New generations are better suited to their environment than the first generation. What is this called?

Descent with modification

Explanation:

We define evolution as descent with modification from a common ancestor.

Evolution only occurs when there is a change in gene frequency within a population over time. These genetic differences are hereditary and can be passed on to the next generation - which is what really matters in evolution: long-term change. In this process, the new generations are better suited to their environment than the first generation because of descent with genetic modification.