A clever argument to support common descent.
Although I have studied evolutionary biology for nearly fifteen years – as both a student and a professional scientist – I had never read On the Origin of Species from cover to cover. During the Christmas holidays, I finally decided to explore this groundbreaking work in detail. Despite its Victorian prose, the book remains remarkably accessible and rich with intriguing ideas. As I worked my way through the chapters, I became inspired to write a series of blog posts related to The Origin of Species, each starting from a specific quote or concept and developing it in light of our current understanding of evolution. This first blog post focuses on a striking comparison in Chapter V, “Laws of Variation,” where Darwin draws parallels between domesticated pigeon breeds and several species within the horse genus.
Multiple Ancestors
Before we can compare pigeons and horses, we first need to understand the origin of domesticated pigeon breeds. In Darwin’s time, most naturalists believed that each pigeon breed descended from a distinct wild species. Darwin challenged this view in the first chapter (“Variation Under Domestication”), where he presented compelling evidence that “all our domestic breeds are descended from the rock-pigeon or Columba livia with its geographical sub-species.” Let’s briefly review the main arguments supporting this single-ancestor hypothesis.
First, Darwin argues that it is extremely unlikely that each domestic pigeon breed descended from a distinct wild species. This scenario would require the existence of seven or eight separate ancestral species. If this were the case, where are these wild ancestors today? Have they all gone extinct? As Darwin himself notes, “the supposed extermination of so many species having similar habits with the rock-pigeon seems a very rash assumption.” Moreover, many of the distinctive traits found in domestic pigeon breeds have no counterparts in any known wild species.
We may look in vain through the whole great family of Columbidae for a beak like that of the English carrier, or that of the short-faced tumbler, or barb; for reversed feathers like those of the Jacobin; for a crop like that of the pouter; for tail-feathers like those of the fantail.
Accepting multiple wild ancestors would therefore require us to assume not only that early humans deliberately (or accidentally) selected a series of exceptionally abnormal species, but also that all of these species subsequently became extinct or remain entirely unknown. Taken together, this chain of assumptions is highly implausible.
Fertile Hybrids
In addition to arguing against multiple ancestral species, Darwin also offers positive evidence in support of a single-ancestor hypothesis. In particular, he draws on observations of both the fertility and morphology of hybrids produced by crossing different pigeon breeds. Based on his own experiments, Darwin reports that “the hybrids or mongrels from between all the breeds of the pigeon are perfectly fertile,” a finding that strongly suggests close relatedness among the breeds.
Morphological evidence further reinforces this statement. Darwin observed that hybrid offspring often display traits characteristic of the ancestral rock pigeon, even when these traits are absent in the parent breeds. He concluded that “we can understand these facts, on the well-known principle of reversion to ancestral characters, if all the domestic breeds are descended from the rock-pigeon.”
Counting Stripes
The final argument supporting a single ancestral origin for pigeon breeds – reversion to ancestral characters – also plays a pivotal role in Darwin’s comparison with horses. In Chapter V, “Laws of Variation,” Darwin extends this reasoning to patterns of variation in both wild and domesticated equine species. He begins with a detailed survey of asses and horses that exhibit striping on the legs or along the back. Rather than reviewing all of his examples here, I will focus on a representative passage that nicely illustrates Darwin’s determination to amass as much empirical evidence as possible.
With respect to the horse, I have collected cases in England of the spinal stripe in horses of the most distinct breeds, and of all colours; transverse bars on the legs are not rare in duns, mouse-duns, and in one instance in a chestnut; a faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace in a bay horse. My son made a careful examination and sketch for me of a dun Belgian carthorse with a double stripe on each shoulder and with leg-stripes. I have myself seen a dun Devonshire pony, and a small dun Welsh pony has been carefully described to me, both with three parallel stripes on each shoulder.
Darwin next considers the “effects of crossing the several species of the horse genus.” Once again, he marshals numerous examples of hybrids that exhibit striping on different parts of the body. In one striking anecdote, he recalls, “I once saw a mule with its legs so much striped that anyone might have thought that it was a hybrid zebra.” He further describes striping in hybrids between an ass and a zebra (“whose legs were much more plainly barred than the rest of the body”) as well as in crosses between a chestnut mare and a male quagga, and between an ass and a hemionus (now known as the onager).
ArtsCult.com | Wikimedia CommonsA Common Parent
Now, we are finally in a position to directly compare pigeons and horses. By invoking the principle of reversion to ancestral characters, Darwin builds a compelling case for the common ancestry of all species within the horse genus. He explicitly draws the reader’s attention back to domestic pigeon breeds, which (despite their striking diversity) are known to descend from a single ancestral species. In that case, the occasional reappearance of ancestral traits in particular individuals or in hybrids is readily explained by shared descent. Darwin argues that the same logic applies to horses: the recurrence of striped patterns reflects common ancestry as well, although unfolding over longer evolutionary timescales.
Call the breeds of pigeons, some of which have bred true for centuries, species; and how exactly parallel is the case with that of the species of the horse genus! For myself, I venture confidently to look back thousands on thousands of generations, and I see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse (whether or not it be descended from one or more wild stocks) of the ass, the hemionus, quagga, and zebra.
A Mere Mockery and Deception
It is fascinating to see how Darwin weaves together multiple lines of evidence (each grounded in a wealth of careful observations) to make a compelling case for the common descent of morphologically diverse species. For me, his writing offers a masterclass in how to construct a rigorous argument for or against a scientific hypothesis. At the end of Chapter V, Darwin contrasts his explanation with the prevailing view of the time (independent creation), underscoring the power of his reasoning. It is a fitting and powerful conclusion to both the chapter and this blog post.
He who believes that each equine species was independently created, will, I presume, assert that each species has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like the other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown cause. It makes the works of God a mere mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the seashore.
References
Darwin, C. (1997). On the origin of species. Wordsworth Editions. (Original work published 1859)
Featured image: Common Pigeon (Columba livia) © Satdeep Gill | Wikimedia Commons
Avian Hybrids 
