Convergent Evolution: Limited Forms Most Beautiful

Chapter 4. Convergent Ecosystems

4 In chemistry, the urge of scientists to order and classify natural phenomena resulted in the well-known periodic table of the elements, which allowed chemists to predict new elements and their chemical properties. . . . Some ecologists wonder whether something like a “periodic table of niches” might be possible. —Pianka (1978, 267) One Ecological Role, Many Convergent Players Imagine a universe in which there are an unlimited number of ways to make a living. In such a universe, each species would have its own unique way of making a living,different from all other species.When we examine the ecological structure of living organisms on Earth, we can clearly see that we do not inhabit such a universe. In our universe the number of ways of making a living, of ecological roles or niches, is demonstrably limited. Multiple species are constrained in their evolution to playing the same ecological role, filling the same ecological niche, as best they can given their own phylogenetic and developmental backgrounds. The ecological role is the same, but the species players are from many different evolutionary pathways that have independently converged on filling that niche. Ecological niche convergence is best recognized by first considering truly bizarre ways of making a living—ecological roles that are so strange that, at first glance, it would seem probable that only one species would have evolved such a restricted pathway. And then one realizes that, astonishingly, the ecological evolution of life on Earth is so constrained that multiple species have converged on that pathway. One such bizarre pathway is that of plants that eat animals—an ecological role so strange that it fascinated Darwin (1875). Carnivorous plants have evolved adaptations to actively trap, kill, and digest animals. Convergent Ecosystems 136 Chapter 4 Although such adaptations seem so improbable that one might think they could have evolved only once in the plants, Darwin’s (1875) own researches convinced him that the carnivorous plants were polyphyletic, and led him to believe that plants had independently evolved carnivory in three separate lineages. Over a century and a quarter later, modern phylogenetic analyses reveal that carnivory has arisen independently in at least six separate plant clades: within the commelinid monocotyledons, the caryophyllalean core eudicots, the eurosids-I eudicots, the ericaleanasterid eudicots, and at least twice in the eurasterids-I eudicots (Albert et al. 1992; Cameron et al. 2002; Ellison and Gotelli 2009). Not only is the trait of carnivory convergent in plants, but the means of carnivory itself is also convergent, in that independent plant lineages have evolved the same mechanism for it. Although six separate plant clades have evolved carnivory, carnivorous plants use only three different types of animal traps: flypaper traps, pitcher traps, and mechanical traps (table 4.1). Carnivorous plants with flypaper traps have leaves or hairs that ooze sticky droplets to capture animals that touch them. Flypaper-type traps have convergently evolved at least five separate times (table 4.1), and it is possible that they have independently arisen more than once in the clade of the Caryophyllales. Within the carnivorous Caryophyllales, the sundews (Droseraceae) are basal; thus, at first one might conclude that flypaper traps are a synapomorphy for all plants in this clade with that trait, and so I have listed its evolution only once in table 4.1. However, higher in the clade, the dewy pines (Drosophyllaceae ) and the African lianas (Triphyophyllum peltatum, Dioncophyllaceae ) may have reevolved flypaper traps a second time, in that the Nepenthaceae are below these two families in the clade of the Caryophyllales , and the Nepenthaceae do not have flypaper-type traps (Ellison and Gotelli 2009). Carnivorous plants with pitcher traps have tubular leaves or flowers with a pool of digestive fluids at the base of the tube. Animals that enter the plant’s tubes fall into the cup at its base to drown and be consumed. In some cases the plant’s tubes have downward-pointing hairs or spines that prevent the prey from trying to climb back out of the tube structure to escape. Pitcher traps have convergently evolved at least four separate times (table 4.1). In some plant lineages, pitcher traps are developmental modifications of preexisting flypaper traps (Asian pitcher plants in the Caryophyllales), whereas in other lineages pitcher traps have evolved independently of any lineage possessing flypaper traps (Australian pitcher plants, Brocchinia bromeliads). While insects are the prey of Convergent Ecosystems 137 choice for most...