Final answer:
Polymorphism within species is maintained by environmental pressures and genetic isolation, ultimately leading to adaptations that ensure survival in contrasting habitats.
Step-by-step explanation:
Polymorphism in contrasting habitats is maintained by the environmental pressures and interactions between different polymorphisms that lead to varying adaptations within a species. For example, beach-dwelling mice may exhibit different colours depending on whether they blend in with the sand or the grass patches, and this variation is preserved because intermediate phenotypes are less likely to survive due to predation. In geographically isolated areas, certain species evolve unique characteristics that are maintained through genetic isolation.