Final answer:
Host shifts by the herbivorous leaf beetles of genus Ophraella are attributed to sufficient genetic variation for evolutionary change, demonstrated through directional selection. This type of natural selection favors phenotypes that are better adapted to new environmental conditions, leading to significant evolutionary shifts within a population.
Step-by-step explanation:
Futuyma and colleagues (1995) demonstrated that host shifts by the herbivorous leaf beetles of the genus Ophraella in feeding on members of the sunflower family are due to the availability of genetic variation sufficient for evolutionary change. Situations where populations experience a change in environment, such as that encountered by beetles or the peppered moth during the Industrial Revolution, often undergo directional selection, which favors phenotypes that are better adapted to the new conditions. For example, peppered moths with darker coloration had a higher survival rate as the soot from factories darkened the tree trunks where they rested and thus were less visible to predators.
The process of directional selection can lead to significant evolutionary shifts within a population, resulting in changes in traits such as coloration, physical structure, and other characteristics that affect an organism's ability to survive and reproduce in a changing environment. Similarly, functional trait analysis, as well as the relationship between specific species and their ecological niches, can provide insight into how species might respond to environmental changes or shifts in resource availability.