C.M. Caruso—Research statement
Understanding natural selection is a fundamental problem in biology because it is the only
mechanism that can cause adaptation. While the theory of natural selection dates back to Darwin,
most estimates of selection in natural populations have been made in the last 25 years.
Consequently, we are only now in the position to answer more general questions about natural
selection. My research addresses two of these questions:
1. What are the ecological causes of selection?-Natural selection differs from other
mechanisms of evolution in being a fundamentally ecological process. Selection occurs when an
ecological factor causes individuals with certain traits to have higher survival or reproduction.
Although understanding the ecological causes of natural selection is necessary to predict whether
organisms will adapt to novel environments, selection is often estimated without identifying its
cause. My research in this area has led to two key findings:
Interspecific competition can influence the strength of natural selection.-Darwin hypothesized
that interspecific competition for limited resources results in character displacement when
species distributions overlap. A key but largely untested prediction of this hypothesis is that
natural selection on resource-related traits differs in the presence and absence of a competitor.
Consistent with this prediction, my Ph.D work demonstrated that the presence of a competitor for
pollination alters the strength of selection on floral traits. My research provides some of the best
support to date for the hypothesis that competition promotes phenotypic diversification by
altering natural selection.
Pollinators may not be the most important cause of selection on floral traits.-Biologists since
Darwin have assumed that mutualistic pollinators are the primary agent of selection on floral
traits. However, antagonists such as herbivores may also exert selection these traits. If so, then
many floral traits may be adaptations to reduce herbivory rather than to increase pollination.
Alternatively, floral traits may evolve in response to diffuse selection by pollinators and
herbivores. We have demonstrated that although pollinators can select on floral traits, there are
populations where much of the selection on these traits cannot be attributed to pollinators. In
some populations, seed herbivores are the cause of selection on floral traits. But even in
populations where plants interact with both pollinators and herbivores, selection is not
necessarily diffuse. Instead, some floral traits can still evolve in response to strong, pairwise
interactions with pollinators.
2. Is adaptive evolution genetically constrained?-Although natural selection is strong in the
wild, organisms are not perfectly adapted to their environments, suggesting that their evolution is
constrained. However, the relative importance of selection versus constraint in shaping adaptive
evolution is unclear. My research in this area has led to two key findings:
Genetic correlations can constrain the evolution of floral displays.-Negative genetic correlations
between traits should constrain their adaptive evolution, but there is only weak empirical
evidence for this prediction. In particular, negative genetic correlations between flower size and
number are hypothesized to constrain the evolution of optimal floral displays, yet few studies
have documented such a trade-off. My work indicates that flower size and number are negatively
genetically correlated in natural populations. This correlation causes the predicted response to
selection for flower size to be less than or opposite in sign to what selection would favor,
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C.M. Caruso—Research statement
indicating a genetic constraint on adaptive evolution of this trait. My work provides some of the
best support to date for trade-offs between flower size and flower number, a common assumption
in models of floral evolution.
Genetic correlations may not constrain the evolution of traits controlling carbon and water
balance.-Correlations among functionally related traits are ubiquitous in both plants and animals.
These correlations could be the result of genetic integration, which prevents the production of
unfit trait combinations, but also constrains evolution in response to any selection for novel
combinations. Alternatively, natural selection against unfit trait combinations could maintain
correlations in the absence of any genetic integration. To distinguish between these two
hypotheses, we estimated genetic correlations between two physiological traits, photosynthesis
and transpiration, which are important determinants of growth and reproduction in plants.
Phenotypic correlations between photosynthesis and transpiration are positive because stomata
must be open to allow CO2 to diffuse into leaves, but water is lost by transpiration via these same
pores. However, we did not find a significant genetic correlation between these traits. This result
suggests that photosynthesis and transpiration are not genetically integrated and thus would not
be constrained from evolving in response to novel selective pressures.
New directions: Evolution of gender dimorphic plants-I have recently begun studying gender
dimorphic plant species because they provide a unique opportunity to examine how ecology and
genetics interact to affect evolution. Although most plants produce hermaphroditic flowers,
gender dimorphism has evolved repeatedly in the angiosperms. Dimorphism is more common in
certain environments, suggesting that there is an ecological context for its evolution. At the same
time, the genetic basis of sex determination in plants is complex and varies among populations.
In collaboration with Andrea Case at Kent State University, I am testing how ecology and
genetics interact to generate sex ratio variation among populations of a gynodioecious (female +
hermaphrodite) species. With funding from the National Science Foundation, we will be
examining whether variation in female frequency among populations can be explained by
variation in fitness among hermaphrodite genotypes, a mechanism known as the cost of
restoration. Models predict that the cost of restoration is a key determinant of variation in
population sex ratios in gynodioecious species. By using a novel approach to estimating cost, our
work will be the first to empirically test this prediction in natural populations.
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