The Geographic Mosaic of Coevolution. John N. Thompson. xii
+ 443 pp. University of Chicago Press, 2005. $75 cloth; $28 paper.
The ability of any species to survive and reproduce is determined to
a large extent by other species—prey, predators, pathogens,
competitors and mutualists. This fact suggests that if we are to
have any hope of understanding why a given species has the
characteristics we observe today, we need to understand coevolution,
the coupled evolutionary changes that occur in sets of interacting species.
Explaining traits based on the mutual evolution of interacting
species dates back to Darwin, who noted that faster deer would
select for increased speed in wolves, and vice versa. Evolutionary
biologist John Thompson has arguably contributed more to our
understanding of coevolution than has any other living scientist. In
The Geographic Mosaic of Coevolution, his third book on
the topic, he elaborates an idea introduced in his second book,
The Coevolutionary Process (1994): the notion that
coevolution must be understood in the context of a physical
environment that is heterogeneous and often spatially discontinuous.
These characteristics mean that the evolutionary forces acting on a
given species will usually differ from place to place and that
evolutionary outcomes will depend on these local differences in
selection combined with movements of individuals and/or genes.
Of course, selection on any characteristic in any organism will vary
in space and time. Tolerance for high temperatures is selected for
more strongly in warmer locations and in hot years. No evolutionary
biologist can completely ignore spatial variation in conditions.
Evolution of characteristics involved in interactions with other
species—coevolution—is certainly no exception to this
To give a purely hypothetical example, some deer may find themselves
in a suburban neighborhood without a wolf within hundreds of
kilometers, whereas others may live in a Canadian national park with
a large and vigorous population of predators. Some may find
themselves coexisting with wolves but also with some other prey
species that the wolves prefer. Others may live in places where
there are no wolves but great speed is required to avoid heavy
traffic. If longer legs confer greater speed at a cost of increased
chance of broken legs, the length of a deer's legs will depend on
local selection for greater maximum speed and on the movement of
individuals to or from populations where speed is less of an
advantage. Wolves experience their own spatially varying selection
for traits that make them faster. Because any selective pressure
that affects speed in wolves is likely to also affect selection on
traits that determine speed in deer that are exposed to those
wolves, understanding the evolution of leg length in either species
requires an understanding of all factors affecting speed in both
species. There is no doubt that leg length or other traits related
to speed should vary spatially across a heterogeneous environment,
provided the genetic exchange between different areas is not so high
as to overwhelm adaptation to local conditions.
Thompson reviews a number of case studies in which the outcome of
coevolution differs from one area to another. For example, some
populations of lodgepole pine trees have cones that resist predation
by crossbills, birds specialized to extract conifer seeds from
cones. However, the cones' adaptations for resisting these birds are
much less developed where red squirrels are the main source of
mortality for pine seeds. Different adaptations are required to
defend cones against red squirrels. Thompson chooses to classify
different locations dichotomously as being coevolutionary
"hotspots" or "coldspots." Areas with many red
squirrels are a hotspot for pine-squirrel coevolution, but a
coldspot for pine-crossbill coevolution.
I have to admit that when Thompson first introduced his
"geographic mosaic" idea, I was rather puzzled by it.
Ecologists and evolutionary biologists have long known that any
species with a significant geographic range will experience
different conditions in different places—different physical
variables, different sets of interacting species and different
characteristics of almost any particular interacting species. Yet
there is no demand that every ecologist or evolutionary biologist
must study geographical variation to understand every single local
population. The key question is, Why should coevolution require a
special "geographic mosaic theory," when the purely
ecological study of between-species interactions does not have this
requirement, nor does the evolutionary study of traits that are not
involved in interactions? What is it about coevolution that makes it
different from these other fields? I am afraid I found no convincing
answer to this question in Thompson's new book.
In any evolutionary study, it is clearly important to be aware of
potential local differences in selection pressures and movements of
individuals for different populations. However, there have been
successful studies of coevolution of predators and prey and of
competitors in environments (such as small lakes) where gene flow
and spatial differences in selection were not key attributes of the
process. This is the case with Dolph Schluter and J. Donald
McPhail's well-known work on evolution in the three-spined
stickleback, work that is summarized quite nicely in Thompson's book.
On the other hand, Thompson is no doubt correct in arguing that many
biologists studying coevolution have devoted too little attention to
spatial variation. Until recently, evolutionary biologists studying
species interactions had not studied spatial variation to the extent
that population ecologists have done. The body of work summarized in
Thompson's new book provides a strong case for correcting this
imbalance by considering the impacts of spatial variation when
thinking about coevolution.
Thompson's own research on the interaction and evolution of a
variety of butterflies and moths with their host plants represents a
model of how fieldwork can help one to understand the nature and
magnitude of current-day interactions. His studies of the
coevolution of yuccas and yucca moths and of swallowtail butterflies
and their host plants are featured in many evolution textbooks. His
work has revealed several cases where interactions that had at one
time been classified as mutualistic have turned out to vary across
the full spectrum from mutualism to neutrality to parasitism. His
recent collaborations with Richard Gomulkiewicz and Scott Nuismer
have laid a firm basis for mathematical modeling of evolving traits
in two or more species distributed across a heterogeneous landscape.
All of this work is presented in a clear and simple manner in the
book under consideration here. However, The Geographic Mosaic of
Coevolution is far more than a review of Thompson's own work or
a discussion of how spatial variation alters the evolution of
interacting species. It is a review of the majority of empirical and
theoretical work on almost all facets of coevolution that has
appeared over the past decade.
Although such a review is needed, and Thompson does a thorough and
insightful job, trying to fit the diverse body of research on
coevolution into a single theory with codified assumptions,
hypotheses and predictions may not be the best way to advance
research in the field. It is not necessary to study everything as a
geographic mosaic. There are also reasons to question the generality
of the five assumptions and three hypotheses used to define the
Geographic Mosaic Theory, and the classification of adaptive
outcomes into seven major trajectories seems restrictive and
somewhat arbitrary. These trajectories are actually reflective of
the rather limited number of species assemblages in which
coevolution has been studied.
Take the example of "coevolutionary displacement," one of
Thompson's seven trajectories. Thompson defines this to be
divergence of competing species, although several researchers in the
field (I am one) have argued for definitions of
"displacement" that include any direction of change,
rather than just divergence (species becoming more dissimilar).
Given that there are easily a dozen mechanisms that in theory are
expected to produce convergence or parallel change in response to
competition, it seems premature to define the only coevolutionary
response to competition to be divergence. In Thompson's scheme,
convergence is restricted to being a consequence of mutualistic
interactions. Both convergence and divergence have also been
predicted as potential outcomes of predator-prey interactions.
Coevolutionary trajectories are likely to be far more diverse than
the limited number of previous studies has suggested.
Other aspects of Thompson's framework can be criticized as well. The
prediction that there will often be hotspots and coldspots of
coevolution appears to be almost tautological; every ecological or
evolutionary process is stronger in some places than in others. And
I would like to know why Thompson chose to retain the historical
restriction of the term "coevolution" to cases with
reciprocal evolutionary responses within both species of a pair of
interactors. The same models and the same message about the
potential importance of space apply to the evolution of any trait
that influences interspecific interactions of a single species
within a biological community, even if there is no coevolutionary response.
Although the entire complex edifice of geographic mosaic theory may
not stand the test of time, there is no doubt that Thompson's
emphasis on the implications of spatially varying selection will
help us to better understand the evolution of interspecific
interactions. His fieldwork and his theoretical collaborations will
be cited for many years to come. There is no more authoritative
source for the latest research in one of the most important areas of
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