Final answer:
Stabilizing selection is a form of natural selection that favors intermediate traits and selects against extreme phenotypes, reducing phenotypic variation and usually occurring in stable environments.
Step-by-step explanation:
The concept you're asking about is known as stabilizing selection. This is a type of natural selection that favors intermediate phenotypes by acting against extreme variations. It results in a narrowing of the phenotypic range within the population, and typically occurs in environments where conditions tend to be stable and extreme traits are disadvantageous. An example of stabilizing selection is the consistent birth weight in human babies, where too low or too high weights are selected against because they are associated with higher mortality rates.