Let's look at dogs again, as they are well understood examples of rapid evolution. They've been domesticated for roughly as long as humans have farmed, and in that time they have changed a great deal. You can see that dog behaviors are derived from the behavioral adaptations of wolves, their ancestral species. There are breeds like the Irish setter that point, and breeds like the Border collie that live to herd other animals. Both breeds show elaborations of behaviors we see in wolves. When wolves first scent a prey, the leading pack members freeze and point rigidly in the direction of the scent. Border-collie herding instinct must also derive from wolf game-herding patterns, but it is greatly exaggerated.
Dogs are much more playful than wolves, and this can probably be understood as retention of juvenile behavior (called "neoteny"). Retaining existing juvenile behavior is accomplished far more easily than evolving a behavior from scratch. Many of the ways in which dogs interact with humans can be understood as a new application of behavioral adaptations designed for a pack—the owner takes on the role of the leader of the pack.
There is no complex behavioral adaptation in dogs without a recognizable precursor in wolves, but that hardly means that all breeds of dogs are the same, or close to it. The testimony of accident statistics is stark: Biting—universal in dogs—is disproportionately distributed among breeds. A survey of U.S. dog attacks from 1982 to 2006 found 1 record of bodily harm attributable to Border collies, but 1,110 records attributable to pit bull terriers.9
While there has probably not been enough time for dogs to develop wholly new complex adaptations, there has certainly been enough time to lose some, sometimes in all breeds, but other times only in a subset of dog breeds. Wolf bitches dig birthing dens; a few breeds of dogs still do, but most do not. Wolves go into season in a predictable way, at a fixed time of the year; a few dog breeds do, but most do not. Wolves regurgitate food for weaned cubs, but dogs no longer do so. Male wolves help care for their offspring, but male dogs do not. Any adaptation, whether physical or behavioral, that loses its utility in a new environment can be lost rapidly, especially if it has any noticeable cost. Fish in lightless caves lose their sight over a few thousand years at most—much more rapidly than it took for eyes to evolve in the first place.
In some sense these are evolutionarily shallow changes, mostly involving loss of function or exaggerations and redirections of function. Although changes of this sort will not produce gills or sonar, they can accomplish amazing things. Dogs are all one species, but as we have noted, they vary more in morphology than any other mammal and have developed many odd abilities, including learning abilities: Dog breeds vary greatly in learning speed and capacity. The number of repetitions required to learn a new command can vary by factors of ten or more from one breed to another. The typical Border collie can learn a new command after 5 repetitions and respond correctly 95 percent of the time, whereas a basset hound takes 80-100 repetitions to achieve a 25 percent accuracy rate.
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