Wade Hazel

A polygenic model for the evolution and maintenance of conditional strategies

We develop a genetic model for conditional strategies which places such strategies in the context of phenotypic plasticity. The model, which treats conditional strategies as polygenic threshold traits, indicates that, given requisite genetic variation in reaction norms, conditional strategies will evolve to their optimum level and be maintained by stabilizing selection, provided environmental variation results in a fitness trade-off for the alternative conditional phenotypes. The precise value for the evolutionary optimum is found to depend primarily on the probability density function of the environmental variation that influences the production of the conditional phenotypes and the magnitude of the fitness trade-offs of the conditional phenotypes across such environmental variation. The model is tested by application to three well-studied conditional strategies. In each case the predictions of the model are in good agreement with the results of these studies.

The Ecological Genetics of Conditional Strategies

We develop a quantitative genetic model for conditional strategies that incorporates the ecological realism of previous strategic models. Similar to strategic models, the results show that environmental heterogeneity, cue reliability, and environment‐dependent fitness trade‐offs for the alternative tactics of the conditional strategy interact to determine when conditional strategies will be favored and that conditional strategies should be a common form of adaptive variation in nature. The results also show that conditional and unconditional development can be maintained in one of two ways: by frequency‐dependent selection or by the maintenance of genetic variation that exceeds the threshold for induction. We then modified the model to take into account variance in exposures to the environmental cue as well as variance in response to the cue, which allows a derivation of a dose‐response curve. Here the results showed that increasing the genetic variance for response both flattens and shifts the dose‐response curve. Finally, we modify the model to derive the dose‐response curve for a population polymorphic for a gene that blocks expression of the conditional strategy. We illustrate the utility of the model by application to predator‐induced defense in an intertidal barnacle and compare the results with phenotypic models of selection.

THE ENVIRONMENTAL AND GENETIC CONTROL OF SEASONAL POLYPHENISM IN LARVAL COLOR AND ITS ADAPTIVE SIGNIFICANCE IN A SWALLOWTAIL BUTTERFLY

Abstract Seasonal polyphenism, in which different forms of a species are produced at different times of the year, is a common form of phenotypic plasticity among insects. Here I show that the production of dark fifth‐instar caterpillars of the eastern black swallowtail butterfly, Papilio polyxenes, is a seasonal polyphenism, with larvae reared on autumnal conditions being significantly darker than larvae reared on midsummer conditions. Both rearing photoperiod and temperature were found to have individual and synergistic effects on larval darkness. Genetic analysis of variation among full‐sibling families reared on combinations of two different temperatures and photoperiods is consistent with the hypothesis that variation in darkness is heritable. In addition, the genetic correlation in larval darkness across midsummer and autumnal environments is not different from zero, suggesting that differential gene expression is responsible for the increase in larval darkness in the autumn. The relatively dark autumnal form was found to have a higher body temperature in sunlight than did the lighter midsummer form, and small differences in temperature were found to increase larval growth rate. These results suggest that this genetically based seasonal polyphenism in larval color has evolved in part to increase larval growth rates in the autumn.

PREDATOR-INDUCED DEFENSE: VARIATION FOR INDUCIBILITY IN AN INTERTIDAL BARNACLE

Phenotypic plasticity is a widespread and often adaptive feature of organisms living in heterogeneous environments. The advantages of plasticity seem particularly clear in organisms that show environmentally cued switches between alternative morphs. Information concerning the presence and nature of variation underlying the induction of these morphs, especially under field conditions, would be valuable. Here we examined the basis for variation underlying a predator-induced defense in an intertidal barnacle (Chthamalus anisopoma). In a previous experiment, juvenile barnacles were exposed to a predatory gastropod (Acanthina angelica). Some of these individuals were induced to develop as a predation-resistant form, but other individuals developed as the default, undefended morph. Here we tested two alternative explanations for this observation. One, the “continuous-sensitivity” model, holds that there is normally distributed genetic variation for sensitivity to the cue. This model predicts that, given suffici...

Habitat complexity drives experimental evolution of a conditionally expressed secondary sexual trait

The conditional expression of alternative phenotypes underlies the production of almost all life history decisions and many dichotomous traits, including male alternative reproductive morphs and behavioral tactics. Changes in tactic fitness should lead to evolutionary shifts in developmental switch points that underlie tactic expression. We used experimental evolution to directly test this hypothesis by rearing ten generations of the male-dimorphic mite Rhizoglyphus echinopus in either simple or three-dimensionally complex habitats that differed in their effects on morph fitness. In R. echinopus, fighter males develop weapons used for killing rivals, whereas scrambler males do not. Populations evolving in complex 3D habitats, where fighters had reduced fitness, produced fewer fighters because the switch point for fighter development evolved to a larger critical body size. Both the reduced mobility of fighter males and the altered spatial distribution of potential mates and rivals in the complex habitat were implicated in the evolutionary divergence of switch point between the habitats. Our results demonstrate how abiotic factors like habitat complexity can have a profound effect on evolution through sexual selection.

Convergent evolution of neuroendocrine control of phenotypic plasticity in pupal colour in butterflies

Phenotypic plasticity in pupal colour occurs in three families of butterflies (the Nymphalidae, Papilionidae and Pieridae), typically in species whose pupation sites vary unpredictably in colour. In all species studied to date, larvae ready for pupation respond to environmental cues associated with the colour of their pupation sites and moult into cryptic light (yellow–green) or dark (brown–black) pupae. In nymphalids and pierids, pupal colour is controlled by a neuroendocrine factor, pupal melanization‐reducing factor (PMRF), the release of which inhibits the melanization of the pupal cuticle resulting in light pupae. In contrast, the neuroendocrine factor controlling pupal colour in papilionid butterflies results in the production of brown pupae. PMRF was extracted from the ventral nerve chains of the peacock butterfly Inachis io (Nymphalidae) and black swallowtail butterfly Papilio polyxenes (Papilionidae). When injected into pre‐pupae, the extracts resulted in yellow pupae in I. io but brown pupae in P. polyxenes. These results suggest that the same neuroendocrine factor controls the plasticity in pupal colour, but that plasticity in pupal colour in these species has evolved independently (convergently).

Microhabitat choice and polymorphism in the land snail Theba pisana (Müller).

Associations between microhabitat, shell banding and apex colour were examined in the land snail Theba pisana. Snails on their summer aestivation sites were sampled from a transect that included a relatively sheltered Acacia habitat and a more exposed open habitat. The frequencies of fully banded snails and snails with dark apexes, as well as the intensity of banding in fully banded snails, were lower in the open habitat than in the Acacia habitat. No differences in microhabitat relative to shell phenotypes were found in samples from the open habitat. However, significant differences in microhabitat were found between effectively unbanded and fully banded snails in the Acacia habitat, with effectively unbanded snails more common in the exposed Acacia canopy and fully banded snails more common beneath the canopy. Air temperatures in the Acacia canopy were consistently higher than below the canopy, while body temperatures of living fully banded and unbanded snails in sunlight indicated that fully banded snails heat more rapidly than do unbanded snails. These results suggest a potential role for both climatic selection and adaptive plasticity in microhabitat choice in the maintenance of variation in shell banding.

AN EXPERIMENTAL TEST OF NATURAL SELECTION FOR PUPATION SITE IN SWALLOWTAIL BUTTERFLIES

The pupae of some swallowtail butterflies are dimorphic-usually green or brown-while those of others are monomorphic. We have suggested (West and Hazel, 1979) that one important factor determining whether a species evolves dimorphism or monomorphism is the type of pupation site that the species uses. The use of a narrow range of cryptic sites in the leaf litter, for example, would favor monomorphic brown pupae, while the use of a variety of sites on exposed stalks, tree trunks, etc. would favor the ability to develop green or brown pupae in response to such substrate characteristics as color or texture (Hazel and West, 1979). This suggestion immediately raised the question why different species prefer different kinds of pupation sites to begin with. Clarke and Sheppard (1972) successfully selected for changes in pupation site preference in Papilio polytes, thereby revealing genetic variation in the trait, and we have shown (West and Hazel, 1979 and unpubl.) that there are some striking differences in natural pupation sites among six eastern North American swallowtails. Among these there are two species which, despite the similar forest habitats of their immature stages, prefer very different pupation sites: Papilio glaucus in the leaf litter and Battus philenor well off the ground on exposed surfaces like tree trunks and cliffs (West and Hazel, 1979). These species differ in two other important respects. Papilio glaucus lays eggs singly and widely spaced on trees (includ-

The evolution of environmentally-cued pupal colour in swallowtail butterflies: natural selection for pupation site and pupal colour

1. Environmentally‐cued pupal colour in swallowtail butterflies has been hypothesized to evolve as a consequence of (a) the evolution of a preference for pupation sites above the ground that vary in colour and (b) natural selection for crypsis on such sites.

Genetic variability and phenotypic plasticity in pupal colour and its adaptive significance in the swallowtail butterfly Papilio polyxenes

Genetic variability and phenotypic plasticity have been suggested as mutually exclusive ways by which adaptation to environmental heterogeneity can be accomplished. The genetic and environmental components of variation in the darkness of brown pupae of the swallowtail butterfly Papilio polyxenes were investigated. Variation in darkness was found to be both genetic and environmental, with the darkness of the pupation site having a significant influence on the darkness of pupae, thus allowing for the production of cryptic brown pupae in nature. These results suggest that the evolution of phenotypic plasticity in a trait need not be to the exclusion of the maintenance of genetic variation in that trait. The results of field experiments show that cryptic pupae have a higher probability of escaping predation relative to noncryptic pupae.