Photo: Papyrus swamp, south-western Uganda.
I am interested in the interacting effects of natural selection, gene flow, and phenotypic plasticity on evolutionary diversification. Divergent natural selection among environments should result in adaptive genetic divergence. Yet, gene flow among selective regimes can constrain local adaptation. These interactions have been well-documented in previous studies. Less attention has been paid to the effects of phenotypic plasticity on adaptive divergence and gene flow. Phenotypic plasticity may permit colonization of novel environments and enhance population persistence by allowing flexible responses to environmental change. An initially plastic response to a novel environment may be followed by genetic changes that enhance the adaptive response (i.e. genetic accommodation). These changes may ultimately lead to reproductive isolation among populations through the process of ecological speciation. Alternatively, high gene flow among environments may result in selection for increased phenotypic plasticity, if the plastic response enhances fitness after dispersal to a new selective environment.
I studied these processes in natural populations of the cichlid fish Pseudocrenilabrus multicolor from Uganda. This species inhabits African lakes, rivers, and swamps that are characterized by differing levels of oxygen. Fish from hypoxic (low oxygen) environments have larger gills and smaller brains than fish from normoxic (high oxygen) environments. Larger gills are expected to be adaptive under hypoxia for enhanced oxygen uptake, and smaller brains are expected under these conditions for increased oxygen allocation for other functions. My work revealed that much of the phenotypic variation observed in nature is due to plasticity. I found gene flow to be high among populations from different oxygen regimes (i.e. no evidence for ecological isolation), and that plasticity is higher in populations among which the potential for dispersal and gene flow is greater. Therefore, plasticity might have permitted expansion into different environments, followed by the evolution of plasticity in this meta-population.
Crispo, E., and Chapman, L.J. (2010) Geographic variation in phenotypic plasticity in response to dissolved oxygen in an African cichlid fish. Journal of Evolutionary Biology 23: 2091-2103. (PDF)
Crispo, E. (2008) Modifying effects of phenotypic plasticity on interactions among natural selection, adaptation and gene flow. Journal of Evolutionary Biology 21:1460-1469. (PDF)
Crispo, E., and Chapman, L.J. (2008) Population genetic structure across dissolved oxygen regimes in an African cichlid fish. Molecular Ecology 17: 2134-2148. (PDF)