Question

Salt is dangerous to land snails. It will rapidly kill them. However, some populations of aquatic, freshwater snail have adaptations that allow them to live in a mix of salt and fresh water. This allows them to live in different parts of a river system. A group of scientists monitored salt concentrations in the habitats of two freshwater snail populations. They recorded the salt level as follows (in ppt, parts per thousand):
Location 1 (Day 1): 0.1 ppt (morning), 0.0 ppt (afternoon), 0.1 ppt (evening)
Location 2 (Day 1): 0.3 ppt (morning), 0.5 ppt (afternoon), 0.2 ppt (evening)
In 3,5 sentences, describe in which population you'd expect to see more adaptations for salt water. Make sure to also explain how such adaptations would become more common in this population.

Answer 1

You would expect more salt‑tolerance adaptations in the population at Location 2, because its measured salinities (0.2–0.5 ppt) are consistently higher than Location 1 (0.0–0.1 ppt) and thus impose stronger selection for traits that tolerate salt. Individuals in Location 2 that carry genetic variants (or physiological plasticity) that improve salt balance or stress resistance will survive and reproduce more than sensitive individuals. Over successive generations those heritable tolerance traits become more common as allele frequencies shift by natural selection. If there is substantial gene flow from Location 1, it can slow this divergence, whereas isolation will accelerate local adaptation.