Which is an example of stabilizing selection and how does it affect variation?

Which is an example of stabilizing selection and how does it affect variation?

• A. An increase in average body size over many generations
• B. Human birth weight—intermediate weights have higher survival; extremes are selected against
• C. Random fluctuations in allele frequencies due to chance
• D. Species diverging due to reproductive isolation

Answer: (B)

Stabilizing selection favors the average form of a trait and reduces variation by selecting against extremes. In humans, birth weight is most successful at intermediate values; babies that are too small or too large have higher health risks, so middle weights become more common over generations and the overall variability in birth weight decreases. This demonstrates how stabilizing selection maintains a trait near its optimum and narrows its range. Other options reflect different evolutionary forces: increasing the average body size would be directional selection shifting the mean upward; random fluctuations in allele frequencies are genetic drift; and reproductive isolation leading to speciation involves divergence, not stabilization of a single trait.

Stabilizing selection favors the average form of a trait and reduces variation by selecting against extremes. In humans, birth weight is most successful at intermediate values; babies that are too small or too large have higher health risks, so middle weights become more common over generations and the overall variability in birth weight decreases. This demonstrates how stabilizing selection maintains a trait near its optimum and narrows its range. Other options reflect different evolutionary forces: increasing the average body size would be directional selection shifting the mean upward; random fluctuations in allele frequencies are genetic drift; and reproductive isolation leading to speciation involves divergence, not stabilization of a single trait.