A century ago, paleontologists typically depicted the evolutionary pattern of fossil horses in North America as a linear sequence from smaller ancestor to progressively larger descendent (Fig. 2), which fit well with a concept popular during the nineteenth century called orthogenesis, or “straight-line evolution” (“ortho-,” Gr., straight, direct; “-genesis,” Gr., origin). This concept, attributed to Haacke (1893) and other European scientists (Simpson 1944), carries along with it notions that evolution is progressive, represents improvement, and is predestined; these were accepted during that time, but, in modern scientific thought, they are not considered as part of the theoretical framework of macroevolution (MacFadden 1992; Gould 2002). Nevertheless, since that time, this pattern of the fossil horse lineage was embodied in museum displays (Clark 2008) and other media that communicated about evolution. Thus, this sequence of fossil horses has been widely recognized as a classic, “textbook” example of evolution (Gould 2002; Dyehouse 2011). More than a half century ago, Simpson (1944, p. 157) stated that: “The most widely cited example of orthogenesis, in any sense of the word, is the evolution of the horse.”
The problem with depicting fossil horses as orthogenetic is that, by the early twentieth century, paleontologists understood that, rather than a simple, straight-line sequence (Fig. 3A), the actual fossil record of horses was a complexly branching tree (Fig. 3B). The latter is exemplified by Gidley’s (1907; Fig. 4 here) branching diagram of the subfamilies of fossil horses in which the Anchitheriinae overlaps in time with the Protohippinae during the late Miocene and the Protohippinae overlaps in time with the Equinae. In the orthogenetic model (and more recent concept of anagenesis), this overlap cannot occur as a macroevolutionary pattern because ancestral and descendant taxa (species) do not overlap in time, i.e., the former is replaced in time by the latter (Fig. 3). Following on the branching pattern, several decades later, Matthew (1930; Fig. 5) depicted the evolution of fossil horses with numerous genera and independent lineages that overlapped in time. This scheme was further elaborated upon by Simpson’s (1951; Fig. 6) now widely cited horse phylogenetic tree, which continues in its essential pattern to the present day (MacFadden 1992, 2005 [Fig. 7]; Franzen 2010). Thus, to summarize our current state of knowledge, instead of a linear sequence in which ancestral species evolve directly into their descendants, the evolutionary tree of horses is bushy, with many species overlapping in time, multiple originations, and frequent extinctions.
Despite this knowledge, to this day, the classic story of horse evolution in museums, books, and other media is still oftentimes depicted as orthogenetic. In addition to being factually incorrect within a modern scientific context of what we know about evolution, orthogenesis brings along with it baggage about evolution being largely progressive, deterministic, and representing improvement (MacFadden 1992; Gould 2002). These notions associated with orthogenesis therefore likely contribute to part of the general public’s misunderstanding, or incomplete knowledge, about fundamental aspects of evolution (MacFadden 1992; Diamond and Scotchmoor 2006; Spiegal et al. 2006; MacFadden et al. 2007). In Jonathan Wells’ (2000) book entitled Icons of Evolution: Science or Myth? Why much of what we teach about evolution is wrong, the author cites fossil horses as one of ten fundamental examples of evolution. He also accurately describes some of the problems associated with the miscommunication and states (Wells 2000, 195): “Since the 1950s, neo-Darwinian paleontologists have been actively campaigning to replace the old linear picture of horse evolution with the branching tree.” As we will see below, this campaign has not been particularly successful.
We assert that, despite its roots in nineteenth century evolutionary theory, the outmoded scientific concept of orthogenesis is still widely communicated to the general public through various media, including natural history museum exhibits. We use fossil horses to test this hypothesis because of their widespread use as fundamental evidence for macroevolution (e.g., Gould 2002).