Final answer:
A trait may not be adaptive due to lack of genetic variation, environmental changes, trade-offs with other traits, and fluctuations in selection pressures. Natural selection, while a primary force in evolution, competes with other forces like genetic drift and gene flow that affect a population's genetic makeup. Evolution is not goal-oriented; it depends on existing genetic variance and environmental context.
Correct option is B. Environmental changes
Step-by-step explanation:
A given trait may not be adaptive despite the action of positive natural selection due to a variety of reasons. One reason is the lack of genetic variation, which can limit the potential for adaptation because there may not be advantageous alleles present for selection to act upon. Another reason is environmental changes; if an environment changes, a trait that was previously beneficial may no longer confer an advantage, or may even become a disadvantage. Additionally, there can be trade-offs with other traits; if a beneficial trait has a negative impact on another trait that's also important for survival or reproduction, this can limit the trait's adaptive value. Lastly, fluctuations in selection pressures mean that different traits may be favored at different times, which can prevent a trait from consistently being advantageous.
Natural selection is one of the forces that drive evolution, but it's important to note that other forces such as genetic drift and gene flow can introduce less advantageous alleles into a population's gene pool. Furthermore, the adaptive success of a trait is conditioned by the organism's entire genotype and the specific environmental context. Natural selection does not work towards a preset ideal; it acts on the population's existing genetic variance, which can be influenced by various factors like mutations and sexual selection.
Correct option is B. Environmental changes