The taxon contains ~20 spieces in a single family, the Podicipedidae (grebes). So far, the intrinsic phylogeny of grebes hasn't been investigated with molecular methods.

Due to convergent evolution, this group of freshwater diving birds exhibits similarity with Gaviidae (loons) and extinct Hesperornithiformes. Several species are poor flyers, and two species are flightless (Rollandia microptera, Podiceps taczanowskii).

References

Fjeldså J (2004), “The Grebes: Podicipedidae”, Oxford University Press, Oxford.

Ogawa LM, Pulgarin PC, Vance DA, Fjeldså J, and van Tuinen M (2015), Opposing demographic histories reveal rapid evolution in grebes (Aves: Podicipedidae), Auk 132, 771-786. (pdf)