Measuring local adaptation can provide insights into how coevolution occurs between predators and prey. Specifically, theory predicts that local adaptation in functionally matched traits of predators and prey will not be detected when coevolution is governed by escalating arms races, whereas it will be present when coevolution occurs through an alternate mechanism of phenotype matching. Here, we analyse local adaptation in venom activity and prey resistance across 12 populations of Northern Pacific rattlesnakes and California ground squirrels, an interaction that has often been described as an arms race. Assays of venom function and squirrel resistance show substantial geographical variation (influenced by site elevation) in both venom metalloproteinase activity and resistance factor effectiveness. We demonstrate local adaptation in the effectiveness of rattlesnake venom to overcoming present squirrel resistance, suggesting that phenotype matching plays a role in the coevolution of these molecular traits. Further, the predator was the locally adapted antagonist in this interaction, arguing that rattlesnakes are evolutionarily ahead of their squirrel prey. Phenotype matching needs to be considered as an important mechanism influencing coevolution between venomous animals and resistant prey.

Sex-specific outcrossing rates and the inbreeding coefficient of adults in two populations of gynodioecious Cucurbita foetidissima were estimated using progeny array data from four allozyme loci to compare the frequencies of self-fertilization and the estimated levels of inbreeding depression to predictions from sex ratio theory. The frequencies of self-fertilization by hermaphrodites in both populations were similar and averaged 73 per cent. The outcrossing rate for females in one population was not different from unity whereas the estimated rate of outcrossing by females in the second population (t = 0.593, SE 0.178) indicated the occurrence of biparental inbreeding. Despite considerable self-fertilization by hermaphrodites, inbreeding coefficients of adult plants in both populations were not different from zero and thus inferred values of inbreeding depression were not different from one. Electrophoretically inferred levels of inbreeding depression are somewhat in excess of the value of 0.71 (SE 0.07) obtained in a previous field experiment which tested first-year survival of selfed and outcrossed seeds in this long-lived perennial. The high frequency of self-fertilization by hermaphrodites combined with severe inbreeding depression provides a strong selective force which, along with increased seed-set by females, is sufficient to maintain observed frequencies of females in natural populations of C. foetidissima.

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