3. Variation under Domestication
The opening chapter of ‘The Origin of Species’ deals with variability within species. Charles Darwin begins his book by providing evidence that is visible to all – the variability of the plants, pets and farm animals that could be found in Victorian England. Ever since people have been domesticating plants and animals there has been a tendency to be choosy about the appearance, hardiness and usefulness of the offspring. If a hunter wanted a dog that could chase down a deer, he would not choose the runt of the litter – he would naturally look for the dog with the longest legs and the slimmest build. If this dog turned out to be a good hunter, he would be sure to breed it with another dog with the same qualities. By means of this type of selection, and a few generations of man, these selected dogs would form a breed. By this method, humanity has been selecting their food plants, their beasts of burden, their fleshy fowls and cattle, their goats and cows for milk – breeding then selectively for generation upon generation, until the resulting animals are starkly different from their original wild stock.
Not happy with anecdotal information, Darwin began to raise pigeons. In the Victorian era, pigeon keeping was a great hobby of the working classes. These pigeon fanciers prided themselves on the variety of distinctly featured pigeons that they could raise. And the variations were immense. Darwin noted that if any of these pigeon varieties were found in nature, they would be considered distinct species – yet the were all varieties of the common rock dove, Columba livia.
From his correspondence and personal observations, Darwin also recognised that certain variations seemed to be linked with other variations. Darwin referred to these as, ” …the mysterious laws of correlation.” Thus long legs correlate with long heads, male white cats with blue eyes are usually deaf and hairless dogs tend to have bad teeth. He also noticed that certain inherited traits could skip generations and be transferred from one sex to both sexes, or sometimes to just the male or female descendants.
Darwin was convinced that these variations were the result of domestication itself, and the effect of environment on the parents:
“WHEN we look to the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us, is, that they generally differ much more from each other, than do the individuals of any one species or variety in a state of nature.” (Ch 1, pg 7)
“…I am strongly inclined to suspect that the most frequent cause of variability may be attributed to the male and female reproductive elements having been affected prior to the act of conception.” (Ch 1, pg 8)
“Nevertheless some slight amount of change may, I think, be attributed to the direct action of the conditions of life—as, in some cases, increased size from amount of food, colour from particular kinds of food and from light, and perhaps the thickness of fur from climate.” (Ch1, pg 11)
From his observations and the information he received from others, Darwin was led to believe that variations occur more often under the influence of domestication then in the wild. Today we understand that there is no difference – in the wild, well adapted species in a stable environment also produce variations, but most of these variations are quickly reabsorbed into the population or eliminated by natural selection.
In Darwin’s time, “The laws governing inheritance (were) quite unknown;…”(Ch 1, pg 13) –Gregor Mendel’s work was still in progress. By the 5th edition of the Origin of Species, Darwin believed that Lamarkian evolution, the “use and disuse” argument or the heritability of acquired characteristics could be a cause of variation. Today we know that variation is caused by random mutation and gene recombination. In domesticated varieties, man’s selection helps ‘fix’ these changes to form breeds, though Darwin recognized that even in well established breeds variations would still occur.