Convergent Evolution: Limited Forms Most Beautiful

Chapter 1. What Is Convergent Evolution?

1 The question “How common is convergence?” remains unanswered and may be unanswerable. Our examples indicate that even the minimum detectable levels of convergence are often high, and we conclude that at all levels convergence has been greatly underestimated. —Moore and Willmer (1997, 1) Recognizing Convergent Evolution A porpoise looks like a fish. It has a fusiform, streamlined body like that of a swordfish or a tuna. It has four fins on the ventral side of its body, instead of four legs. It has a large fin at its posterior end, instead of a tail. And it even has a vertical fin centered on its back, so it looks very much like a shark when it is swimming through the water toward you. Astonishingly, all appearances to the contrary, a porpoise is not a fish; it is a mammal. It possesses all the distinctive combination of mammalian traits: a porpoise is placental, gives live birth, nurses its young with specialized mammary glands, has an endothermic metabolism, has three bones in its inner ears, has mammalian milk teeth, reproduces via internal fertilization, and so on. But it has lost other traits that are found in most mammals: it has no legs, no tail, no fur, and has instead evolved fins like those of a fish in place of legs and a tail. The porpoise inherits its mammalian traits from its mammalian ancestors, and the possession of these traits indicates to us that the porpoise belongs to the evolutionary lineage, or clade, of the Mammalia. These traits are synapomorphies, derived traits that are inherited from a common ancestor and that define membership within a particular clade. The fins of the porpoise are not directly inherited from fish ancestors; they are independently derived convergent traits. That is, porpoises have evolved fins that have converged on the morphology of the fins that What Is Convergent Evolution? 2 Chapter 1 are seen in the fish, and, even though the fins of the porpoise look very much like those of a fish, they are in fact not fish fins but rather are mammal fins. Distinguishing synapomorphic traits from convergent traits is critical in the recognition of convergent evolution. In the literature (particularly the older literature), synapomorphic traits are sometimes called secondary homologous traits, and convergent traits are called homoplastic traits (Lecointre and Le Guyader 2006). Thus, the critical determination, for the purpose of recognizing convergent evolution, is sometimes referred to as the determination of secondary homology versus homoplasy (a term that I have always found to be most uneuphonious—it sounds like some type of disease). There are actually three ways in which homoplastic traits—traits that look similar but that are not inherited from a common ancestor—might arise in evolution: convergence, parallelism, and reversion. Let us consider a hypothetical example in which we know the evolutionary relationships between six species, illustrated by the cladogram given in figure 1.1. Species 1, 2, and 3 all possess synapomorphy S, and thus belong to the monophyletic clade S. Species 4, 5, and 6 all possess synapomorphy T, and thus belong to the monophyletic clade T. Although clade S and clade T have diverged in their evolution, they nevertheless evolved from a common ancestor that evolved the derived trait R, a trait that all six species still possess by inheritance; thus, trait R is a synapomorphy for the larger monophyletic clade R, which contains both clade S and clade T. So far, so good. Now let us suppose that in clade S, the new trait Z arises by evolutionary modification of the preexisting trait A, as seen in species 3 (figure 1.2). However, in clade T a new trait very similar to trait Figure 1.1 Cladogram of evolutionary relationships between six hypothetical species. What Is Convergent Evolution? 3 Z also arises, but by evolutionary modification of the preexisting trait B, as seen in species 6 (figure 1.2). Trait Z is thus a convergent trait, a homoplastic trait, having evolved independently in species 3 and species 6. If we were to mistakenly think that trait Z was a synapomorphic trait, we would mistakenly think that species 3 and species 6 were closely related sister species, since they both possess trait Z, and we would thus mistakenly include species 3 and species 6 in the new clade Z. The erroneous clade Z would thus be a polyphyletic clade. If, for...