Final answer:
Animals have different structures that perform similar functions due to convergent evolution, such as gills and lungs. Convergent evolution occurs when unrelated organisms adapt to similar environmental conditions and develop analogous structures.
Step-by-step explanation:
Animals have different structures that perform similar functions due to the process of convergent evolution. Convergent evolution occurs when unrelated organisms adapt to similar environmental conditions and evolve similar structures to carry out the same function. These structures are called analogous structures. For example, gills in fish and lungs in mammals are both respiratory organs that allow for the exchange of oxygen and carbon dioxide, but they evolved independently in these two groups of animals.