What is an example of stabilising natural selection?

Stabilising natural selection is a type of natural selection that favors the average individuals in a population, leading to a reduction in variation and maintaining the status quo of a trait. Increased birth weight in babies is a prime example because babies that are born at an average weight tend to have a higher survival rate compared to those that are either significantly underweight or overweight. Babies that are too small may have higher mortality due to complications, while those that are too large may encounter difficulties during birth. As a result, the trait of having an optimal birth weight is favored, illustrating how stabilising selection works to maintain traits that are advantageous for survival and reproduction.

In contrast, the other options do not represent stabilising selection. For instance, the lighter peppered moths surviving indicates a shift in population toward a specific trait due to changing environmental conditions, which aligns more with directional selection. Long necks in giraffes suggest a gradual increase in a favorable trait over time, often linked to competition for resources, showcasing another form of directional selection. The development of new species from a common ancestor is representative of divergent evolution, which does not pertain to stabilising selection as it involves changes that lead to variation and new traits rather than the maintenance of existing traits.