Guppies are small fish that live in South American rivers. They can have different-sized spots on their bodies.

Question

The river bottoms are covered in rocks. Guppies with spots that are the same size as the rocks on the bottom are harder for bigger fish to see and catch.

The diagram below shows a population of guppies that live in a river. At time 1, the population had the same number of guppies with small and large spots. At time 2, after many generations, there were many more guppies with small spots and fewer guppies with large spots in the population.

2 diagrams, each containing rows of fish icons. The first diagram is titled Population at Time 1 and contains 2 rows of 4 fish icons each, one labeled Small Spots and one labeled Large Spots. The second diagram is titled Population at Time 2 and contains 2 rows of fish icons. The first row is labeled Small Spots and contains 7 fish icons. The second row is labeled Large Spots and contains 2 fish icons. An arrow points from the first diagram to the second diagram and is labeled: After Many Generations.
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7. How did the environment change between time 1 and time 2? How did the population change?
Responses

a
You cannot tell how the environment changed. With each generation, more guppies passed on the gene for small spots to their offspring.
You cannot tell how the environment changed. With each generation, more guppies passed on the gene for small spots to their offspring.

b
The rocks became smaller. With each generation, more guppies with small spots survived long enough to pass on the gene for small spots to their offspring.
The rocks became smaller. With each generation, more guppies with small spots survived long enough to pass on the gene for small spots to their offspring.

c
The rocks became smaller. Guppies with small spots are more likely to survive, so the guppies with large spots changed to have small spots.
The rocks became smaller. Guppies with small spots are more likely to survive, so the guppies with large spots changed to have small spots.

d
The rocks became smaller. Guppies with small spots are more likely to survive, so both kinds of guppies passed on the gene for small spots to their offspring.

Answer 1

The most appropriate response to how the environment changed between time 1 and time 2, along with how the population changed, would be:

a You cannot tell how the environment changed. With each generation, more guppies passed on the gene for small spots to their offspring.

This response focuses on the observation that the frequency of guppies with small spots increased over generations, suggesting natural selection favoring this trait without making assumptions about the specifics of environmental changes (like the size of rocks).