by Greg Mayer
Late summer is state- and county-fair time in much of the United States, and besides all sorts of odd foods (more on that below), these fairs afford a good opportunity to examine the phenomena of variation under domestication. These phenomena were of great interest to Darwin in helping him formulate his theory of descent with modification. He studied the amount and kinds of variation, the causes and inheritance of variation (less successfully), and, perhaps most important, the results of artificial selection. Besides his discussion in the the Origin, Darwin wrote a two volume monograph on the matter. That selection by breeders for slight variations could lead to large and permanent differences among domestic varieties provided Darwin with the vera causa he needed for natural selection.
On Friday I went to the Wisconsin State Fair, and headed straight for the small animal barn to see the pigeons. As we’ve noted before, Darwin’s own most extensive animal breeding experiments were with pigeons, and I like to see some of the many varieties up close and personal. The one above is very close to the wild form of Columba livia in color and pattern. The next few are also racing pigeons, but with varying colors and patterns.
[JAC note: As I recall, one of the readers for John Murray, the publisher of The Origin, said, when vetting the manuscript, that Darwin should jettison all the dull stuff on natural selection and evolution and just leave in the interesting stuff about pigeons!]
So far we’ve seen birds bred for behavior, and for color and pattern. Some pigeon breeds are bred for aesthetic criteria, and these often have variant feather shapes and number. Tail feather number is a taxonomically important trait in birds, so changing tail feather number is in some senses a “big” change.
The next birds show feathering on the feet. This is also a trait developed in some chicken breeds.
The next bird has a crest of feathers on its neck and a ruffle of feathers on its forehead, as well as feathered feet.
The next pigeon I find the most interesting, because it has a distinctive head shape, quite different from that of wild pigeons; head and bill shape are of great adaptive and taxonomic significance in birds. It also has a neck crest and ruffled chest feathers; I think it’s a “turbit”.
Darwin was also struck by head shape variation in domestic pigeons, and illustrated this in his Variation of Plants and Animals Under Domestication:
So, Darwin learned from pigeons not only that selection was efficacious in changing the hereditary characters of pigeons, but also that the modifications possible extended to many characters (color, pattern, behavior, size, shape, feathers, skeleton), including those of great ecological and taxonomic importance. Thus, selection could account for adaptively and taxonomically important characters, and, given enough time, he argued, these could add up to large differences between groups.
[JAC addition: In the first edition of Darwin's Origin, right in chapter 1 ("Variation under domestication"), Darwin establishes the crucial principle that, since animal and plant breeding are invariably successful, there must be ample genetic ("heritable") variation in populations. Were this not the case, natural selection could not be efficacious and evolution of any sort couldn't occur. Here's a famous quote (my emphasis) from that chapter:
The great power of this principle of selection is not hypothetical. It is certain that several of our eminent breeders have, even within a single lifetime, modified to a large extent some breeds of cattle and sheep. In order fully to realise what they have done, it is almost necessary to read several of the many treatises devoted to this subject, and to inspect the animals. Breeders habitually speak of an animal's organisation as something quite plastic, which they can model almost as they please. If I had space I could quote numerous passages to this effect from highly competent authorities.
The breeder as sculptor! So one of the tests of Darwin's theory was to see if one could take wild plants and animals and, using artificial selection, transform them in a desired direction. That would establish that those wild populations had genetic variation for things like size, color, behavior, body shape, and so on. As Greg notes, all of these changes have been effected via selection on pigeons, descended from the rock dove. He doesn't mention behavior, but look up "roller pigeons."
We now know that genetic variation is ubiquitous, removing one of the possible objections to the theory of evolution. In Drosophila (the fruit flies I work on), there have been hundreds or even thousands of selection experiments on traits ranging from bristle number to behavior, and I'm aware of only three that did not change the population in the desired direction. Two of those three were done by me, involving selection on directional asymmetry (e.g., flies whose right eyes were bigger than the left, and vice versa.). I haven't yet published the second "failure."]
Back to Greg:
And now for the food! The quintessential foodstuff of the Wisconsin State Fair is the cream puff (although there was some controversy this year about the source of the cream: from an Illinois-based dairy cooperative, but supposedly only from Wisconsin members of that cooperative, and actually prepared in Iowa). Regardless, they were delicious.
As regards this next advertised food, I don’t know what it is, and I didn’t find out!