Hat tip to Troy Britain for this link to a paper about the evolution and development of feathers. It just struck me as rather nice. I think this is a fairly well-known paper as it outlines hypothetical intermediate stages of feathers, some of all of which have been observed, as an evolutionary sequence.
Although feathers are diverse in form, they all share a common developmental origin in the cylindrical feather follicle. Indeed, since a single follicle can produce feathers of striking structural diversity during the life of a bird, the tremendous structural diversity of feathers is best understood in terms of the control of feather development within the follicle. The development of feathers is also essential to any discussion of feather origins because any complete theory of the origin of feathers must account not only for structure of feathers themselves but [also] for the structure and complexity of the follicle.
“Development and Evolutionary Origin of Feathers,” (1999) by Richard O. Prum. (Journal of Experimental Zoology (MOL DEV EVOL) 285:291–306)
It never occurred to me, as it has to this scientist, that the same follicle producing different kinds of feathers, e.g. a chick’s down vs. adult feathers or perhaps winter and summer feathers, is significant when we think about evolution.
Now, read Troy Britain’s article about creationist ignorance.