A scientist studies a population of lizards isolated on a tropical island over many years. During the study, several rats are accidentally introduced to the island. The rats prey on the lizards. The scientist notices that the lizards with the longest legs run the fastest and are more likely to outrun the rats. Over several more generations, the scientists finds that while the range of leg lengths in the lizards changes little, the average leg length has increased.

Question

Which two statements explain the scientist's observations?
A. Lizards needed to run faster to escape the rats, so they developed longer legs.
B. By running faster to escape the rats, the lizards made their legs slowly grow longer over time.
C. Predation by rats caused new mutations in the lizards, which caused their legs to grow longer.
D. Lizards with longer legs are more likely to survive and reproduce than lizards with shorter legs.
E. There was already variation in the genes controlling leg length in the lizard population before the rats arrived.

Answer 1

The two statements that explain the scientist's observations are:

D. Lizards with longer legs are more likely to survive and reproduce than lizards with shorter legs.

E. There was already variation in the genes controlling leg length in the lizard population before the rats arrived.

These statements highlight that the lizards that could run faster (due to longer legs) had a survival advantage, which increased the average leg length in the population over generations, as offspring inherited the traits of the more successful lizards. Additionally, the presence of genetic variation prior to the introduction of the rats allowed natural selection to act on leg length.