“But evolution is hit-or-miss proposition, and there’s much more miss than hit. The majority of genetic mutations have no effect whatsoever, but a very small percentage result in an adaptive improvement every few million years or so. The genetic accident that caused the first two-legged dinosaurs to sprout feathers 120 million or 130 million years ago provided one such improvement. Because feathers (like fur) provide good insulation, feathered dinosaurs in chilly climates had an adaptive advantage over their lizard-skinned cousins. These fluffy dinos lived longer and had more kids, and eventually in some parts of the world feathers became the norm rather than the exception.
As with so much in evolution, the true potential of feathers revealed itself very slowly. When they first appeared 100 million years ago, feathers were more like scraggly strands of hair than the majestic plumage found on birds today. The same process of slight, incremental genetic mutation that gave rise to feathers in the first place went to work in causing these feathers to grow to larger sizes. In addition to keeping dinosaurs warmer (which increased their chances for survival and allowed them to produce more offspring), this development also conferred an aerodynamic advantage.”
http://www.sparknotes.com/home/biology/article/from_scales_to_feathers.html