When males and females of a population look or act differently, it is referred to as
sexual dimorphism
sexual selection
diversifying selection
a cline

Sexual dimorphism

Explanation:

A dictionary definition of sexual dimorphism is:

When sexes differences of the same species exhibit different characteristics beyond the their sexual organs.

Examples of this:  

Bigger famales and small males in spiders.

Male lions have mane, while females dont.

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