WHAT DARWIN FORGOT TO MENTION ABOUT ORCHIDS

Abstract

It has been frequently highlighted (e.g., Ghiselin 1984) that the first of Darwin's books presenting detailed supporting evidence for the theory of natural selection after The Origin was his very carefully crafted treatise on orchids. There, he strived to gather evidence that, quite often, morphological traits of orchids were well suited for insect pollinators removing and depositing pollinia. But it has also been remarked that what Darwin did not do was to explain how orchid flowers come to be different (Wilson and Thomson 1996, p. 88). Ever since Darwin's time, attempts at elucidating the processes and mechanisms involved in the extraordinary diversification of angiosperm flowers, for which orchids represent the unquestioned epitome, have figured prominently in the research program of plant biologists. Animal pollinators have been quite often implied in explanations of angiosperm diversification, for at least the following good reasons: among animal-pollinated lineages, taxonomically distinctive traits are predominantly reproductive (Grant 1949); a suggestive temporal match exists, in geological time, between the radiations of angiosperms and several groups of animal pollinators (Grimaldi 1999); and there is compelling evidence that the radiation has been more rapid in lineages of animal-pollinated plants (Dodd et al. 1999). There seems no question that, for immobile adult plants, enlisting the service of animals in predictably circulating male gametes between anthers and pistils of conspecific individuals represented a decisive evolutionary breakthrough. Thus, at the core of plant-animal pollination systems are, on one side, the ability of flowering individuals to advertise themselves and, on the other, the ability of animals to receive, manage, interpret, and remember the range of visual (flower colors and shapes) and chemical (floral scents) signals being broadcast by plants to identify themselves and inform prospective floral visitors about their spatial location and possible rewards. Animal pollination is, therefore, an essentially communication-based phenomenon, with plants as the signal producers and animals as the receivers. This is why the cognitive aspects involved in pollination may provide some critical hints to the still elusive problem of angiosperm diversification and, more specifically, to the actual role played by animal pollinators in the process. This possibility, however, has been so far hindered by the factual divorce between predominantly behaviorally and plant-oriented research practices. Evolutionary plant ecologists interested in