Man, this guy didn’t know anything.
I don’t mean that as an insult. Darwin, as he admits, knew almost nothing about inheritance, about how variation is produced, or about the origins and history of domesticated plants and animals. You’d think that would be a handicap in using domestication as an analogy for evolution.
And yet, in chapter 1 of the Origin, ‘Variation Under Domestication’, Darwin uses what little knowledge he has so deftly that nowhere do you feel his conclusions are outstripping his data. This, believe me, is quite a skill, both in a scientist and a writer. What, he asks, is the minimum we can infer from what we know?
Well, here’s what his readers can be sure of at the end of the chapter:
- Domestic animals and plants vary – most obviously between different breeds of the same species.
- Domestic animals and plants have changed through time.
- This change results from humans breeding selectively from individuals that carry a valuable trait, because offspring tend to look like their parents.
And that’s about it — hard to argue with any of that, then or now. Darwin does speculate about mechanisms: most variation, he suggests, comes from within the organism, by “the conditions of life … [acting] on the reproductive system”. You could see this as analogous to mutation, although I think that would be stretching inference beyond knowledge. There’s also a shorter discussion of inheritance, the laws governing which are “quite unknown”.
But none of this speculation is load-bearing — the only essentials are right before our eyes. Darwin begins, with great care, by laying his foundations. Probably because no one else had laid any for him to build on.
If it was down to you to invent biology, where would you begin? Darwin takes the time-honoured path of sacrificing realism for tractability, and studies a simplified and controlled version of nature: farming. He recognized that animal breeders were the biotechnologists of his day, and possessed the nearest thing to a body of experimental biological knowledge.
Once he’s latched onto domestic species, Darwin zooms in still further and begins with what is in effect a model organism: “Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons.”
I think I can guess how those deliberations went: ‘Hmm. I need something that I can keep in reasonably large numbers. It’d also be nice if they bred quickly. (Otherwise it’ll take me, like, 20 years to publish.) That rules out sheep, pigs, cattle and so on. Rabbits? They all look pretty much alike — it’d be nice if they were glaring differences between breeds. I need to be reasonably confident that modern varieties are descended from a single wild species. And I’d really like an excuse to hang around in insalubrious London gin palaces. I know — pigeons!’
(This image is taken from Darwin’s ‘Variation in Animals and Plants under Domestication’ of 1868. I found it on Steven Carr‘s page.)
These deliberations, after all — speed of breeding, ease of rearing, high variability — lie behind others’ choice of other species as models, such as the fruit fly Drosophila.
A large part of the ‘Pigeons’ section involves Darwin arguing that the different breeds are all descendents of the rock dove, Columba livia. He spends much of the rest of the chapter arguing that, for most domesticated species, the different varieties of each domestic plant and animal species are usually descended from a single wild ancestor species.
You can see why this is crucial — it’s the evidence that a single ancestor can give rise to many new forms, be they breeds or species (what we now call adaptive radiation). Darwin also notes that breeders create novelties by selecting and breeding within a variety, rather than by crossing varieties — our first whiff of the equally crucial concept of reproductive isolation.
This belief in common ancestry apparently ran against received opinion: “all the breeders of the various domestic animals and the cultivators of domestic plants, with whom I have ever conversed, or whose treatises I have read, are firmly convinced that the several breeds to which each has attended, are descended from so many aboriginally distinct species”.
Again, this shows how little Darwin, or anyone else, had to go on. In this chapter’s Darwin-gets-it-wrong moment, he says “I do not believe … that all our dogs have descended from one wild species.” Whereas DNA evidence now points to them all being descended from wolves, and first domesticated in East Asia.
DNA sequencing and archaeology haven’t cleared everything up, though. For many domesticated species, the number of domestication events and their timing and location has been controversial. To give one example, the number of times pigs were domesticated has recently been revised upward.
Lacking DNA, or any record of humanity older than ancient Egypt, Darwin has to argue from plausibility. For example, when Britain has “hardly one peculiar [i.e. endemic] mammal”, he asks, is it likely that each of the country’s sheep breeds is descended from a different wild ancestor?
He is also resourceful in his sources, willing to truffle out evidence from anywhere:
“Savages now sometimes cross their dogs with wild canine animals, to improve the breed, and they formerly did so, as is attested by passages in Pliny.”
“The principle of selection I find distinctly given in an ancient Chinese encyclopaedia.”
And, as evidence of ancient artificial selection, he mentions that “from passages in Genesis, it is clear that the colour of domestic animals was at that early period attended to.”
Next up, on Wednesday, ‘Variation under nature’, a chapter that Darwin described as ‘short & dry‘.