What does stabilising selection ultimately affect in a population?

Stabilising selection is a type of natural selection that acts to reduce variation in a trait and promotes intermediate phenotypes. This type of selection occurs when individuals with extreme traits have lower fitness compared to those with traits closer to the average. As a result, stabilising selection helps maintain the status quo of trait distribution in a population, leading to a decrease in genetic diversity by reducing the frequency of extreme traits.

For example, in a population of birds where the average beak size is optimal for food acquisition, individuals with very small or very large beaks may struggle to survive because their beaks are not well-suited to the available food resources. Over time, individuals with intermediate beak sizes will become more common, while those with extreme sizes will be selected against. Therefore, stabilising selection ultimately affects genetic diversity by reducing the representation of extreme traits in the population.