Question

Salt is dangerous to land snails. It will rapidly kill them. However, some populations of aquatic, freshwater snail have adapatations 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 adaptations for salt tolerance in the Location 2 population because its habitat repeatedly experiences higher and more variable salt concentrations (0.2–0.5 ppt versus near 0 ppt at Location 1). Traits that improve osmoregulation, ion transport, or salt-avoidance behavior would give individuals a survival and reproductive advantage there. Through natural selection, those tolerant individuals leave more offspring, so the alleles underlying salt tolerance increase in frequency over generations. Restricted gene flow between sites would reinforce these local adaptations, while high gene flow could dilute them.