Lizard Body Size Along an Elevation Gradient
A lizard species is sampled at five elevations: 200, 600, 1000, 1400, and 1800 m. Mean adult body mass decreases from 12.0 g at 200 m to 7.5 g at 1800 m. Common garden experiments raising lizards from each elevation at a constant 25 degrees C show that offspring from high elevation reach 8.0 g, while low-elevation offspring reach 11.5 g. Reciprocal transplant shows high-elevation lizards survive poorly at low elevation (40% vs 75% for residents), and low-elevation lizards survive poorly at high elevation (45% vs 80% for residents). The gradient spans a temperature difference of about 10 degrees C and a growing season that is 60 days shorter at the top.
1. Distinguish the roles of phenotypic plasticity and local adaptation in the body-size cline.
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The field cline (12.0 to 7.5 g) could reflect plasticity, genetic differentiation, or both. The common garden at constant temperature shows that offspring from high elevation still reach smaller size (8.0 vs 11.5 g), so a genetic component exists. However, the common garden values differ from field values, indicating environmental effects (plasticity) also contribute. The reciprocal transplant shows home-site advantage: residents survive better at their own elevation than transplants do, which is the signature of local adaptation. Together, the data support local adaptation with a genetic basis for smaller size at high elevation, plus plasticity that shifts size further in the field. The 10 degrees C and 60-day differences are plausible selective agents, but the transplant data are the strongest evidence for adaptation rather than plasticity alone.
2. The common garden used 25 degrees C for all populations. Is this a fair test? What confound could it introduce?
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A single common temperature is standard for common gardens, but 25 degrees C may be near the optimum for low-elevation lizards and stressful for high-elevation lizards. If high-elevation lizards perform poorly at 25 degrees C, their smaller size could reflect stress rather than genetic adaptation. This is a genotype-by-environment interaction confound: the rank order of sizes might change at a lower temperature. A fairer design would raise each population at multiple temperatures (e.g., 20, 25, 30 degrees C) and compare reaction norms. If high-elevation lizards remain smaller across temperatures, the genetic interpretation is strengthened. If the difference disappears or reverses at cooler temperatures, plasticity or thermal stress explains the pattern. The current design cannot separate these possibilities.
3. A critic says the transplant results are confounded by capture and handling stress. How would you test this?
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Use a sham transplant: move lizards within their home elevation (to a nearby enclosure) and handle them identically to transplanted animals. If sham-transplanted residents survive as well as undisturbed residents, handling and capture are not the cause of the transplant mortality. Also include a common-garden enclosure at an intermediate elevation where both populations are tested, and measure stress markers (corticosterone) across treatments. If corticosterone is elevated equally in all handled groups but survival differs by origin and site, the home-site advantage is not explained by handling stress. Finally, randomize capture times and body sizes across treatments to rule out condition bias.