Artificial selection has also been responsible for the great variety of breeds of cats, dogs (figure 22.14), pigeons, cattle and other domestic animals. In some cases, breeds have been developed for particular purposes. Greyhound dogs, for example, were bred by selecting for maximal running abilities, with the end result being an animal with long legs and tail (the latter used as a rudder), an arched back (to increase the length of its stride), and great muscle mass. By contrast, the odd proportions of the ungainly basset hound resulted from selection for dogs that could enter narrow holes in pursuit of rabbits and other small game. In other cases, breeds have been developed primarily for their appearance, such as the many colorful and ornamented varieties of pigeons or the breeds of cats.
Domestication also has led to unintentional selection for some traits. In recent years, as part of an attempt to domesticate the silver fox, Russian scientists each generation have chosen the most docile animals and allowed them to reproduce. Within 40 years, the vast majority of foxes born were exceptionally docile, not only allowing themselves to be petted, but also whimpering to get attention and sniffing and licking their caretakers. In many respects, they had become no different than domestic dogs! However, it was not only behavior that changed. These foxes also began to exhibit different color patterns, floppy ears, curled tails, and shorter legs and tails. Presumably, the genes responsible for docile behavior have other effects as well (the phenomenon of pleiotropy discussed in the last chapter); as selection has favored docile animals, it has also led to the evolution of these other traits.
Can Selection Produce Major Evolutionary Changes?
Given that we can observe the results of selection operating over relatively short periods of time, most scientists believe that natural selection is the process responsible for the evolutionary changes documented in the fossil record. Some critics of evolution accept that selection can lead to changes within a species, but contend that such changes are relatively minor in scope and not equivalent to the substantial changes documented in the fossil record. In other words, it is one thing to change the number of bristles on a fruit fly or the size of a corn stalk, and quite another to produce an entirely new species.
This argument does not fully appreciate the extent of change produced by artificial selection. Consider, for FIGURE 14
Breeds of dogs. The differences between these dogs are greater than the differences displayed between any wild species of canids.
example, the breeds of dogs, all of which have been produced since wolves were first domesticated, perhaps 10,000 years ago. If the various dog breeds did not exist and a paleontologist found fossils of animals similar to dachshunds, greyhounds, mastiffs, chihuahuas, and pomeranians, there is no question that they would be considered different species. Indeed, these breeds are so different that they would probably be classified in different genera. In fact, the diversity exhibited by dog breeds far outstrips the differences observed among wild members of the family Canidaesuch as coyotes, jackals, foxes, and wolves. Consequently, the claim that artificial selection produces only minor changes is clearly incorrect. Indeed, if selection operating over a period of only 10,000 years can produce such substantial differences, then it would seem powerful enough, over the course of many millions of years, to produce the diversity of life we see around us today.
Artificial selection often leads to rapid and substantial results over short periods of time, thus demonstrating the power of selection to produce major evolutionary change.