To determine which statement accurately describes natural selection, let's analyze each option:

#### Solution By Steps

***Step 1: Analyze Option A***

Option A states that natural selection is the primary mechanism for evolution. This is correct as natural selection is a process where organisms with traits better suited to their environment tend to survive and reproduce more, leading to the gradual change in species over time, which is the essence of evolution.

***Step 2: Analyze Option B***

Option B suggests that natural selection must act on structural traits. While natural selection can act on structural traits, it is not limited to them. It can also act on behavioral, physiological, and other types of traits that affect an organism's survival and reproduction.

***Step 3: Analyze Option C***

Option C claims that natural selection acts to cause overpopulation. This is incorrect. Natural selection does not cause overpopulation; rather, it is a process that favors traits in organisms that enhance their survival and reproductive success in their specific environment, which can help manage population sizes.

***Step 4: Analyze Option D***

Option D states that natural selection causes learned skills to become heritable traits. This is not accurate. Natural selection acts on traits that are heritable, not learned skills. Learned skills are not passed down genetically; they are acquired through experience and cannot be naturally selected.

#### Final Answer

The statement that accurately describes natural selection is Option A: It is the primary mechanism for evolution.