On a global scale, species are constantly being moved from their areas of origin to new locales. Such range extensions can occur naturally, but are frequently aided (intentionally or non-intentionally) by humans (Mack <em>et al.</em> 2000). As a result, the marked world-wide increase in animal, plant, and microbial immigrations, has been found to roughly track the increase in human commerce (Mack <em>et al.</em> 2000). The principal vectors of human-mediated marine invasions are ballast water (Williams <em>et al.</em> 1988), mariculture (Minchin 1996), sediment held in ballast tanks (Carlton 1985), and ship hull fouling (Minchin 1996)
Environmental temperature variation generates adaptive phenotypic differentiation in widespread populations. We used a common garden experiment to determine whether offspring with varying parental origins display adaptive phenotypic variation related to different thermal conditions experienced in parental environments. We compared burst swimming performance and critical thermal limits of African clawed frog (Xenopus laevis) tadpoles bred from adults captured at high (∼ 2000 m above sea level) and low (∼ 5 m above sea level) altitudes. Maternal origin significantly affected swimming performance. Optimal swimming performance temperature had a >9°C difference between tadpoles with low altitude maternal origins (Topt: pure- and cross-bred 35.0°C) and high altitude maternal origins (Topt: pure-bred 25.5°C, cross-bred 25.9°C). Parental origin significantly affected critical thermal limits. Pure-bred tadpoles with low altitude parental origins had higher CTmax (37.8±0.8°C) than pure-bred tadpoles with high altitude parental origins and all cross-bred tadpoles (37.0±0.8 and 37.1±0.8°C). Pure-bred tadpoles with low altitude parental origins and all cross-bred tadpoles had higher CTmin (4.2±0.7 and 4.2±0.7°C) than pure-bred tadpoles with high altitude parental origins (2.5±0.6°C). Our study shows Xenopus laevis tadpoles’ varying thermal physiological traits is the result of adaptive responses to their parental thermal environments. This study is one of few demonstrating potential intraspecific evolution of critical thermal limits in a vertebrate species. Multi-generation common garden experiments and genetic analyses would be required to further tease apart the relative contribution of plastic and genetic effects to the adaptive phenotypic variation observed in these tadpoles.
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