Final answer:
The evolution of larger body sizes in small mammals on islands, a phenomenon known as island gigantism, results from the absence of large predators and the need to maintain body temperature. Conversely, large mammals may become smaller, a phenomenon known as island dwarfism, due to resource limitations and energetic efficiency. These evolutionary adaptations to island living conditions lead to the biodiversity distinct from mainland species.
Step-by-step explanation:
The phenomenon of small mammals on islands evolving larger body size compared to their mainland counterparts is often attributed to a concept known in evolutionary biology as island gigantism or insular gigantism. This is a form of evolutionary divergence where, in the absence of large predators on islands, small mammals face fewer threats and can afford to grow larger. This size increase can be advantageous as larger animals lose heat more slowly and can better maintain their internal temperature in colder climates, which may be crucial to survival in island environments such as those near the Arctic Circle. Additionally, larger sizes may enable them to exploit a wider range of food resources and reduce competition.
Conversely, large mammals on islands might evolve to be smaller through a similar process known as island dwarfism. This occurs because resources on islands are often scarcer, and smaller body sizes require less energy to sustain, giving a survival advantage in these constrained environments. The evolutionary changes in body size, whether towards gigantism or dwarfism, reflect the ecological conditions of islands which can dramatically differ from those on the mainland.
Examples of this principle can be seen in the diverse marsupial populations in Australia and the finches of the Galápagos, where isolation and unique island conditions have led to endemic species that are quite distinct from their mainland ancestors. Such changes are part of the broader patterns of evolution where species adapt to their specific environments over time, leading to great biodiversity.