Free Full House by Stephen Jay Gould Page B

million years. This particular pathway cannot be interpreted as a summary of the bush; or as an epitome of the larger story; or, in any legitimate sense, as a central tendency in equine evolution. We have chosen this little sample of a totality for one reason alone: Equus is the only living genus of horses, and therefore the only modern animal that can serve as an endpoint for a series. If you are committed to depicting the evolution of any living group as a single pathway from an ancestral point to an item of current glory, then I suppose that the story must be told in this conventional way. But when we consider more comprehensive models of evolution, we must call such pictures into question.
    We therefore arrive at my favorite subject of ladders versus bushes, or, in the context of this book, individual pathways chosen with prejudice versus entire systems (full houses) and their complete variation. As the Bible says about wisdom, so too may we state about the proper iconography of evolution: "She is a tree of life to them that lay hold upon her." Evolution rarely proceeds by the transformation of a single population from one stage to the next. Such an evolutionary style, technically called anagenesis, would permit a ladder, a chain, or some similar metaphor of linearity to serve as a proper icon of change. Instead, evolution proceeds by an elaborate and complex series of branching events, or episodes of speciation (technically called cladogenesis, or branch-making). A trend is not a march along a path, but a complex series of transfers, or side steps, from one event of speciation to another. The evolutionary bush of horses includes many terminal tips, and each leads back to Hyracotherium through a labyrinth of branching events. No route to Hyracotherium is straight, and none of the numerous labyrinthine paths has any special claim to centrality (see Figure 10). We run a steamroller right over a fascinatingly complex terrain when we follow the iconographic convention for displaying the pathway from Hyracotherium to Equus as a straight line.

    FIGURE 10 The more complex branching evolution of horses as depicted by Bruce MacFadden in 1988.
    So why do we engage in such distortion, and why have horses become the standard example of an evolutionary "trend"? At this point in the argument, we encounter the irony that I have called "life’s little joke" (see Gould, 1987). We choose horses because their living species represent the endpoint of such an unsuccessful lineage. The situation is even "worse," and fully subject to generalization: our bias against considering the variation of full systems, and trying instead to depict trends as "entities moving somewhere," virtually guarantees that all our standard examples of evolutionary movement and "progress" must feature failing groups, so reduced from earlier bushiness that only a single twig—life’s little joke—survives as a relic of former glory.
    What are the real success stories of mammalian evolution? We can answer this question without ambiguity, at least in terms of numerous species and vigorous radiation: rats, bats, and antelopes (or, in more formal terms, the orders Rodentia, Chiroptera, and the family Bovidae among the artiodactyls). These three groups dominate the world of mammals, both in numbers and in ecological spread. Yet has anyone ever seen an iconographic depiction of their success?
    We never feature these groups because we do not know how to draw their triumph. Evolution, to us, is a linear series of creatures getting bigger, fancier, or at least better adapted to local environments. When groups are truly successful, and their tree contains numerous branches, all prospering at once, we can designate no preferred pathway—and we therefore have no convention for depicting, or even (really) for conceiving, their evolution. But when an evolutionary bush has been so pruned by extinction that only one lineage survives—a twig from an earlier arborescence, a sliver