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Convergent phenotypes often result from similar underlying genetics, but recent work suggests convergence may also occur in the historical order of substitutions en route to an adaptive outcome. We characterized convergence in the mutational steps to two independent outcomes of tetrodotoxin (TTX) resistance in separate geographic lineages of the common garter snake ( Thamnophis sirtalis) that coevolved with toxic newts. Resistance is largely conferred by amino acid changes in the skeletal muscle sodium channel (NaV1.4) that interfere with TTX-binding. We sampled variation in NaV1.4 throughout western North America and found clear evidence that TTX-resistant changes in both lineages began with the same isoleucine-valine mutation (I1561V) within the outer pore of NaV1.4. Other point mutations in the pore, shown to confer much greater resistance, accumulate later in the evolutionary progression and always occur together with the initial I1561V change. A gene tree of NaV1.4 suggests the I1561V mutations in each lineage are not identical-by-decent, but rather they arose independently. Convergence in the evolution of channel resistance is likely the result of shared biases in the two lineages of T. sirtalis-only a few mutational routes can confer TTX resistance while maintaining the conserved function of voltage-gated sodium channels. [ABSTRACT FROM AUTHOR]

A trend for larger males to obtain a disproportionately high number of matings, as occurs in many animal populations, typically is attributed either to female choice or success in male-male rivalry; an alternative mechanism, that larger males are better able to coercively inseminate females, has received much less attention. For example, previous studies on garter snakes (Thamnophis sirtalis parietalis) at communal dens in Manitoba have shown that the mating benefit to larger body size in males is due to size-dependent advantages in male-male rivalry. However, this previous work ignored the possibility that larger males may obtain more matings because of male-female interactions. In staged trials within outdoor arenas, larger body size enhanced male mating success regardless of whether a rival male was present. The mechanism involved was coercion rather than female choice, because mating occurred most often (and soonest) in females that were least able to resist courtship-induced hypoxic stress. Males do physically displace rivals from optimal positions in the mating ball, and larger males are better able to resist such displacement. Nonetheless, larger body size enhances male mating success even in the absence of such male-male interactions. Thus, even in mating systems where males compete physically and where larger body size confers a significant advantage in male-male competition, the actual selective force for larger body size in males may relate to forcible insemination of unreceptive females. Experimental studies are needed to determine whether the same situation occurs in other organisms in which body-size advantages have been attributed to male-male rather than male-female interactions. [ABSTRACT FROM AUTHOR]

Examines the geographic pattern of genetically determined resistance in the garter snake Thamnopis sirtalis over much of its ecological interaction with toxic newts of genus Taricha. Importance of geographic structure to coevolutionary interactions between predators and prey; Predictions of the geographic mosaic process; Evidence which suggests that selection is essential in determining population variation in resistance.

Presents information on a study which involved a large overwintering aggregations to quantify sources and determinants of mortality in a garter snake population in central Canada. Materials and methods; Sources of mortality; Biases involved with each cause of mortality.

Presents information on a study which examined the adaptive evolution of growth rate in the garter snake Thamnophis elegans. Methodology of the study; Results and discussion on the study.

Presents a study which evaluated the factors in the predatory-prey interaction between toxic newts and their resistant garter snake predators. Experimental procedures; Results and discussion.

Deals with a study which compared the genetic variance-covariance matrices for meristic traits in two populations of garter snakes, Thamnophis elegans. Samples of snakes and scoring of traits; Matrix comparisons; Results and discussion.

Presents a study which investigated whether resistance to the neurotoxin of a prey and locomotor performance were negatively associated in a species of garter snakes. Genetic basis of the trade-off; Methodology of the study; Results and discussions.

Do mating preferences evolve at the same rate in males and females? In allopatric species, male and female mate preferences evolved at similar rates, while in sympatric species, female mate preferences developed more rapidly than that of males. Consequently, these conclusions lend support to the interference hypothesis, which states that in sympatric species, evolution of female mate preferences in one species reduces the selection pressure for evolution of male mate preferences in the other species. [Extracted from the article]