Auks are superficially similar to penguins having black-and-white colours, upright posture and some of their habits. Nevertheless, they are not closely related to penguins, but rather are believed to be an example of moderate convergent evolution. Auks are monomorphic (males and females are similar in appearance). Extant auks range in size from the least auklet, at 85 g (3 oz) and 15 cm (5.9 in), to the thick-billed murre, at 1 kg (2.2 lb) and 45 cm (18 in). Due to their short wings, auks have to flap their wings very quickly in order to fly. Although not to the extent of penguins, auks have largely sacrificed flight, and also mobility on land, in exchange for swimming ability their wings are a compromise between the best possible design for diving and the bare minimum needed for flying. This varies by subfamily, the Uria guillemots (including the razorbill) and murrelets being the most efficient under the water, whereas the puffins and auklets are better adapted for flying and walking.