Which are valid examples of directional selection? (choose all that apply)

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BlackTom AI · Accepted Answer

When evaluating which scenarios illustrate directional selection, we look for changes in a trait's average value in a population over time in one particular direction. 
Option: 'Human birth weight' – This is typically described as being under stabilizing selection, where extreme values (very small or very large babies) are selected against and the average birth weight is favored. Therefore, this is not directional selection. 
Option: 'Dark-colored peppered moths increasing in polluted environments' – This is a classic example of directional selection. Pollution darkens tree barks, making the dark morphs better camouflaged and more likely to survive and reproduce, shifting the population mean toward a darker color. 
Option: 'Antibiotic resistance in bacteria' – This embodies directional selection as well. When antibiotics are present, bacteria with resistance alleles have a survival advantage and increase in frequency, moving the population mean toward resistance. 
Option: 'Color polymorphism in seedcrackers' – Color polymorphism can be maintained by balancing or frequency-dependent selection, or by different microhabitats; it does not necessarily show a consistent shift in the average color in one direction over time. Therefore, this is not inherently directional selection.

Sp25 BIOL D006C 02Y Ecology And Evolution Exam 3

Which are valid examples of directional selection? (choose all that apply)

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