Feather - Evolution

Evolution

The functional view on the evolution of feathers has traditionally focused on insulation, flight and display. Discoveries of non-flying Late Cretaceous feathered dinosaurs in China however suggest that flight could not have been the original primary function. There have been suggestions that feathers may have had their original function in thermoregulation, waterproofing or even as sinks for metabolic wastes such as sulphur. Recent discoveries are claimed to support a thermoregulatory function, at least in smaller dinosaurs. While feathers have been suggested as having evolved from reptilian scales, there are numerous objections, and more recent explanations have arisen from the paradigm of evolutionary developmental biology. Theories of the scale-based origins of feathers suggest that the planar scale structure was modified for their development into feathers by splitting to form the webbing; however, the developmental process involves a tubular structure arising from a follicle and the tube splitting longitudinally to form the webbing. The number of feathers per unit area of skin is higher in smaller birds than in larger birds, and this trend indicates their important role in thermal insulation, since smaller birds lose more heat due to the relatively larger surface area in proportion to their body weight. The miniaturization of birds also played a role in the evolution of powered flight. The coloration of feathers is believed to be primarily evolved in response to sexual selection. In many cases the physiological condition of the birds (especially males) is indicated by the quality of their feathers and this is used (by the females) in mate choice. A study published in 2006 confirmed the presence of information for producing feathers in the genes of the American Alligator, but these instructions are suppressed during embryological development, causing the alligator hatchling to express only scales. The presence of a homologous trait in both birds and crocodilians indicates it was inherited from a common ancestor. This may suggest that avian feathers, dinosaur protofeathers and pterosaur pycnofibres are all variations on the same basal archosaur integument, and place the origin of feathers much further back in history than previously thought.