Representative publications:

Pfennig, D. W. & Murphy, P. J. 2003. A test of alternative hypotheses for character divergence between coexisting species. Ecology 84: 1288-1297.

Pfennig, D. W. & Murphy, P. J. 2002. How fluctuating competition and phenotypic plasticity mediate species divergence. Evolution 56: 1217-1228.

Pfennig, D. W. & Murphy, P. J. 2000. Character displacement in polyphenic tadpoles. Evolution 54: 1738-1749.

Overview:
I am interested in the ecological and developmental mechanisms that cause the origin of new species (speciation).  Regarding the former, ever since Darwin evolutionary biologists have hypothesized that a major factor in the origin of species is competition for resources.  Yet, the role of resource competition in promoting divergence has been difficult to demonstrate empirically.  Species that respond to competitors through phenotypic plasticity are ideal for evaluating if and how competition promotes phenotypic divergence between interacting species.  A causal link can be established between the presence of competitors and character change if, in the presence of a competitor, an individual facultatively expresses an alternative phenotype less like its competitor’s phenotype.

I have been using spadefoot toad tadpoles to study whether and how competition promotes species divergence (for an overview of the spadefoot system, see the previous section on Evolution and Development of Alternative Phenotypes).  In ponds containing two spadefoot species, one species, Spea multiplicata, tends to develop into the omnivore morph, whereas the other species, S. bombifrons, tends to develop into the carnivore morph.  Each species produces both morphs, however, in ponds where the other species is absent. 

These two species differ in competitive abilities: S. multiplicata carnivores are competitively inferior to S. bombifrons carnivores, whereas S. bombifrons omnivores are competitively inferior to S. multiplicata omnivores. These competitive differences appear to reflect differences between species in their foraging behavior. In particular, because they spend more time actively searching in the water column, S. bombifrons tadpoles forage more efficiently for shrimp. In contrast, S. multiplicata spend more time grazing on the pool margins and bottom and, thus, forage more efficiently for detritus.

Resource competition leads to divergent trait evolution (“character displacement”).  In controlled laboratory populations, S. bombifrons (the superior competitor for shrimp, but the inferior competitor for detritus) develops in the presence of S. multiplicata almost exclusively into carnivores, whereas S. multiplicata (the superior competitor for detritus, but the inferior competitor for shrimp) develops in the presence of S. bombifrons almost exclusively into omnivores (Fig. 6).

Fig. 6 (click to enlarge)

This pattern of character displacement has not only lead to the evolution of enhanced differences between species; it has also generated heritable divergence in carnivore production between high and low elevation populations of S. multiplicata in southeastern Arizona.  At high elevations, where S. bombifrons are absent, S. multiplicata maintain plasticity to express the carnivore morphology.  In contrast, at lower elevations, where S. bombifrons are present, S. multiplicata have evolved reduced propensities to express the carnivore morphology (Fig. 7).  Moreover, this pattern of divergence in morph production is “replicated” in different populations that experience similar selective pressures (“parallel divergence”).  Thus, selection imposed by competitors has lead repeatedly to the evolution of a canalized omnivore phenotype, which is distinct from the competitors’ carnivore phenotype.

Fig. 7 (click to enlarge)
 

Once populations begin to diverge in morph production, hybrids produced by matings between populations should be competitively inferior to offspring produced by matings within populations.  Selection to avoid costly mismatings and mate assortatively (i.e., mate with individuals from the same population) should then contribute to speciation by generating reproductive isolation between populations.  Hence, by creating contrasting selective pressures between populations that do and do not occur with competitors, competition may trigger speciation.  My current research is aimed at examining the key events early in the speciation process.

Finally, I am interested in examining whether developmental plasticity generally accelerates evolutionary diversification.  I am specifically interested in testing the hypothesis that developmental plasticity (polyphenism) facilitates character displacement, which, in turn, promotes speciation.  This research, therefore, points to a largely unexplored role for development in evolution.  In contrast with the conventional view that development primarily constrains evolution, this research suggests that development can be an important diversifying force in evolution.