Genomic data provide an answer to this long-standing debate?
In 1868, Charles Darwin wrote that “we have not such good evidence with fowls as with pigeons, of all breeds having descended from a single primitive stock.” This statement is rather surprising if you read The Origin of Species – published in 1859 – where he argued that all domestic chicken breeds descend from a single ancestor: the Red Junglefowl (Gallus gallus, then named Gallus bankvia). Where did the uncertainty in 1868 about the origin of domestic chickens come from? A recent paper by Hein van Grouw and Wim Dekkers in the Bulletin of the British Ornithologists’ Club reconstructed the historical events leading up to Darwin’s uncertainty.
Our story begins with French naturalist Georges–Louis Leclerc, Comte de Buffon who believed that different chicken breeds can be traced back to several ancestral wild species (i.e. a polyphyletic origin). For example, he stated that most European breeds descended from the wild Red Junglefowl, while seven giant chicken breeds could be traced back an unknown wild ancestor. Several decades later, Coenraad Jacob Temminck (director of the State Museum of Natural History in Leiden) received a single foot of very large fowl from Indonesia. He assumed it belonged to a large species of wild junglefowl and described it as the Jago Cock (Gallus giganteus). Could this be the ancestor that Buffon predicted?
Edward Blyth, the curator of the Asian Society of Bengal in Calcutta, rejected the polyphyletic origin of chicken breeds. He argued that the varieties of domesticated chickens had evolved by artificial selection from a single wild ancestor: the Red Junglefowl. This monophyletic theory, published in 1851, caught the attention of Charles Darwin because it supported his analogy between artificial selection by humans and natural selection. If humans can create such diverse creatures in a few generations, imagine what nature can do over millions of years! The two naturalists corresponded about the domestication of chickens and in one letter Blyth firmly buried the polyphyletic ideas of Buffon and Temminck, writing: “My very decided opinion, that we may seek in vain for wild types of G. giganteus.”
In The Origin of Species, Darwin credits Blyth for the monophyletic idea: “Mr. Blyth, whose opinion, from his large and varied stores of knowledge, I should value more than that of almost any one, thinks that all the breeds of poultry have proceeded from the common wild Indian fowl.” So, why did Darwin change his mind a few years later? Following the First Opium War between the Qing dynasty and the United Kingdom (1839-1842), several large chicken breeds reached the English naturalists from Asia. After studying these peculiar specimens, Darwin was unsure whether some morphological characters could have been the result of artificial selection from a single ancestor. Perhaps different domestic chicken breeds did originate from different ancestors, such as the Jago Cock?
Darwin continued to investigate the origin of chicken breeds and studied the skeletons and skulls of several breeds. In one breed – the Cochin – he observed certain features that did not occur in Red Junglefowl or any of the other breeds. Despite the aberrant morphology of the Cochin breed, Darwin focused on the similarities between the other breeds and concluded that these chicken breeds probably had a monophyletic origin.
The Cochin, with its deeply furrowed frontal bones, peculiarly shaped occipital
foramen, short wing-feathers, short tail containing more than fourteen feathers, broad nail to the middle toe, fluffy plumage, rough and dark-coloured eggs, and especially from its peculiar voice, is probably the most distinct of all the breeds. If any one of our breeds has descended from some unknown species, distinct from G. bankiva [Gallus gallus], it is probably the Cochin; but the balance of evidence does not favour this view.
The Genomic Picture
The morphological studies of Darwin and other naturalists did not completely settle the debate about the polyphyletic or monophyletic origin of chicken breeds. The uncertainty is obvious in Darwin’s quotes cited above. Nowadays we can turn to genomic data to answer questions about the domestication of certain animals and plants. And that is exactly what Raman Akinyanju Lawal and his colleagues did. They sequenced the genomes of 53 domestic chickens and several individuals of four wild species: the Red Junglefowl, the Grey Junglefowl (G. sonneratii), the Ceylon Junglefowl (G. lafayettii) and the Green Junglefowl (G. varius).
Phylogenomic analyses revealed that the domestic chickens clustered with the Red Junglefowl (see figure below), suggesting that they all originated from a single ancestor. However, D-statistics pointed to gene flow between between domestic chickens and the Grey, Ceylon, and Green Junglefowl species (see this blog post for details on D-statistics). A closer look at particular introgressed regions suggested that they were recently exchanged between the species. For example, some introgressed tracts were relatively long and have thus not been broken down by recombination. Taken together, these patterns show that domestic chicken breeds originated from a single ancestor (the Red Junglefowl) and received genetic material from other species later on. So, while the origin of domestic chickens is monophyletic, their current genetic make-up is polyphyletic.
Lawal, R. A., et al. 2020). The wild species genome ancestry of domestic chickens. BMC Biology, 18(1), 1-18.
van Grouw, H., & Dekkers, W. (2020). Temminck’s Gallus giganteus; a gigantic obstacle to Darwin’s theory of domesticated fowl origin?. Bulletin of the British Ornithologists’ Club, 140(3), 321-334.
Featured picture: Feral rooster on Kauaʻi © Frank Schulenburg | Wikimedia Commons
The papers have been added to the Galliformes page.