No abstract

No abstract

Abstract.-The butterfly Colias eurytheme requires body temperatures above 30'C for flight. When cold, it orients its exposed wing undersides to present maximum surface area to sunlight; when too warm, it orients for minimum exposure. Dark-winged color forms heat faster in sunlight than light ones. The seasonal color polymorphism of Colias appears to have been evolved to maximize solar heating in cold seasons and minimize overheating in warm seasons.Temperate zone organisms experience great seasonal variation in many environmental parameters. An organism limited in occurrence by one (or more) of these might adapt to this condition in several ways. It might restrict especially sensitive parts of its life cycle to favorable seasons or suspend activity altogether in unfavorable seasons. It might simply evolve broader tolerence to the limiting forces, or it might retain narrow tolerances but evolve some means of changing the limits of these from season to season. The latter might be done by establishment of a genetic polymorphism in the population, so that one morph would be maximally adapted to each season, and frequencies would cycle seasonally, as in certain Drosophila chromosomal polymorphisms.1 Alternatively, all individuals might show the same phenotype at one time but alter it from season to season to increase adaptation to each. Bearers of such a seasonal shift mechanism would not be selectively disfavored in some seasons as would, for example, a warmadapted genetic-morph in a cool season.Such a seasonal shift is exhibited by the common "sulfur" butterflies, Colias eurytheme Boisduval and C. philodice Latreille. Summer broods of these closely related species2 display clear, unclouded orange or yellow color, due to pteridine pigments,3 on the hindwing undersides. Spring and fall broods show an intense darkening of this area due to replacement of the pteridines by black melanin. Ae,4 after a suggestion by Remington,' showed that the day length to which the immature stages are exposed controls the adult pigment. Long photoperiods cause larvae to produce summer-form adults, whereas sibling larvae reared under a short photoperiod produce dark spring-fall forms.In the normal adult Colias' resting position, the wings are folded over the back, exposing the undersides; thus, thorax and abdomen are closely covered by that part of the hindwing which is most darkened in the short-day form. This suggested that the dark form might absorb more solar energy than its light counterpart and thus be better adapted to the cool climate of spring and fall. Development of thermistor probes so small as to monitor Colias' temperatures without impeding their normal physical activity6 has made possible the test of this suggestion. 767

Phosphoglucose isomerase genotypes in the butterfly Colias differ dramatically in biochemical properties. These differences were evaluated earlier, using metabolic network theory, to predict, successfully, their effects on glycolytic metabolism and hence on Colias flight capacity and several consequent fitness components in the wild. Female egg-laying, not previously studied, also depends on flight, so female fecundity is now predicted to differ among these genotypes. An experimental design incorporating the thermal ecology of Colias confirms these predictions in a cool habitat. Thus female fecundity differences among animal enzyme polymorphs have now been found. Quantitative reconstruction of the selection regime for phosphoglucose isomerase genotypes in Colias can now begin. The most heat-stable genotypes are the least fecund, suggesting that global warming, if it occurs, may have severe impacts, through population genetics, on demography of thermally sensitive creatures.

Knowledge of both prokaryotic and eukaryotic organisms is essential to the study of molecular evolution. Their common ancestry mandates that their molecular functions share many aspects of adaptation and constraint, yet their differences in size, ploidy, and structural complexity also give rise to divergent evolutionary options. We explore the interplay of adaptation, constraint, and neutrality in their evolution by the use of genetic variants to probe molecular function in context of molecular structure, metabolic organization, and phenotype-environment interactions. Case studies ranging from bacteria to butterflies, flies, and vertebrates emphasize, among other points: the importance of moving from initial recording of evolutionary pattern variation to studying the processes underlying the patterns, by experiment, reconstructive inference, or both; the complementarity, not conflict, of finding different performance and fitness impacts of natural variants in prokaryotes or eukaryotes, depending on the nature and magnitude of the variants, their locations and roles in pathways, the nature of molecular function affected, and the resulting organismal phenotype-environment interactions leading to selection or its absence; the importance of adaptive functional interaction of different kinds of variants, as in gene expression variants versus variants altering polypeptide properties, or interaction of changes in enzymes' active sites with complementary changes elsewhere that adjust catalytic function in different ways, or coadaptation of different steps' properties in pathways; the power afforded by combining structural and functional analyses of variants with study of the variants' phenotype-environment interactions to understand how molecular changes affect (or fail to affect) adaptive mechanisms "in the wild." Comparative study of prokaryotes and eukaryotes in this multifaceted way promises to deliver both new insights into evolution and a host of new and productive questions about it.

We examined intra-population variation in oviposition preference in the pierid butterfly, Colias eurytheme. Females' preferences were tested in the laboratory, using two-way choice tests between the potential hosts, alfalfa and vetch. There were consistent differences in oviposition preference among females within a population. Larval and adult experience had little or no effect on females' preference. These results suggest that the intra-population variation in oviposition preference is genetically based, but the results of experiments designed to estimate the heritability of oviposition preference were not conclusive. We suggest that intra-population variation in host selection characters may play a key role in shifts to new host plants.

No abstract

The sex-limited "alba" genetic polymorphism in wing color of Colias butterflies has been studied with respect to potential selective pressures on this locus. Alba female pupae, carrying at least one dominant A allele, redirect resources, used by aa pupae for pigmentation, to other metabolic ends. Associated with this reallocation, alba, A-, female adults eclose earlier, retain more larva-derived resources in their fat bodies for somatic maintenance and for reproduction, and, in some conditions, mature their eggs faster than do aa females. Alba females are also less attractive to males than are aa females and mate less frequently. Evolutionary implications of these results are discussed. Most species of sulfur butterflies, Colios (Lepidoptera, Pieridae), show the "alba" genetic polymorphism, sex-limited in expression, for adult pteridine wing pigmentation. Females carrying a dominant allele A at a single autosomal locus are white instead of the species-specific red, orange, or yellow displayed by their homozygous recessive aa sisters and by all males. This genetic mechanism, first demonstrated by Gerould (1, 2), is the same throughout the genus (3, 4). North American Colios have higher A frequencies in colder-habitat populations although, at any one place, sympatric species may have quite different frequencies (5, 6).A sufficient evolutionary explanation of this polymorphism would include: (a) analyzing the mechanistic basis of the phenotypic differences among genotypes; (b) identifying the selective pressures acting on the phenotypic differences; (c) determining the mode of selection (heterosis? frequency dependence? etc.) and the importance of factors such as genetic drift; and (d) estimating selection coefficients in experiments or natural conditions. a and b must precede c and d, both logically and to restrict the latter tasks to a feasible scale.Descimon (7) and Watt (8) began study of the biochemical mechanism of alba. They found that alba females sharply decrease the input of precursor to wing pteridine synthesis. This shifts the kinetic balance of the pathway so that only colorless pteridines accumulate in the wings. This diversion of nitrogen-rich pigment material to other uses in the closed metabolic system of the developing pupa may be a major advantage of alba in the wild (8), especially since herbivorous insects in general (e.g., refs. 9 and 10), Lepidoptera (e.g., refs. 10,11,and 12), and Colias' close relative Pieris most particularly (10) are nitrogen-limited in growth. An early hypothesis (13) that Afemales fly in cooler parts of the day than do aa females, or otherwise differ in their adult thermal biology, has been refuted by field behavioral (14) and thermal-ecological (8) studies. It has been suggested that the morphs might be differently attractive to males because of their color difference (8). Evidence to test these ideas further and to integrate laboratory and field perspectives has been lacking until now.Here we report field study of two Colias species-C. alexandra Edwards and C...