Final answer:
The difficulty in studying adaptation without a closely related species lies in identifying the nuances of evolutionary changes, including rates of extinction and genetic diversity, that can be misinterpreted without a comparative framework.
Step-by-step explanation:
In the study of adaptation and evolutionary biology, determining the rate and mechanisms of adaptation can be difficult without a closely related species for comparative analysis. For instance, the cited example of the European crab Carcinus maenas initially suggested rapid adaptation to cold-water due to morphological changes. However, later genetic analyses suggested a different scenario involving the introduction of a new genetic lineage that was already pre-adapted to cold conditions, rather than in situ adaptation of the local crab population. Furthermore, in the process of adaptive radiation and speciation, understanding if and how species adapt to their new environment or how they may hybridize and lose distinct characteristics often requires the support from modern genetics. The comparison-and-contrast reasoning strategy is used to discern differences and similarities which can shed light on evolutionary processes, but without closely related species for comparison, researchers may miss critical nuances in the rate of extinction, genetic diversity, and evolutionary changes happening within a group of species.