Evolution and Development of Alternative Phenotypes
 

Representative publications:

Frankino, W. A. & Pfennig, D. W. 2001. Condition-dependent expression of trophic polyphenism: effects of individual size and competitive ability. Evolutionary Ecology Research 3: 939-951.

Pfennig, D. W. 1992. Polyphenism in spadefoot toad tadpoles as a locally-adjusted evolutionarily stable strategy. Evolution 46: 1408-1420.

Pfennig, D. W. 1992. Proximate and functional causes of polyphenism in an anuran tadpole. Functional Ecology 6: 167-174.

Overview:
A central goal of evolutionary biology is to understand why living things are so diverse.  One way to achieve this goal is to study the process of diversification within species, where diversity is ubiquitous and often pronounced.   Therefore, in order to understand the origins and evolution of diversity, I study organisms that display polyphenism (for examples, see Fig. 1).  In particular, much of my research has focused on spadefoot toad tadpoles (genus Spea), which develop into either a small-headed omnivore morph or a large-headed carnivore morph (Fig. 3).  I have been using these tadpoles as a model system to address the following two questions: 1) What selective factors favor alternative phenotypes? 2) How do alternative phenotypes arise developmentally?

Fig. 3 (click to enlarge)

Regarding the first question, alternative phenotypes are maintained, in part, as an adaptive response to environmental heterogeneity.  Spadefoots breed in temporary rain-filled pools, and carnivores are favored in the most ephemeral ponds (Fig. 4) where shrimp are most abundant and where the carnivore's rapid growth and development increase the chance that tadpoles will metamorphose before the pond dries.  Omnivores, conversely, are favored in longer lasting pools, because they tend to metamorphose in better condition.  However, in ponds that are intermediate in duration, competition for food becomes paramount.  Experimental manipulation of morph frequencies within natural ponds reveals that competition is keenest among phenotypically similar individuals (Fig. 5).   Because detritus (the chief food source for the omnivore) and shrimp (the chief food source for the carnivore) are both limited in natural ponds, natural selection favors alternative phenotypes as an adaptive means to reduce competition for food.

Fig. 4 (click to enlarge)

Fig. 5 (click to enlarge)

Regarding the second question above, alternative phenotypes often arise developmentally through environmental-dependent gene expression (polyphenism).  For example, Spea tadpoles are born as omnivores, but they may develop into carnivores if they ingest shrimp early in life.  However, certain families are more prone than others to produce carnivores when given shrimp, and there is evidence that a major gene regulates the polyphenism, possibly by causing developmental reorganization of preexisting phenotypes (“heterochrony”).

Spea tadpoles, therefore, are a model for understanding the evolution and development of alternative phenotypes. Moreover, because alternative phenotypes may represent an important phase of evolution (West-Eberhard 2003), such alternatives also provide an ideal setting in which to explore how variation within species can cause diversity between species (see Ecology, Development, and the Origin of Species).