Well, maybe not all but hopefully a whole lot, at least what I can remember and what I know now. Sorry to disappoint but I do not know it all but there are a ton of links at the end that get us a lot closer to the truth.
I am old enough to remember when "Indian necks" were standard and genetic hackle was just becoming more commonly used and available. Hoffman, Metz, and Whiting, Hebert, Ewing, Spencer, Keough, Conranch (now Clearwater), and Collins others were but a few producing birds that slowly were becoming more commercially available. Previously it was Indian necks for most tyers and those in the East might have been able to find Darbee and Dette birds which are the lines that gave rise to most of the other commercial lines. Later, local producers supplied necks to tyers - saddles were an afterthought and were generally not dry fly quality. Most saddles were sold relatively inexpensively as "bugger saddles".
Artificial selection can produce pretty crazy extreme variation and it can all happen quite quickly. From wolves, we have teacup dogs and Great Danes, Mastiffs, and Wolfhounds. Most of this divergence has occurred over the past 15,000 years, a mere blip in the history of the Earth. Artificial selection is responsible for your food - both plants and animals; your pets; and your lawn, flower beds, trees, and shrubs. We select for the traits we want - yield, disease resistance, drought or flood tolerance, color or shape, quality of the meat - basically any traits we want so long as the genetic information to allow for those changes is present in the species. Artificial selection occurs so much more quickly than does natural selection where - "only the strong survive". When we make their survival a near certainty, we get the characters we want much more quickly. Just think how far fly tying hackle has come in the last two decades.
Artificial selection gave us our "fancy chicken feathers" and the gains we have seen in the last few decades is pretty amazing. Grab an older neck from your collection and put it up against a modern neck. The modern neck will be much larger with many more feathers on it. The feathers will be longer and tie more flies. And each feather will have more and stiffer barbs. In fact, for traditional tyers, the hackles today are too good to tie some of the old Catskill patterns. Fortunately artificial selection allows breeders like Charlie Collins and others to maintain the qualities of the Dette and Darbee birds that some tyers seek. Whiting's Heritage line aims to fill that need as well.
However, artificial selection is not perfect. First, the genetics must be there to be able to be used by breeders. There are no rainbow colored capes or saddles because those genes do not exist in chickens. CRISPR gene editing and other genetic engineering techniques are emerging and they may be change that limitation as we have seen in a number of crops. Second, many traits are linked together due to their proximity on the chromosomes so when choosing for one trait, you may have a hard time not having another trait come with the trait you would like. Chickens (Gallus domesticus) have 78 chromosomes - coincidentally, the same number of chromosomes as dogs and wolves. This high chromosome count - you may remember humans have 46 chromosomes - means there are fewer linked traits which aids the speed and flexibility of artificial selection.
For the tyer, there are some downsides to the amazing dry fly hackles we have today. It used to be that the necks were selected for and saddles were an afterthought. Today the saddles fetch a higher price. As we select for longer and stiffer dry fly feathers, the soft hackles (hen necks and saddles) get stiffer as well. In fact, there are a few BWO dry-ish flies I tie with hen "soft" hackles and they float just fine on our spring creeks. Increasingly specialized products from different lines are being produced to provide tyers with better feathers for buggers, deceiver-like tails, flatwing streamers, and other applications where long but not-so-stiff feathers are preferred. But dry fly hackle is king - it is where producers make most of their money, I am sure.
The Renaissance is Upon Us
Fly tyers have never had it so good when it comes to fancy chicken feathers. If you go back and look at your old dry fly capes and saddles from days of yore, you will see that not only are today's feathers longer with stiffer and denser barbs but the stems today are worlds better. There is not a great reason for a rooster to have stems that are softer and easier to wrap around a hook shank. But years of selective breeding has produced stems that are supple and do not twist, both of which were issues with genetic hackles in days of yore (OK, I'll stop with that). If you are newer to tying, you may have no idea how good you have it!
Hackle has gotten so good and so consistent today that it is hard to buy hackle that is not a pretty decent value. Some of the rarer and more unique colors that fetch, in my mind, ridiculous prices, are about the only exception. Many of these cree, "champagne", rusty dun, and other sought after colors and variants are probably never tied with but instead enter someone's collection. There are more colors and options available today than every. We have access to dry fly necks and saddles today. And while those lines have changed, new lines for buggers, flatwing streamers, and other specialized uses. Want to tie traditional Catskill style dry flies? Charlie Collins is still producing great birds originally from the Darbee stock. Sideling Hill, which is carrying on the Collins stock (for more, read this...), are making some great hackle though they are a bit harder to find in stock (Etsy shop). I am sure there are other great options for those that are more into tying and fishing more traditional flies.
These are the good old days for those of use that have a thing for fancy chicken feathers. There is a greater diversity at a higher quality than ever before of dry fly hackle. Today's tyer has great options for both capes (necks) and saddles and many of them come at a great bargain. Whiting hackle has been the standard but a number of other producers are growing quality hackle you may find a bit more inexpensive to buy. Gone are the days of needing more than one feather for a dry fly. As are the days of thick and sometime twisting stems. Today's dry fly hackle is not exactly a miracle but the result of very specific choices that breeders made over the decades to provide us with the hackle we can buy today.
I certainly can not write all about dry fly hackle as I (sort of) promised. I wrote earlier about my thoughts on collecting and not tying with these quality necks and saddles. Dr. Tom Whiting is without question that name most associated with the breeding and production of dry fly hackle. There are a number of articles, including those written by Dr. Whiting himself. I would start with his Poultry Breeding page. Once you have read that, here are a few more links that ought to keep you busy for quite awhile:
Whiting Farms - Fly Tying Feather Facts
Fly Tying: What is "Genetic Hackle"? (MidCurrent)
Tim Camissa video - Differences Between Saddle Hackle and Capes
Savage Flies video - Fly Tying Materials for Beginners - Hackle Primer
Chicken Parts (J. Stockard)
Kelly Galloup video - Dry Fly Hackle Explained
Kelly Galloup video - Whiting Farms Episode 1: Introduction to Whiting Dry Fly Hackle
Kelly Galloup video - Whiting Farms Episode 2: How Whiting Dry Fly Hackle is Graded
Kelly Galloup video - Whiting Farms Episode 3: Colors and Phases of Dry Fly Hackle
Kelly Galloup video - Whiting Farms Episode 4: What Dry Fly Hackle is Right For You?
Kelly Galloup video - Whiting Farms Episode 5: Introduction to Whiting American Rooster Hackle
FlyFishFood video - Whiting Farms: How the World's Best Hackle is Made
FlyFishFood video - Dry Fly Hackle Genetic Lines Explained
Nordic Angler video - In-Depth Hackle Explained: Whiting Feathers, Roosters, Hen Saddle and capes