VARIATIONS ON A THEME-THE ECOLOGY AND EVOLUTION OF WITHIN-PLANT DIVERSITY

Abstract

Review of: Herrera, C.M. 2009. Multiplicity in Unity: Plant Subindividual Variation and Interactions with Animals. University of Chicago Press. 437 Pages. ISBN 978-0-226-32794-5. Paper. $40. Biologists engage in a love–hate relation with variation. On one hand, interspecific diversity and differences among conspecifics motivate most biological hypotheses. On the other hand, variation that is peripheral to a hypothesis is typically dismissed as noise that confounds understanding. However, there is a fine line between love and hate, which is crossed repeatedly in one direction during the development of biology, as variation that was once ignored as mundane is recognized as interesting and so comes to stimulate its own hypotheses. In his recent book on variation within individual plants, Carlos Herrera strives to lead readers across this line, making the case that by influencing plant interactions with animals components of such variation can be adaptive and so warrant broader attention. As unitary organisms, we biologists tend to view the limited variation within our own bodies, such as between the sides of our faces, and those of other unitary organisms either as inconsequential or negatively as a sign of developmental instability (e.g., Moller 2006). This perspective is reinforced by recognition that a body’s cells share the same genome, so that stochastic withinindividual variation cannot be transmitted to offspring. Even if somatic mutation creates genetic variation within a body, its transmission is unlikely because of the independence of somatic and germinative cell lines within unitary organisms. In contrast, modular organisms, such as vascular plants, corals, and bryozoans grow by the reiterated production of somatic components, each of which has the capacity to make reproductive organs. This characteristic body plan creates opportunities for variation among reiterated organs within individuals that are not possible for unitary organisms. Such variation is readily observed by overlaying leaves from the nearest house or garden plant or ordering the grapes within a bunch from smallest to largest. Furthermore, to the extent that systematic patterns of phenotypic variation among organs are genetically determined, they represent traits that can be subject to selection. Thus, at least some aspects of the variation within modular organisms could represent adaptations to specific environments, including interactions with herbivores and agents of pollen and fruit dispersal. In his book, Herrera builds on an extensive, scholarly review of literature from Theophrastus to 2007 (supported by 78 pages of references) to argue that within-individual variation and its ecological and evolutionary implications deserve much more attention than they have received to date. After a largely historical introduction, Herrera develops his argument in four sections encompassing nine chapters. In the first section, Herrera devotes a quarter of his overall text to convincing doubting readers that the traits of leaves, flowers, fruits, and seeds often vary extensively within plants. He achieves this objective through the weight of evidence drawn primarily from the ecological and agronomic literature. In this section, Herrera establishes that virtually all above-ground traits vary within plants (Chapter 2: