Box 2 (continued)
Coadaptation involves a series of mutual adjustments between interacting entities determined by the frequency of interaction, the impact on fitness (i.e.,
survival, mating success, or fecundity), and the relative evolutionary potential
(e.g., population size, generation time, genetic variation). Among the interactions that caused strong coadaptation or coevolution in strictu sensu are antagonistic associations such as parasite-host and plant-herbivore interactions and
mutualistic associations. These relationships are intrinsically dynamic, and may
endure for million years, as has happened with flowering plants and pollinating
insects [67, 166]. Pollinator hawk-mots and orchids are a classic example of fine
reciprocal fitting between traits involved in a specialized association of plants
and insects (see Nilsson et al. [128], Boberg et al. [23]).
4
The “Escape and Radiate” Model of Coevolution
Much of what we know about coevolution has come from the study of one of the main
antagonistic and widely specialized associations, the plant-herbivore interaction. The
conceptual framework for the study of plant-herbivore interaction was constructed
based on phylogenetic patterns among host plants and phytophagous insects. Using
ecological data from butterflies of the superfamily Papilionoideae and their host plants,
Ehrlich and Raven [48] documented a conserved phylogenetic pattern of host use,
mediated by secondary metabolites characteristic of the host-plants. This pattern was
placed in a theoretical context, known as the “escape and radiate” hypothesis. The
authors proposed an “arms race” model of coevolution [38] that describes one of the
ways in which coevolution can occur (mainly diffuse, see [166]). The model suggests
that in response to reciprocal selection pressures exerted by plants and herbivores: (1)
plants evolve novel defensive traits to avoid or reduce herbivory and (2) herbivores
evolve counter-defenses to cope with mostly chemical barriers impose by plants. As a
consequence, plants can (3) escape from herbivores and radiate, and herbivores, by
surpassing chemical defenses of plants could thus (4) radiate on diversified host-plants.
The “escape and radiate” hypothesis predicts corresponding patterns of speciation and
diversification, reflected in taxonomic congruence. Coevolutionary metaphors of the
“escape and radiate” by an “arms race,” encompass the outcomes of coevolution and
the mechanisms behind it, that function at different hierarchical scales. The “arms race”
metaphor describes the process (i.e., reciprocal natural selection) behind patterns that
emerged either at ecological or phylogenetic scales. Their predictions are usually tested
within populations by means of trait-based analyses. The “escape and radiation”
metaphor entails the resulting patterns of natural selection. This prediction is only
tested across phylogenies and often uses taxon-based analyses [101]. Phylogenetic and
ecological evidence supporting particular postulates of the coevolutionary model is
discussed below.
3 Coevolution: Plant-Herbivore Interactions and Secondary Metabolites of Plants
53
Coadaptation involves a series of mutual adjustments between interacting entities determined by the frequency of interaction, the impact on fitness (i.e.,
survival, mating success, or fecundity), and the relative evolutionary potential
(e.g., population size, generation time, genetic variation). Among the interactions that caused strong coadaptation or coevolution in strictu sensu are antagonistic associations such as parasite-host and plant-herbivore interactions and
mutualistic associations. These relationships are intrinsically dynamic, and may
endure for million years, as has happened with flowering plants and pollinating
insects [67, 166]. Pollinator hawk-mots and orchids are a classic example of fine
reciprocal fitting between traits involved in a specialized association of plants
and insects (see Nilsson et al. [128], Boberg et al. [23]).
4
The “Escape and Radiate” Model of Coevolution
Much of what we know about coevolution has come from the study of one of the main
antagonistic and widely specialized associations, the plant-herbivore interaction. The
conceptual framework for the study of plant-herbivore interaction was constructed
based on phylogenetic patterns among host plants and phytophagous insects. Using
ecological data from butterflies of the superfamily Papilionoideae and their host plants,
Ehrlich and Raven [48] documented a conserved phylogenetic pattern of host use,
mediated by secondary metabolites characteristic of the host-plants. This pattern was
placed in a theoretical context, known as the “escape and radiate” hypothesis. The
authors proposed an “arms race” model of coevolution [38] that describes one of the
ways in which coevolution can occur (mainly diffuse, see [166]). The model suggests
that in response to reciprocal selection pressures exerted by plants and herbivores: (1)
plants evolve novel defensive traits to avoid or reduce herbivory and (2) herbivores
evolve counter-defenses to cope with mostly chemical barriers impose by plants. As a
consequence, plants can (3) escape from herbivores and radiate, and herbivores, by
surpassing chemical defenses of plants could thus (4) radiate on diversified host-plants.
The “escape and radiate” hypothesis predicts corresponding patterns of speciation and
diversification, reflected in taxonomic congruence. Coevolutionary metaphors of the
“escape and radiate” by an “arms race,” encompass the outcomes of coevolution and
the mechanisms behind it, that function at different hierarchical scales. The “arms race”
metaphor describes the process (i.e., reciprocal natural selection) behind patterns that
emerged either at ecological or phylogenetic scales. Their predictions are usually tested
within populations by means of trait-based analyses. The “escape and radiation”
metaphor entails the resulting patterns of natural selection. This prediction is only
tested across phylogenies and often uses taxon-based analyses [101]. Phylogenetic and
ecological evidence supporting particular postulates of the coevolutionary model is
discussed below.
3 Coevolution: Plant-Herbivore Interactions and Secondary Metabolites of Plants
53