You can start with 2 birds and have as much genetic veration, as if you would have out crossed. You do not need to start with this large and vast amount of diversity. Sure the more you have the better! But that does not mean you can not get it with out only have two or three birds. The key is numbers! The more you breed the wider the selection will be! If you start with only two you will need to hatch out LOTS. (Thousands!) of chicks to get genetic diversity. And using just a few breeding techquines you can do this while increasing genetic diversity, while not increasing how in bred they are. This is not information i am making up. Conversations with a great breeder and friend (Also a Biologist). You know you could end up with duplicates, sure. But 99% of breeds are not to the point where we need to worry about it. Also i have taken note that only slight devation's in the same strain but bred by seperate breeders is enough. Coupled with correct breeding strategics it's better then outcrossing.
I am wondering if you have bred chickens for any length of time? Even though i have only breeding for six years, there are somethings you just find out. The first year of breeding i developed a new line of Blue Rosecombs. Never got the to SQ before i dispresed of them because i lost interest in the flighty little breed
Last comment on this. I guarantee, GUARANTEE that a trained geneticist wants genetic diversity which only come rom large numbers. I watched geneticist From large genetics genetics company that purchased a strain of birds and demand to acquire hatching eggs from as many birds in production as possible for MAXIMUM GENETIC diversity. Yes, commercial breeding programs are different but genetics is genetics. Two hens will not give you the same genetic diversity as 100 birds of the same breed. Fact, you will never get the same genetic diversity in two hens as 10, or 20, or 50 and so on.
A good breeder of exhibition poultry stock will then breed AGAINST genetic diversity because thy will want the maximum number of their offspring to be of good to great quality. I saw 45 Silver Sebrights brought to one show by the same breeder and the APA judge commented "this is like separating a bunch of peas from the same pod". This was quite a compliment to the breeder because it demonstrated his success, his line had become very homogeneous genetically, which is the goal of all quality breeders, for all offspring to be great!!!! So if that breeder discovers he lacks a specific trait, he will have to go outside of his line to get it. In cases where entire breeds or a persons access to that breed becomes limited in his type of line, crossing outside the breed MIGHT be necessary.
Your last comment, wondering if have I bred chickens at all or for any length of time. Uh, yes. I got my first chickens to breed 36 years ago and I haven't gone a single year of that time when I haven't bred chickens or other birds. Pulled a hatch two weeks ago with 200+ chicks, this is the minimum I hatch at least twice each month. I am not ONLY regurgitating from a book here.
Anyway, to each his own. I am all for the breeding and preservation of Fowl of any kind. Chickens, pheasants, jungle Fowl, Trogapons, the list could go on. Good luck everyone......