For snail species that evolve larger shells in response to predation (which may or may not include these dogwinkles), what is likely to limit the size of the shells? Use your knowledge of biology and your experience in this experiment to propose a likely reason why shells that evolved to be larger do not continue to evolve to be even larger than the size they currently are. In other words, what might limit the evolution of yet larger shells?

The correct answer is: The limited variability in the size of shells in the initiating population.

Explanation:

The shell size in snail species is determined by a number of variants (alleles) of the gene that is responsible for determining the size of their shell. The number of variants of the gene encoding for shell size is limited. However, the different variants may encode for different shell size, say from 5 millimetre to 20 millimetre. All these variants are present in the original population of snail but in different proportions. However, if predation tend to target the snail species with smaller shell size (5 millimetre to 10 millimetre) then evolution would tend to increase the frequency of snails having the larger shell variants (15 millimetre to 20 millimetre). This will result in a population of snails with increased shell size. However, an increase in shell size over 20 millimetre is not possible because no allelic variants encoding for a shell size greater than 20 millimetre is available in the population.