What is a chicken?

We refer to them by the species name Gallus gallus domesticus, but there was a time before they had any connection to us. The wild species is Gallus gallus, also known as the Red Jungle Fowl, and it can still be found running around the wilds of south-east Asia.

There is genetic evidence that modern chickens arose from multiple independent domestication events. The diversity of alleles found in domestic chickens encompasses those found in wild populations of G. gallus spread through India (G. g. murghi), Burma (G. g. spadiceus), and Tailand (G. g. gallus). This is best explained by the early incorporation of Red Jungle Fowl from different regions into the common pool of chickens being cared for by people.

It turns out that there are three other related species of jungle fowl (grey, Ceylon, and green) roaming the area of south-east Asia. A trait found in domesticated chickens that causes yellow skin on the legs and feet is due to an allele which shows most similarity to an allele found in the Grey Jungle Fowl.

A. Green stars indicate putative domestications.
B. Domesticated chicken.
C. Red Jungle Fowl. (Range in red in A.)
D. Grey Jungle Fowl. (Range in grey in A.)

At least four different populations across two (of what we consider) separate species contributed to modern domesticated chickens.

How could the process of domestication start in multiple places at the same time? Well... it can't, but it can happen close enough in time to be indistinguishable to modern researchers.

It is a common pattern in domestication for the idea of domesticating an animal or plant to spread faster than the newly domesticated organism can spread. This results in multiple independent domestication of a single species, or of similar species, found across a wide area.

Cattle appear to have been domesticated two or three times (from Bos tauros, B. indicus, and possibly B. africanus). Sheep and goats appear quite distinct to us now, but when they were domesticated, they were very similar creatures.

Chile peppers have been domesticated at least five times (Capsicum annum, C. chinense, C. frutsecens, C. bacatum, C. pubescens). Squash were domesticated at least five times (Curcurbita pepo, C. moschata, C. maxima, C. mixta, C. ficifolia). Carrots (Daucus carota), parsnips (Pastinaca sativa), celery (Apium graveolens), parsley (Petroselinum crispum), Dill (Anethum graveolens), and chervil (Anthriscus cerefolium) all belong to the family Apiaceae and look very similar in their wild state.

So.  What is a chicken?

It is an example of how the rapid spread of ideas through human culture impacts the process of wild things becoming integral to our civilization.

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References

1. http://en.wikipedia.org/wiki/Red_junglefowl
2. Multiple domestication : http://www.biomedcentral.com/1471-2148/8/174
3. Hybrid between red and grey jungle fowl : http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000010 
4. Cattle : http://archaeology.about.com/od/domestications/qt/cattle.htm
5. Chile peppers : http://archaeology.about.com/od/cbthroughch/qt/Chili-Peppers.htm
6. Squash : http://en.wikipedia.org/wiki/List_of_gourds_and_squashes
7. Apiaceae : http://science.jrank.org/pages/1240/Carrot-Family-Apiaceae-Edible-species-in-carrot-family.html