For many people, the horse family remains the classic example of evolution. As more and more horse fossils have been found, some ideas about horseevolution have changed, but the horse family remains a good example ofevolution. In fact, we now have enough fossils of enough species in enoughgenera to examine details of evolutionary change.Evolution does not occur in a straight line toward a goal, like a ladder; rather,evolution is like a branching bush, with no predetermined goal. Horsespecies were constantly branching off the evolutionary tree and evolvingalong various unrelated routes. There’s no discernable straight line of horseevolution. Many horse species were usually present at the same time, withvarious numbers of toes, and adapted to various diets. In other words, horseevolution had no inherent direction. We only have the impression of straightline evolution because only one genus happens to still be alive, whichdeceives some people into thinking that the one genus was somehow thetarget of all the evolution. Instead, that one genus is merely the last survivingbranch of a once mighty and sprawling bush.Tracing a line of descent from Hyracotherium to Equus reveals severalapparent trends: reduction of toe number, increase in size of cheek teeth,lengthening of the face, and increase in body size. But these trends are notseen in all of the horse lines. On the whole, horses got larger, but somehorses then got smaller again. Many recent horses evolved complex facialpits, and then some of their descendants lost them again. Most of the recenthorses were three-toed, not one-toed, and we see a trend to one toe onlybecause all the three-toed line have recently become extinct. Additionally,these traits do not necessarily evolve together, or at a steady rate. Thevarious morphological characters each evolve in fits and starts, and did notevolve as a suite of characters. For example, throughout the Eocene, the feetc...
