Darwinists and intelligent design theorists agree on some things. For example, thanks to millennia of human breeding programs, there are hundreds of horse breeds ranging from miniature horses to huge draft animals.
Horses were domesticated in Central Asia some 6,000 years ago. Today, most horse breeds result directly from human selection programs. But there is good evidence to suggest that, even before humans domesticated them, horses and their relatives, including zebras, donkeys, and other equine species, had a complicated and multi-branched genealogy with criss-crossing bloodlines on several continents.
Here is where Darwinists and ID theorists part company: Darwinists argue that the horse is simply a product of natural selection acting on random mutation. ID theorists leave open the possibility that the horse's complicated genealogy is evidence of design.
Darwinist Philosophy Limits Explanations
As Jonathan Wells points out in Icons of Evolution, claims about horse evolution have not fared well.
Almost as soon as Othniel Marsh, a Yale professor, published a drawing showing the supposed path of horse evolution from a four-toed ancestor to a one-toed (hoofed) animal in 1882 his notion of a linear pathway was challenged. Fossil evidence actually suggests that horse evolution is not linear, it's more like a tree with many branches, some dead.
But Marsh's suggested path bolstered the idea that there was a direction to evolution. This could be due to design by a creator or an internal mechanism driven by natural laws, in charge of evolution. This view, common in Darwin's time, is called orthogenesis (meaning 'straight' and 'origin') But no biological mechanism that leads to any specific evolutionary goal has been found.
Today, Darwinists reject the concept of goal-directed evolution. Yet that rejection of a goal is philosophical, not scientific. For example, Dawkins defined biology in his 1986 book, The Blind Watchmaker, as "the study of complicated things that give the appearance of being designed for a purpose."
If actual design is rejected in principle no evidence can be cited for it. But given the horses’ excellent ability to return to and adapt to their feral state and thrive in harsh climates, they do seem to be elegantly constructed.
Horse evolution is not a very good illustration of Darwinist theory
Horses are thought to have originated in North America where fossil remnants of their family tree go back 55 million years.
By the time horses went extinct in North America over 10,000 years ago, they filled forest and plains niches on several continents. It’s not entirely clear why they went extinct in North America, but it may have had something to do with the ice age that overtook the continent during that period.
Modern horses in North America, whether wild or tame, do not descend from the original North American animals. They descend from domesticated horses brought to the continent by Europeans. The Spanish conquistador Hern·n CortÈz was the first European to bring the horse back to North America, in the late 16th century.
The horse's family tree includes many extinct species. Genus Equus (horses, donkeys, asses, and zebras) is the only living branch in this bushy tree. Modern and extinct horses have adapted to geography and climate by changing their size and diet. Ancient horses even roamed continents to find better territory.
Still, the belief first popularized in the 19th century that there is a single linear evolutionary sequence of horses, extending from tiny fox-sized forest dwellers to modern horses, has remained popular with textbook writers.
But, as Bruce J. MacFadden Curator of Vertebrate Paleontology at the Florida Museum of Natural History, points out, horses did not evolve in a straight line from the fox-sized forest browser to modern horses.
Until recently, horses were thought to have moved progressively from a forest diet of leaves, to a plains diet of grass. But fossilized teeth show that some horses fed on grasses while their contemporaries fed on leaves. Some species ate both grass and leaves. And some apparently returned to a forest diet after they had already made a transition to a plains diet.
In Fossil Horses from "Eohippus" (Hyracotherium) to Equus: Scaling, Cope's Law, and Evolution in Body Size, Bruce MacFadden points out that horses also violate another expectation of Darwinism. Cope's Law states that species tend to grow larger over generations. But horses did not always become larger. Some species actually got smaller.
Even today we see a huge diversity in size among horses. Miniature horses may be as small as 18 inches (45 cm) high while the Shire stallion may be almost 6 feet high (178 cm) and weigh as much as 1100 kilograms. Twenty-first century horses adapt to Canadian muskeg, semi-desert prairie, and North African deserts with surprising ease.
But even after centuries of domestication, horses often escape human handlers. Wild horses have re-adapted to harsh climates, meager diets, and rough terrain. So Darwinists and ID theorists agree that horses have adapted to different environments. In the case of domesticated horses, design (in the form of artificial selection) played a role; in the case of feral horses, it did not.
But is the eminently adaptable horse itself the result of design? While ID is open to exploring that possibility, Darwinism rejects it outright.