Re to the question, there is a very definite 'type'.
Size, is very uniform, comb types, what they have and dont have, diversity of colors, i.e. other colors havent been selected out by selecting for one or another. You find this in OEGBs also. A breed with many colors. Also uniform in size and conformation as the Icelandic Chicken is. There is a recognizeable type.
The Icelandic Chicken has a very strong survivablity that other chickens do not have, shown in part in the ability to get much of their food in ranging. They still brood their chicks, a trait lost in many other breeds/varieties.
Their gestation period is usually shorter then other chickens, a trait with real survival value in a country where harsh cold snaps come often in the spring and summer.
These traits and more they exhibit and pass on to their offspring, one of the hallmarks of a breed.
The research that is of the Icelandic Chicken has happened to a small degree in Iceland by the university there. It is a fact that their genes differ greatly from modern breeds. As soon as I can find a translated paper I will post a link to it here. Meanwhile I will maintain and grow my small flock, as so many others have done over the past thousand years. Much of what we learn about the Icelandic Chicken is going to be learned right here in America. Every flock is a lab. And over time we will build data bases. The School of the Icelandic Chicken is right here, as are the students and teachers and researchers.
This is one of the most fascinating breeds of chickens that I have encountered in my fairly long life. I have kept a number of breeds of chickens, Rhode Island Reds, several colors of Wyandottes, Barred Rocks, OEGBs, Blue Andalusions, some others, and a few mutts. All of these were good chickens, and well worth having. There are of course many others out there, in this country we have a wealth of breeds and non breeds to have and to raise. They are different from Icelandic Chickens. If you donot use selection very carefully in just a few generations they will change markedly in appearance, away from known standards of the breed, they willNOT reproduce themselves to a known type.
The Icelandic otoh, reproduces itself, and has for over a thousand years. Without a standard. That itself is unique enough for me to want to reproduce them .
I do think that there will be some changes in them, some flocks will become more uniform in colors for the owner preferring one color over another, a pity but it will be. Some flocks will become noticeably larger in size as that is selected for, there will be some differences in flock appearance because of owner selection, but, if we are very careful to keep out other influences we will still have recognizeable Icelandic Chickens.
We will still have Icelandics, and we will know much more about them then we do now, and much more every year.
Life is good, and better with Icelandic Chickens and Packgoats !