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.

Diagram titled Population Before containing 2 rows of fish icons. The first row is labeled Medium Spots and contains 9 fish icons. The second row is labeled Large Spots and contains 2 fish icons.
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The diagram above shows a population of guppies that lived in a river. Scientists had previously only seen guppies with medium or large spots in this river; they had never seen any guppies with any other spot sizes.

The diagram below shows how the population changed after many generations.

Diagram titled Population Before containing 2 rows of fish icons. The first row is labeled Medium Spots and contains 9 fish icons. The second row is labeled Large Spots and contains 2 fish icons.
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Karina says the change in the guppy population was caused by a mutation.
Miles says the change was caused by a change in the environment.
Zora thinks both Karina and Miles are correct.
Who do you think is right? Explain what happened to the guppies over many generations.

Answer 1

To understand the change in the guppy population over many generations, we need to consider both the concepts of mutation and environmental influence.

1. Mutation: Mutations are random changes in the genetic material of an organism that can create new traits. In the case of the guppies, a mutation could lead to the development of new spot sizes that were not previously present. If these new traits provided an advantage for survival (e.g., smaller spots that help guppies blend in with the rocks), then guppies with these mutations might survive better and reproduce more, leading to an increase in that trait over generations.

2. Change in Environment: The environment plays a crucial role in natural selection. If the environment changes—such as a shift in the size of rocks on the riverbed—this could influence which guppies are most successful at survival. For instance, if the rocks become smaller, guppies with smaller spots may become less visible to predators and therefore more likely to survive and reproduce.

Given that both mutation and environmental change can lead to shifts in population traits, it is likely that both Karina and Miles are correct. Mutations could have introduced new size variations in spots, while changes in the environment (like the nature of the river bottom) influenced which of those variations were favorable for survival.

In conclusion, the change in the guppy population was likely a result of both genetic mutations that created new spot sizes and environmental factors that selected for the guppies with traits that offered them better camouflage against predators. Therefore, Zora's statement that both Karina and Miles are correct reflects a comprehensive understanding of the factors influencing the guppies' evolution over generations.