How can loons and grebes look similar without being closely related?

Loons and grebes appear similar due to a process called convergent evolution, rather than a shared evolutionary history.1

While these birds were once grouped together in the taxonomic order Colymbiformes, modern ornithology has determined they are not closely related and has placed them into separate orders: Gaviiformes for loons and Podicipediformes for grebes.1

Their physical similarities—such as their streamlined bodies, specialized diving capabilities, and legs positioned far back on the body—are analogous traits. These features evolved independently in both groups as adaptations to similar ecological niches and environmental pressures associated with an aquatic, diving lifestyle. Because they face similar demands for swimming and hunting underwater, they have developed comparable body plans despite their distinct ancestral origins.12345