Final answer:
The selection differential (S) will be the highest in a population undergoing natural selection because it measures the strength of selection and is linked to how certain traits that affect an organism's fitness are favored over time.
Step-by-step explanation:
The selection differential (S) will be the highest in a population undergoing natural selection. In this context, S indicates the strength of selection, where R represents the response to selection, and h^2 symbolizes the heritability of the trait in question. Natural selection favors individuals with advantageous traits that enhance survival and reproductive success in a given environment, leading to evolutionary change over time. It happens when there is variation in traits that affect an organism's fitness, and these traits can be passed on to the next generation.
On the other hand, the Hardy-Weinberg equilibrium describes a population in which allele frequencies do not change over time, meaning there is no evolutionary change and hence no natural selection. Therefore, in a population at Hardy-Weinberg equilibrium (S = 1), the selection differential would not be as high as in a population undergoing directional selection, where certain traits become more common.
Logistic growth and concepts of r- and K-selection relate to how populations grow and are regulated by environmental factors, but they do not directly determine the magnitude of selection differential. However, these concepts are important in understanding the dynamics of populations under different ecological pressures.