Which of the following mechanisms contributes to variation in physical traits in response to environmental changes?

Phenotypic plasticity is the correct choice because it refers to the ability of an organism to change its physical traits in response to environmental conditions without altering its genetic code. This means that individuals can exhibit different traits based on varying circumstances, such as temperature, availability of food, or other environmental factors.

For instance, a plant may grow taller in response to limited sunlight, developing longer stems to reach for light, while another plant in a sunnier environment may remain shorter and stockier due to sufficient light. This adaptability allows organisms to optimize their survival and reproduction in changing environments, demonstrating the concept of phenotypic plasticity effectively.

In contrast, genetic adaptation involves changes in the genetic makeup of a population over time, which can lead to trait variation but does not occur as rapidly as phenotypic plasticity. Reproductive isolation is a mechanism that leads to speciation and affects genetic variation, but it does not directly relate to how an individual might change its appearance based on environmental factors. Natural selection works on existing variation in traits but it does not create the immediate or flexible responses seen in phenotypic plasticity.