Final answer:
The difference between an obligate and facultative trait is that an obligate trait is always expressed, whereas a facultative trait's expression depends on environmental conditions. The reaction norm helps us understand the range of expression of a trait and can indicate if the trait is facultative.
Step-by-step explanation:
The difference between an obligate trait and a facultative trait relates to their expression in response to environmental conditions. An obligate trait is always expressed, regardless of the environment, while a facultative trait is expressed only under certain environmental conditions. The reaction norm of a trait shows the range of potential expressions of that trait across different environments. A trait with a wide reaction norm is likely to be facultative, showing variation depending on the environment. In contrast, an obligate trait would show little to no change across different environments.
For example, if we consider the case of plant species with different leaf sizes adapting to moisture availability, the size of the leaves is a facultative trait. In a moist climate, large leaves can be beneficial as they absorb more sunlight, but in dry conditions, smaller leaves that conserve water are selected for. This change in trait expression in response to different environmental conditions demonstrates a facultative trait.
In conclusion, the correct answer to the student's question is A) The obligate trait is always expressed, while the facultative trait depends on environmental conditions.