Chickens are the most popular poultry worldwide and are now used for both meat and egg production. There is only a sporadic information in the literature on chicken taxonomy; and there is a lack of a single concise document that deals with different issues related to this important topic. The present document contains detailed information on the tree taxonomy of chickens. In addition, process of domestication and its effect on chicken genetic diversity, physiology and behaviour are also discussed. The major conclusions are as follows: 1) Chickens are classified as: order: Galliformes, family: Phasianidae, genus: Gallus (junglefowl). Four species of junglefowl are recognized. These are: a) Gallus gallus (red junglefowl), b) Gallus varius (green junglefowl), c) Gallus sonneratii (grey junglefowl) and d) Gallus lafayetii (Ceylon junglefowl). The current chickens that are used for both meat and egg production commercially are domesticated fowl and are the descendants of the red junglefowl species. 2) Domestication process is a complicated one and involves intensive breeding and selection programmes. During this process, genetic diversity is still maintained, yet reduced, so careful attention is needed in the future to maintain the diversity of specialized chicken species. 3) Domestication resulted in basic changes in the behaviour, physiology and production of the bird, but still there are some similarities between the ancestor and the current chickens.

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