Science Assessment

A hundred years ago, a network of streams connected three lakes: Lake Unknown, Lake Mystery, and Lake Forgotten. These streams allowed trout to travel freely between the three lakes and interbreed.

Due to environmental changes, the streams dried up, so trout in each lake can no longer breed with trout from the other two lakes. Biologists are interested in how this change has affected the frequencies of different alleles of Gene Z. The chart below shows the allele frequencies of Gene Z in the three populations.

Which sequence of events provides the simplest and most likely explanation of how Allele 3 appeared in Lake Unknown, but not in the other two lakes?

1. Step 1: The streams dried up.

   Step 2: Allele 3 appeared in the Lake Unknown population because of gene mutations.

2. Step 1: Allele 3 appeared in the Lake Unknown population because of gene mutations.

   Step 2: The streams dried up.

3. Step 1: The streams dried up.

   Step 2: Allele 3 appeared in the Lake Unknown population because of a change in environment. No gene mutations occurred.

4. Step 1: Allele 3 appeared in the Lake Unknown population because of a change in the environment. No gene mutations occurred.

   Step 2: The streams dried up.