For Galápagos marine iguanas, body size is not correlated with survival, but small iguanas can sprint faster than large iguanas. If a new predator species e.g., cats) that preferably catches and eats slower iguanas is introduced to the island, iguana body size is likely to in the absence of other factors; the iguanas would then be under selection.

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Quizplus · Accepted Answer

With the introduction of a predator species that prefers slower iguanas, larger iguanas (with slower sprint speeds) will be at a disadvantage and less likely to survive and reproduce. Therefore, over time, the population will evolve to have smaller body sizes with faster sprint speeds, resulting in a directional selection towards smaller body sizes.

For Galápagos Marine Iguanas, Body Size Is Not Correlated with Survival