Question

read this passage about two inherited traits in pond snails.

pond snails live in freshwater environments such as ponds, lakes, and rivers. a pond snails shell may be narrow or round. each shape is better at protecting a snail against a different kind of predator.
a narrow shell provides better protection against crayfish, which attack a snail at the opening of its shell. narrow shells have smaller openings, so snails with narrow shells are harder for crayfish to reach during an attack.
a round shell provides better protection against carp, which crush shells with their teeth. round shells are harder for carp to crush.

the pond snails environment changed to have significantly more carp. after this change, over a time period long enough for natural selection to occur, the percentage of the pond snail population with narrow shells decreased.

use the theory of natural selection to construct a possible explanation for how the pond snail population adapted to its changed environment.

having a round shell was ____ advantageous than having a narrow shell after the environment changed. so the alleles for round shells were passed on with __ frequency than the alleles for narrow shells. as a result, the proportion of the population with round shells __. over time, the population adapted such that the percentage of snails with shells that were easier for carp to crush ____.
a narrow shell (left) and a round shell (right)

Explanation:

1. After the environment had more carp, round shells offered better survival protection against carp predation, making them more advantageous than narrow shells.
2. Snails with round shells survived more often, so their round shell alleles were passed to offspring at a higher frequency than narrow shell alleles.
3. As more round-shelled snails reproduced, the proportion of round-shelled snails in the population increased.
4. Narrow shells are easier for carp to crush, so as natural selection favored round shells, the percentage of these vulnerable narrow-shelled snails decreased.

Answer:

Having a round shell was more advantageous than having a narrow shell after the environment changed. So the alleles for round shells were passed on with higher frequency than the alleles for narrow shells. As a result, the proportion of the population with round shells increased. Over time, the population adapted such that the percentage of snails with shells that were easier for carp to crush decreased.