The coevolutionary process: The effects of population structure on a predator-prey system

The common garter snake Thamnophis sirtalis and the rough skinned newt Taricha granullosa are engaged in a coevolutionary arms-race in the Pacific. Northwest of the United States. T. granulosa has developed high levels of tetrodotoxin (TTX), a deadly neurotoxin, for defense: T. sirtalis has evolved high levels of resistance to TTX in response to elevated TTX toxicity. The geographic mosaic theory of coevolution (GMTC) predicts that the strength of coevolution differs between spatially separated populations, resulting in interpopulation phenotypic variability. Gene flow among populations acts to homogenize phenotypes across populations, potentially resulting in maladaptation depending upon the strength of selection and the rate of gene flow. It is critical to know these strengths to understand the evolutionary dynamics of coevolving traits.; I researched the three different aspects of coevolution: selection, geographic variability, and genetic remixing. Chapter 2 presents a technique to measure the strength of selection from specific sources, which can be used to quantify reciprocal (coevolutionary) selection. Analyses of simulated data sets emphasize the importance of the correlation between interacting phenotypes to generate strong selection. The relationship between resistance in adult and neonate snakes was investigated in Chapter 3. Mass adjusting the dose of TTX given to an adult garter snake yields quantitatively equivalent estimates of resistance. Tetrodotoxin resistance in Thamnophis couchii was undocumented prior to Chapter 3. Chapters 4 and 5 focus on the measurement of population structure (an indirect estimate of gene flow) and phenotypic change between populations in snakes and newts. Phenotypic measurements utilized the methods in Chapter 3. Three transects were chosen for fine-scale measurements. Neighboring populations showed little structure in both species, possibly due to Pleistocene glaciation. Isolation-by-distance was seen on a broad geographic scale. These studies show that unbalanced gene flow generates maladaptation in this system and that the degree of maladaptation is geographically variable. Putative coevolutionary hot spots seem to influence maladaptation. Hot spots must be verified using proper methods (described in Chapter 2). Maladaptation affects snakes more than newts as predicted by the life-dinner principle. Chapter 5 verifies several predictions made by the GMTC.