Which of the following is an example of adaptive radiation?

Adaptive radiation refers to the evolutionary process where organisms diversify rapidly into a wide variety of forms to adapt to different environments or ecological niches. The example of Darwin's finches is a classic illustration of adaptive radiation. These finches originated from a common ancestral species and evolved into multiple species with distinct beak shapes and sizes, each specialized for different types of food available in their specific environments on the Galápagos Islands. This diversification allowed them to effectively exploit various ecological niches, showcasing how environmental pressures drive evolution and speciation.

In contrast, the other options do not demonstrate adaptive radiation. The rapid evolution of mammal species might suggest evolutionary change but does not necessarily imply a diversification into multiple forms each adapted to specific niches like the finches. Genetic similarity of populations in the same habitat indicates a lack of diversification among them rather than adaptive radiation. Lastly, the extinction of unique species in isolated environments speaks to the loss of biodiversity rather than an increase in it through adaptive mechanisms. Thus, the diversification of Darwin's finches exemplifies adaptive radiation due to the significant variation and specialization driven by environmental factors.