Lizard Body Size on a Snake-Invaded Islet
In 2010, 180 wall lizards were released on a 2.5 ha islet that had no native predators. A snake arrived in 2013, probably in a cargo crate. Researchers measured adult snout–vent length (SVL) each year. The 2010 founders averaged 62 mm. In 2018 they marked 120 adults; the mean SVL of all marked adults was 63 mm. Over the next 12 months, snakes took 30 of the marked animals; the 30 taken averaged 54 mm, while the 90 survivors averaged 66 mm. In 2019 the adult mean SVL was 68 mm. The population is estimated at 60–80 adults, but a 2016 cyclone reduced it to about 25. A laboratory experiment raised offspring of long and short parents on identical diets; offspring SVL tracked the mid-parent value with a slope of 0.45. A student writes: 'The lizards grew longer because they needed to escape the snakes. This proves natural selection caused the change.'
1. Calculate the selection differential and the predicted response to selection per generation, and compare with the observed change. State any assumptions.
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At the 2018 census the mean of all marked adults was 63 mm; the 30 animals taken by snakes averaged 54 mm, so the survivors' mean is 66 mm. The selection differential is S = 66 − 63 = 3 mm. The laboratory mid-parent–offspring slope estimates heritability, h² ≈ 0.45. The predicted response per generation is R = h²S = 0.45 × 3 = 1.35 mm. The observed change from 62 mm in 2010 to 68 mm in 2019 is 6 mm over 9 years. If wall lizards breed at about 2 years, that is roughly 4.5 generations, giving about 1.33 mm per generation, close to the prediction. The agreement is suggestive, not proof: it assumes h² and S stayed constant, that the 2010 and 2019 samples are comparable, and that drift and gene flow were negligible.
2. To what extent does the evidence support the student's claim? Address the phrase 'needed to' and the possible role of genetic drift.
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The evidence supports selection as a plausible cause, but not the student's wording. Natural selection is not a response to need; it is the consequence of some variants leaving more offspring. Here the ingredients are present: SVL varies, the laboratory slope shows that part of that variation is heritable (h² ≈ 0.45), and smaller lizards were more likely to be taken (54 mm versus 66 mm survivors). But the population crashed to about 25 animals in 2016. With such a small effective population size, genetic drift can shift the mean by chance, and the bottleneck may have changed allele frequencies independently of snake predation. The founders were also a small sample. The claim 'this proves natural selection caused the change' is too strong; selection and drift are both consistent with the data. Genetic markers or replication on other islets would help separate them.
3. Design one additional study to test whether the increase is genetic rather than plastic, and explain what result would support plasticity.
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A common-garden experiment would separate genetic change from phenotypic plasticity. Collect eggs or pregnant females from the islet and raise the offspring in identical laboratory conditions, with and without chemical cues from snakes. If all offspring grow to the same SVL regardless of parent SVL and cue, the islet change was plastic. If offspring of long parents are longer even under identical conditions, the difference has a genetic component. A complementary approach is to compare museum specimens from 2010 with 2019 animals raised on the same diet, or to use a reciprocal transplant between the islet and a snake-free islet. The key is to hold the environment constant so that any remaining difference in SVL can be attributed to inherited differences rather than to the snakes changing behaviour, growth or diet.