restricted access 13. Adaptive Radiation: Phylogenetic Constraints and Ecological Consequences
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174 A fundamental question in evolutionary biology is how adaptive radiation proceeds on continents, where most of it occurs. The question is most pressing when insects are considered , especially in phytophagous taxa, which represent over 25% of terrestrial biodiversity. Each taxon has, no doubt, followed a unique trajectory through time, but unifying themes should reveal some general patterns and processes, even if the answers recognize that with different starting points, different end points will result (cf. MacArthur 1972). Given the great diversity of insect herbivores , we should expect both divergent and convergent modes of adaptive radiation, which therefore require a pluralistic approach to their explanation. Adaptive radiation is the relatively rapid evolutionary divergence of species in a lineage into a series of rather different adaptive zones, with each zone occupied by species with similar ecological niches. Thus, for insect herbivores, adaptive zones may involve adaptation within a single phyletic line to living in or on, and feeding upon, different plant modules such as leaves, stems, roots, flowers, or fruits. The ecological niches within any adaptive zone may be defined by the host-plant species that is exploited, so that related insect herbivore species may speciate across related host-plant species. Adaptive radiation is a centerpiece of evolutionary biology because its study necessarily unites and integrates major aspects of the biological sciences, including ecology, evolution , behavior, systematics, and physiology. It forms a central theme in evolutionary biology. And yet, for major taxa of insect herbivores, such as the Hymenoptera, Hemiptera (Homoptera), or Lepidoptera, we have very little in the way of a conceptual framework with which to develop hypotheses and to detect patterns. One surprising condition, perhaps, is that much of ecology has never embraced the evolutionary synthesis. Central themes in ecology such as distribution, abundance, and population dynamics have proceeded largely without the evolutionary point of view. Indeed, some ecologists would assert that population dynamics is purely an ecological subject , as if evolutionary approaches, especially employing the evolutionary background of a species or larger taxon, and the ground plan, or Bauplan, of the lineage, exert no influence in ecology. Although these ecologists would admit that species are adapted to their environment, they would not acknowledge that such adaptation has its own constraints (Ligon 1993), and that these constraints impact the ecology of the species. An adaptive move in one direction will limit alternative evolutionary options. For example, increasing the length of the ovipositor of a parasitoid wasp in response to more deeply concealed hosts in wood lessens the adaptive advantage for attacking unconcealed hosts. The bene- fits and costs of an adaptation and its attendant constraint must be kept in mind, as well as the context of adaptation represented by the ground plan of the species or group, as espoused from Darwin (1859) to Gould (2002): “recognizing organisms as products of history, rather than objects created in their present state” (Gould 2002, p. 99; see also Darwin 1859, pp. 485–486). A similar theme is adopted by Thompson (2005), who emphasizes that “species are phylogenetically conservative in their interactions” (p. 12), and “how the combination of phylogenetic conservatism and ecological opportunity shapes the structure of interaction webs remains one of the least understood aspects of community ecology” (p. 33). The term “adaptive radiation” obviously highlights the main evolutionary advantages in a lineage through time, but inevitably constraints are involved, either explicitly or more cryptically. A balanced view of adaptations and constraints , and the explicit recognition of both, provide a blending opportunity in the field of adaptive radiation. We can embrace heretofore refractive parts of ecology into the evolutionary synthesis—in particular, distribution, TH I RTE E N Adaptive Radiation: Phylogenetic Constraints and Ecological Consequences PETER W. PRICE abundance, and population dynamics. For, if we are to compare the adaptive radiation of lineages, and to search for pattern and mechanism, then ecological characteristics of species and higher taxa are of central concern. The Phylogenetic Constraints Hypothesis We have advocated an approach to adaptive radiation and insect herbivore population dynamics that we call the phylogenetic constraints hypothesis (Price et al. 1990; Price 1994, 2003). The hypothesis developed here argues that the Bauplan of a taxonomic group ultimately influences the ecology of that group in terms of distribution, abundance, and population dynamics. We emphasized phylogenetic constraints because we were faced with the puzzle of why a particular insect herbivore species we studied showed such relatively stable, or latent, population dynamics, when the majority of studies on variation in numbers were...