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Why are grebes no longer classified with loons in the order Colymbiformes?

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Slavonian, or horned, grebe (Podiceps auritus) — Grebe Slavonian, or horned, grebe (*Podiceps auritus*)

Grebes and loons are no longer classified together in the order Colymbiformes because later scientific scholarship determined that the two groups are not closely related.1

While they share a superficial resemblance and similar physical traits, ornithologists concluded that these similarities are the result of convergent adaptation—where unrelated species evolve similar features to survive in comparable environments—rather than a shared evolutionary background.12

As a result, the order Colymbiformes was suppressed, and the birds were reclassified into separate, distinct orders:
* Loons: Now classified in the order Gaviiformes.12
* Grebes: Now classified in the order Podicipediformes.13