring as a result of individual selection,
and any adaptation that occurs can only be adaptation of the individual to its environment. When a
community is spatially subdivided, that is, it is a ‘metacommunity’, a much richer range of evolutionary
forces becomes possible [1].
Evolution within a single community can only proceed by selection among units within that community.
Thus, the primary evolutionary change of interest will
be classical coevolution. Such coevolution occurs
when there are reciprocal selective pressures on traits
in separate species [2], a process that, at least for individual selection, is well understood (e.g. [3]). In this
coevolutionary process, the selection that is being
exerted on one species is a function of the character
state of its ‘partner’ species; however, this partner
species is effectively a part of the environment.
Although the partner species is part of the environment, it is of course true that it will evolve over time;
thus both species will be evolving to the continually
changing environment provided by their partner
* [email protected]
One contribution of 13 to a Theme Issue ‘Community genetics: at
the crossroads of ecology and evolutionary genetics’.
species. Thus, processes that have been described as
‘coevolutionary arms races’ can occur. When a community is divided into isolated ‘sub-communities’ to
form a metacommunity, coevolution can occur
through processes that are qualitatively distinct from
the simple pairwise individual selection described by
Janzen [2].
2. FACTORS AFFECTING METACOMMUNITY
EVOLUTION
In a metacommunity, there are three factors I will discuss that may affect metacommunity evolution. First,
the subdivision of the community into smaller communities will mean that not all communities will
have identical species compositions, and that evolution
can proceed by changing the presence and absence or
the abundance of individual species, irrespective of
genetic changes that may occur within the individual
species. Second, the species that are present in a community will have smaller population sizes, and there
will be opportunities for them to differentiate among
communities either by genetic drift, or by selection
in communities that differ in their composition.
Finally, with a subdivided community, there is the
potential for multi-level selection, both at the singlespecies group level and at the whole community
level. These different processes are not independent;
for example, shifts in the species present in a community will change the population sizes of the other
species, and the selection pressures acting in the communities; thus, changes in community composition
can potentially affect the within-species differentiation
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Review. Metacommunity evolution
of populations through selection and drift. Similarly,
changes in genetic differentiation and species composition will change the nature and response to
multi-level selection.
These three factors are tied together by a fourth
factor, and that is how communities are founded. To
introduce this it is convenient first to consider singlespecies groups. Wade [4] made a distinction between
populations founded via a ‘migrant pool’ model and
a ‘propagule pool’ model. In the migrant pool
model, individuals migrate out of their natal population and then group randomly to form new
populations. In contrast, in the propagule pool
model, groups of individuals migrate from the natal
population and together found a new population.
The difference between these two models is telling.
In the migrant pool model, migration is a homogenizing force that tends to reduce variance. By randomly
sampling from a set of natal populations any genetic
differences that arose among those populations is
reduced by the migration process. In contrast, in the
propagule pool model, because groups of founders
migrate from a single natal population to found a
new population, genetic differences among populations are preserved, and because the founder
propagules are a small subset of the natal population they may even enhance the differentiation of
populations [4].
Moving this model to the community level, there
are two levels at which ‘propagules’ can be recognized.
As with the single-species populations, individual
migrants or propagules may migrate to found new
communities. In addition, propagules may consist of
single species migrating independently of other species
(single-species propagule pool), or they may consist of
multi-species assemblages (multi-species propagule
pools; figure 1). As with the single-species migration
structure, the pattern of migration of communities
can profoundly affect the response to selection. The
importance of this distinction between single-species
propagule pools and multi-species propagule pools
will be discussed below.
The potential for multi-species propagule pools
depends on the type of community being considered.
At one extreme, there are large communities, such as
forest communities consisting of many species of
trees, shrubs, vertebrates, arthropods and other organisms. Such communities are unlikely in the extreme to
migrate together to found a new community. In the
middle may be communities such as the arthropod
communities on trees [5]. In most cases, these communities probably are colonized by migrant pools
and single-species propagule pools; however, there is
the potential that community colonization by a multiple-species propagule can occur. For example, a
multi-species propagule could result from a phoretic
species riding on its host. Finally at the opposite
extreme are communities that show adaptations that
specifically promote the movement of propagules of
intact communities. Examples of this include lichens
that have isidia or soridia, small packets of fungal
hyphae and algal cells that are wind dispersed [6],
Atta ant queens that carry packets of symbiotic fungi
that they use to found their new colony [7], and
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Figure 1. Three models of migration. (a) Migrant pool—
individuals migrate independently to found the next generation. Each ‘offspring’ community is the result of mixing of
individuals and species from many ‘parental’ communities.
(b) Single-species propagule pool—groups of the same
species migrate to found new populations. Within each offspring community, the constituent species each come from
one community (or a few communities), but the different
species come from different communities. (c) Multi-species
propagule pool—groups of two or more species migrate to
form new populations. The entire offspring community or
a multi-species subset of the offspring community comes
from a single parental community.
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burying beetles that carry with them phoretic mites
[8]. Finally, real communities may well be founded
by a mixture of migration patterns, with some species
migrating in as individuals (migrant pools), others
coming in as single-species propagules, and still
others arriving as multi-species propagules.
3. EVOLUTION IN METACOMMUNITIES
Once communities are formed, evolution in a metacommunity can occur by selection and drift acting
within individual communities, but it can also occur
through a process unique to the metacommunity
structure. Selection and drift acting within communities should proceed following the standard
population genetics theory. In brief, genetic drift and
potentially random extinctions will randomly change
the genetic structure and the species structure of
each community. Selection acting on individual-level
fitness differences will then act within the framework of this individual community. In addition, it is
important to recognize that the random forces of
drift and random extinction, and colonization will
cause the different communities to vary in the specific species present in each community, and in the
genetic makeup of those species. As a result, to the
extent that individual selection results in adaptation
to the specific details of the community in which a
population is found, selection will act differently in
different communities, and drive increased community
divergence.
If migration levels are high, species may adapt to the
weighted average of the community types rather than
to the individual communities. As a consequence,
a species may adapt relatively rapidly to common community types, but only vary slowly to rare community
types. This will be particularly interesting if there is a
negative genetic correlation between the fitnesses in
the different community types. This is a situation
that is similar to the evolution of phenotypic plasticity
models explored by Via & Lande [9]. In this case,
a species potentially may fail to adapt to the rarer
community types because of the negative genetic correlation. As an example, consider that a predator
may not be able to adapt to a particular prey species
if the prey species occupies only a few communities,
even if that prey species is abundant where it does
occur. Thus, the prey species may avoid being the
focus of predator adaptation by being globally rare,
even if it is locally abundant.
No models are available demonstrating that interactions among species can lead to selectively driven
diversification among communities, although a similar
effect is seen in the diversification of single-species
populations when there is gene interaction (dominance
or epistasis). In a series of models [10,11] I defined the
local average effect of an allele to be the subpopulation-specific average effect of an allele on the
phenotype of an individual measured as a deviation
from the metapopulation mean. Note that this differs
from the standard concept of average effect [12] in
that an allele has only a single average effect, but
may have different local average effects in each
subpopulation of a metapopulation. In these models,
Phil. Trans. R. Soc. B (2011)
C. J. Goodnight
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I demonstrated that when there were only additive
effects the local average effect of an allele (adjusted
for the population mean) was constant; however,
when there was gene interaction it was no longer constant. This means that alleles that have a positive effect
on the phenotype in one subpopulation may have a
negative effect in other subpopulations. As a result,
even if selection is acting in the same manner in all
subpopulations, when there is gene interaction it
can become a diversifying force causing demes to
differentiate genetically.
A similar effect will be seen in a metacommunity. If
there are indirect genetic effects [13,14] between
species this will have the potential to change the
local average effects of alleles in one species based
on the presence or absence of other species in the community, or because of different genetic strains of other
interacting species. If communities are established by a
migrant pool or a single-species propagule pool then
these indirect genetic effects will behave as environmental effects. This is because migrants leaving one
community and entering a new community will experience a new multi-species genetic environment. Thus,
the interspecies’ indirect effects will be experienced
as a genotype by environment interaction. By contrast,
if migration occurs as a result of multi-species propagule pools then the propagules have the potential to
carry the interspecies indirect genetic effects with
them. In this case, because interacting species move
together between communities, the interaction is
transferred with the propagule, and the interaction
can move from being a part of the environment to
being a heritable component of variation.
When this variation among communities is coupled
with extinction and recolonization [15] or differential
migration among communities [16], it is possible
that community-level selection can occur [17– 20].
Following a definition consistent with contextual
analysis, community selection occurs when the fitness
of an individual is a function of community membership ([21], see also [22– 25]). Alternatively, following
Wade [26], community selection may be defined as
the differential survival and/or reproduction of communities. These two definitions are consistent with
each other; however, the contextual analysis definition
is both more general and more controversial.
There have been two experimental studies of the
response to community selection, one by Goodnight
[17,18] examining the response to selection in twospecies communities of the flour beetles Tribolium
castaneum and Tribolium confusum, and the second by
Swenson et al. [19,27], examining the response of
communities of water and soil micro-organisms to
selection on community-level traits. Importantly, the
Tribolium experiments focused on communities with
fixed species compositions, and thus only genetic
changes could occur, whereas the micro-organism
communities used in the experiments of Swenson
et al. were diverse communities in which the response
to selection appeared to occur primarily through
changes in community composition. Both of these
sets of experiments used a multi-species propagule
pool. By contrast, Wilson [20] theoretically examined
community evolution with a migrant pool.
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4. EVOLUTION WITH FIXED SPECIES
COMPOSITION
Goodnight’s [17,18] selection experiment using
Tribolium illustrates several important features of
community-level selection [1]. In this experiment,
there were eight different treatments, each with two
replicates, for a total of 16 independent selection
treatments. Each selection treatment was a metacommunity consisting of 10 two-species communities,
and the ‘communities’ consisted of two species of
Tribolium flour beetles, T. castaneum and T. confusum.
These two species are fierce competitors, and the
environmental conditions were set so that the two
species were almost equal in competitive ability [28].
In continuous culture, one of the two species would
have driven the other to extinction; however, competitive exclusion was prevented by transferring 16
individuals of each species to the next generation.
In each community, four traits were measured.
These were population size in T. castaneum, population
size in T. confusum, emigration rate in T. castaneum and
emigration rate in T. confusum. Each of the independent selection treatments consisted of community
selection either up or down for one of the four traits.
Note that although these are traits measured on the
individual species, because these two species are
strong competitors all of these traits have the potential
to be influenced by heritable factors in both species.
Selection was performed by measuring the four
traits, ranking the 10 communities in a metacommunity by the value of the trait under selection in that
treatment, and choosing the five most extreme communities to be found in the next generation. Each of
the surviving communities founded two communities
in the next generation using a multi-species propagule
pool model.
The main result of this study was that community
selection was effective. A significant response to selection was seen for all four traits (e.g. figure 2). This
alone was important because community selection
has much in common with group selection, which
has often been considered to be less effective than
individual selection (e.g. [29], but see [4]). There
were a number of other interesting results from this
experiment as well.
First, a number of correlated responses to selection
were observed. These included both within species,
and more significantly, between species correlated
responses to selection. For example, selecting on population size in T. confusum led not only to an increase in
T. confusum population size, but also to an increase in
T. castaneum emigration rate (figure 3). These interspecies correlated responses can only occur if there are
between-species indirect genetic effects. That is, a
response to selection, in a species such as these without
cultural inheritance, will necessarily involve changes in
gene frequency in the individual species. An interspecies correlated response to selection implies that at a
minimum a genetic change in one species is affecting
traits in both species. This provides direct evidence
that community selection can act on interspecies
indirect genetic effects [14].
Second, in a follow up study to the selection experiment, I examined the genetic basis of the observed
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Figure 3. The correlated response of T. castaneum migration
rate to selection on T. confusum population size. Solid lines represent selection for increased T. confusum population size,
dashed lines represent selection for decreased T. confusum
population size. (Reprinted from [17]).
responses to selection [18]. To do this I took the
intact communities from the selection experiments
and subjected them to three different treatments.
The first treatment was simply the intact community
with both species present. In the second treatment,
I set up single-species populations from the twospecies communities, and in the final treatment, I set
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selected community
T. castaneum T. confusum
test strain
T. castaneum
test strain
T. confusum
assay A
assay B
assay C
Figure 4. Three assays performed in Goodnight [18] on the
selected communities. Assay A, intact community; assay B,
single-species populations; assay C, ‘reconstructucted’ communities with one species having come from the selection
experiment and the second species having come from a
naive test strain. (Reprinted from [18]).
up two-species communities; however, only one of the
species was from the selection treatment, with the
second species having come from a naive test strain
(figure 4). The intact communities confirmed that all
of the traits showed a significant response to selection
(assay A). By contrast, the response to selection was
not observable in any of the single-species populations
(assay B). This indicates that the ecological or genetic
structure of the community was required for the
responses to selection to be observed. From this
assay alone it is not possible to tell whether the lack
of response to selection is because of an evolved interaction, or because there is a genotype by environment
interaction such that the responses to selection are only
observed in the intact community. The reconstructed
communities (assay C) were important because they
restored the ecological setting of the communities, but
nevertheless disrupted the genetic setting of the selected
communities. In this assay, an interesting divergence of
results was observed. In the treatments in which there
was selection on emigration rate, the response to selection was restored in the reconstructed communities.
This indicates that much, if not all, of the genetic basis
for the response to selection was in the species selected,
and was only expressed in the community setting. By
contrast, in the treatments in which there was selection
for population size, the response to selection completely
disappeared in the reconstructed communities.
This indicates that the response to population size
resided entirely in the interaction between the two
species, and that the selected strains of both species
needed to be present for the response to selection to be
expressed.
It is interesting to compare this with another study
on similar, but unselected, communities. In this study,
Goodnight & Craig [30] examined the effects of coexistence on competitive outcome. They compared the
competitive outcome of communities that had been
allowed to coexist for 18 generations by individual selection with communities that had been maintained for the
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C. J. Goodnight
1405
18 generations as separate single-species populations and
had been placed in two-species communities for the first
time. We found that there was substantial genetic variation among communities, and that the communities
would respond rapidly to community-level selection,
but that there was no detectable difference between the
coexistence and the single-species communities. This
study indicates, that in agreement with other studies
[31,32], there is no evidence that competitive ability
can evolve by individual selection. However, there is
substantial evidence that community selection would
be successful in changing competitive outcome.
The conclusion that community selection, but not
individual selection, can modify competitive outcome
makes sense if it is recognized that competition is
an interaction between individuals. As a consequence,
there is a strong potential for interspecies genetic interactions (interspecies epistasis) and indirect genetic
effects that affect competitive ability to be high. In
his classic work on what are now called indirect genetic
effects Griffing [33], considering only single-species
populations, demonstrated that individual selection
could not act directly on indirect genetic effects; however, group selection was able to act on indirect effects.
As a consequence, for traits involving substantial interactions among individuals, individual selection would
typically be ineffective or even counterproductive,
whereas group selection could act on these interactions
and would therefore be far more effective. In a similar
manner it makes sense that, consistent with the lack or
response to coexistence, selection within communities
could not act on interspecies indirect effects and
epistasis, whereas community selection could act
on these interactions, and would lead to a response
to community selection.
5. EVOLUTION THROUGH SPECIES
REPLACEMENTS IN METACOMMUNITIES
In the experiments described above, the number and
identity of the species were fixed, and any evolutionary
change occurred as a result of genetic changes within
the species. It is also possible for evolution to occur
through changes in the species present in the communities. Wilson [20] developed a model in which
metacommunities diverge and evolve as a result of
changes in species abundances. In his model, species
were not allowed to evolve, but communities could
evolve by extinctions and changes in the abundance
of the constituent species. Wilson examined a situation
in which sub-communities within the metacommunity
were established by randomly sampling a fixed number
of individuals from a pool of 10 species. Because of the
random sampling, the sub-communities consisted of a
subset of the 10 species with the initial population
density for each species also being random. In some
simulations, all 10 species were forced into each community by assuming that there was a minimum
background density maintained by continual lowlevel migration. Species interactions were initially
modelled using Lotka – Volterra competition equations
involving competitive interactions among all of the
species within the community weighted by their frequency. In a second set of simulations, the simple
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two-species interaction coefficients were replaced by
three species interaction coefficients. Thus, for
example, the interaction term ahij was defined as the
effect of species j on species i in the presence of species
h (see [20] for the details of this model). The final
interaction between a pair of species (a†ij, the effect
of species j on species i) was taken as the weighted
average of the three way interactions taken across all
of the species present in the community. An important
result of this model was that the initial founding conditions of the sub-communities strongly affected the
within-community dynamics, and the same set of
interaction coefficients could develop communities
that shared no species in common. It is apparent that
in these simple models there were multiple stable communities, and which species set a community evolved
towards depended on the initial founding conditions.
This becomes particularly apparent because, as mentioned above, in many of the simulations all species
were forced to be present at a minimal frequency.
Thus, the different community compositions evolved
in spite of the fact that all species were present at
some level in all communities.
Wilson [20] also examined the potential for community-level selection by examining the effects of
multiple iterations of random community formation
and dispersal back into a global migrant pool. In the
simulation he identified a focal species, and made
the probability of extinction of the sub-community a
function of the frequency of the focal species in the
community. He found that when he exerted this selection pressure, the frequency of the focal species rapidly
declined, and that the global frequencies of the other
species adjusted such that there was a high probability
that communities that suppressed the focal species
would be formed.
This model is important in that it illustrates that
community evolution can occur strictly by species
replacements with no genetic changes taking place
within the species themselves. Importantly, in the
simulations in which the community-level selection
was applied he found that this higher level selection
force could lead to adaptive evolution in the form of
changes in global species abundances that favoured
communities that suppressed the focal species that
was disrupting the community.
In a separate study, Swenson et al. [19,27] experimentally examined a situation that was analogous to
that modelled by Wilson. Their study reports on two
independent experiments; however, the first experiment examining the evolution of soil communities
illustrates the results adequately. In this study, the
sub-communities were obtained by sampling soil
from a forest and using sterilized water to extract a
representative biotic community from the soil. Each
sub-community consisted of a single pot of sterilized
soil inoculated with soil extract, and 50 surface-sterilized Arabidopsis seeds. The trait selected was the
above-ground biomass of Arabidopsis (figure 5). Each
generation, after assessing the above-ground plant biomass in the up treatment, the three pots with the
highest (or lowest in the down treatment) aboveground biomass were selected, and the soil from
these pots was combined and used to inoculate
Phil. Trans. R. Soc. B (2011)
sterilized soil used in the next generation. The plants
for each generation were drawn from a constant
stock population. Thus, the plants were genetically
constant, and any response was due to changes in
the soil community composition.
As with all multi-level selection experiments [34],
there was a rapid response to selection. Unfortunately,
soil communities are difficult to measure, and
Swenson et al. [19] were not able to measure the
species composition of the soil communities, but
they were able to demonstrate that the soil nutrients
in the up and down lines were significantly different,
with the up line having higher levels of potassium,
zinc and phosphorus than the down line. Even without
the species composition data, their results are consistent with the Wilson model. That is, it is very probable
that the response to selection that they observed took
place primarily by species replacements, although
genetic changes within species cannot be ruled out.
6. CONCLUSIONS
These laboratory studies and models of community evolution make several important points. Most important is
to recognize that the members of a community interact,
and because of these interactions, the effects of genetic
changes may be seen in species other than the species
in which the genetic changes occurred. The consequences of these indirect effects will depend on the level at
which the selection is acting, and on the manner in
which new communities are founded. We can recognize
three possible levels of selection: individual selection,
single-species group selection and multi-species group
selection, and three forms of migration: migrant pool,
single-species propagule pool and multi-species propagule pool. This results in nine possible combinations of
selection and migration pattern. It is excessive to discuss
all nine of these possibilities; however, it is worth
contrasting several of the possibilities.
First, it is instructive to contrast individual selection
and migrant pool migration with community selection
and a multi-species propagule pool migration. If selection is acting at the individual level, and communities
are founded by migrant pool migration then indirect
genetic effects and genetically based interactions will
be the evolutionary equivalent of environmental effects.
As Griffing [33] has pointed out, in many cases these
indirect effects will be negatively correlated with the
direct effects. In other words, in many cases, a response
to individual selection in one species will have negative
fitness consequences for other interacting species, and
quite probably for the community as a whole. Thus,
we can expect that in many cases pure individual selection will be disruptive to community function and
cohesiveness. By contrast, community selection acts
on the community as a whole. Not surprisingly, it can
act on all of the genetic effects and interactions that
are internal to the community [35]. If, furthermore,
communities are founded by multi-species propagule
pool migration, then these interactions will be transferred intact between generations. Thus, we should
expect community-level selection to favour a balance
of direct and indirect effects that maximizes the overall
fitness of the community.
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C. J. Goodnight
Review. Metacommunity evolution
1407
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Figure 5. Results of Swenson et al. [19] ecosystem selection experiments expressed as deviations from the overall means.
(a) Difference between above-ground biomass of Arabidopsis thaliana grown in soils selected for high and low biomass with
6.0 g inoculum. (b) Difference in above-ground biomass with 0.06 g inoculum. Open upward-pointing triangles are communities selected for high value of the trait, solid downward-pointing triangles are communities selected for low values of
the trait. Stars indicate significant differences. (Reprinted from [19]).
If selection is acting solely at the level of the individual, then it is interesting to contrast the migrant pool
with the multi-species propagule pool migration. From
the perspective of community evolution, this is most
interesting from the perspective of coevolution. If
migration is based on a migrant pool (or single-species
propagule pool), classical reciprocal individual-based
coevolution can potentially occur, but such coevolution
will be one species adapting to the metacommunity average for the interacting species. If migration takes the
form of a multi-species propagule pools, the same,
generally, negative effects of individual selection on
community would be expected. However, the multispecies propagule pool should allow coevolution
between pairs of specific strains of interacting species,
rather than to the average of the competitive pair to
occur. Goodnight [36] looked for, but was unable to
detect, this kind of coevolution in competing strains of
T. confusum and T. castaneum. Although this one experiment failed to detect such coevolution, the possibility
that it can occur cannot be ruled out.
Phil. Trans. R. Soc. B (2011)
At the opposite extreme it is useful to compare the
effects of community selection under a migrant pool
(or single-species propagule pool) model with the
response under a multi-species propagule pool model
of migration. Community selection, can act on any genetic effects that are within the community. Goodnight
[18] showed that in his two species Tribolium system,
migration and population size were qualitatively different in how they responded to community selection. In
the case of migration, the response to selection was
observed not only in the intact communities, but also
in the reconstructed communities. In these reconstructed communities only the selected strain of the
species expressing the trait was present, with the competing species being a standard laboratory strain. This
suggests that migration would have responded to selection under any of the three migration models. That is,
even if the community is broken up, each generation
community selection will still be effective at changing
the migration rate. By contrast, Goodnight [18] only
observed a response to selection for population size in
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1408
C. J. Goodnight
Review. Metacommunity evolution
the intact communities. This indicates that the
response to selection resides in a genetic interaction
between the two species, and that the response to community selection would be far greater when the two
species migrate together in a multi-species propagule
pool than in either a migrant pool or in a single-species
propagule pool. Thus, in the migrant pool form of
migration, community evolution should be limited to
adaptations that are due to genetic changes that are
entirely within a single species. On the other hand, a
multi-species propagule pool will allow multi-species
genetic interactions to evolve in which interacting
genes affecting community-level fitness are distributed
across two or more species. It is this sort of distributed
response to community selection that contributed to
the response to population size in Goodnight’s [18] study.
Although less is known about evolution through
species replacements, it is reasonable to suspect that
the same general issues arise. As an adaptive process,
community evolution by species replacements can only
occur through community-level selection. Pure individual selection and single-species group selection are
forces that act within species, and species replacements
are necessarily a multi-species process. Nevertheless,
the distinction between the migrant pool and the
single- and multi-species propagule pools will influence
the evolutionary process. Particularly relevant here is
Wilson’s [20] simulation in which community-level
selection was imposed that favoured reducing the frequency of a focal species. In this simulation, Wilson
observed adaptive changes in community composition
that suppressed the frequency of the focal species. It is
tempting to speculate that this process might have
been faster had he used a multi-species propagule pool
model of migration rather than a migrant pool model,
the reasoning being that sets of species that were particularly good at suppressing the focal species would migrate
together in the multi-species propagule pool. This may
have allowed the evolution of favourable multi-species
interactions in a way that would not have been possible
with the migrant pool.
Much remains to be done examining the metacommunity genetics, and selection and evolution at the
community level. There have been only two sets of
experiments [17 – 19,27] examining community-level
selection, and only one model of community selection
[20]. These experiments and models have established
that evolution by community selection can be an effective force in the laboratory. What is needed is some
sense as to how strong community-level selection is
in natural populations. Specific experimental protocols
for studying community selection in natural populations have not been described; however, methods,
such as contextual analysis (e.g. [37– 40]), that have
been developed for studying single-species group
selection in natural populations, should be readily
adaptable to studying community selection. As important is examining the heritability of community-level
traits. This will be complicated by the difficulty of
defining how the heritability of such traits should be
measured, but an understanding of the mechanisms
by which communities are founded would go a long
way towards addressing this problem. It will not be
an easy task to study community selection in nature;
Phil. Trans. R. Soc. B (2011)
however, the question of how important it is in evolution
is an empirical one that can only be answered by empirical studies of natural populations.
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