This order of freshwater diving birds contains but a single family, the Podicipedidae or grebes. So far the intrinsic phylogeny of this group of 20 species hasn't been investigated with molecular methods. Based on convergence, grebes exhibit similarity with loons, family Gaviidae (and extinct Hesperornithiformes). Some species are poor flyers, and two species are even flightless (Rollandia microptera, Podiceps taczanowskii). 

References/Literature

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)