Ward B. Watt

ADAPTIVE SIGNIFICANCE OF PIGMENT POLYMORPHISMS IN COLIAS BUTTERFLIES. I. VARIATION OF MELANIN PIGMENT IN RELATION TO THERMOREGULATION

The identification of the selective forces acting on natural variation in a population of organisms may be a formidable task. Only a few such identifications have been made, as in the studies of Cain and Sheppard (1954 et seq., summarized by Ford, 1964) and others on polymorphism in Cepaea snails, or the studies of Kettlewell (summary: Kettlewell, 1961) on industrial melanism in moths. The wing pigmentation of pierid butterflies presents many opportunities for this sort of analysis. Much natural variation in this system has been noted in the genera Pieris and Colias (Bowden, 1961, 1963; Gerould, 1923; Remington, 1954). These insects deposite two kinds of pigment in their wing scales: melanin, which is black, and pteridines, which may be white, yellow, orange, or red. The wings of different Colias species may possess any of the latter shades as

THERMOREGULATORY STRATEGIES IN COLIAS BUTTERFLIES: THERMAL STRESS AND THE LIMITS TO ADAPTATION IN TEMPORALLY VARYING ENVIRONMENTS

As a case study of adaptive strategies in temporally varying environments, thermoregulation in three populations of Colias butterflies along an elevation gradient in Colorado is studied in relation to the fluctuating meteorological environment. Emphasis is placed on short time scale (15-300 s) variation in air temperature and wind speed and its role in determining elevational patterns of body temperature, flight activity, and thermal stress due to overheating. A stochastic, linear filter model of an organism in a variable environment is used which views the adaptive process as the adjustment of the organisms filter. The relation-ship between this filter model and a transient energy balance model of the butterfly is examined to show how the thermoregulatory mechanisms of adaptation determine the filtering properties of the organism. Heat shocks at 45⚬ C significantly decrease survivorship and fecundity in Colias. Time series analysis indicates that short-term variation in wind speed and air temperature under sunny, midday conditions is significantly greater at higher elevation sites. Negative cross-correlations between wind speed and air temperature at certain time scales amplify the probability of overheating in Colias. Simulation and field results show that Colias from higher elevation populations are more sensitive, in terms of body temperature response, to a given level of wind speed variability, because of their higher wing solar absorptivities. As a result of these factors, variation in body temperature under sunny, midday conditions is significantly greater for butterflies in higher elevation Colias populations, regardless of behavioral thermoregulation. While mean body temperatures under these conditions are 2⚬ C-3⚬ C higher for low elevation than for high elevation Colias, the maximum body temperatures experienced in these populations are similar. These results are consistent with the hypothesis that microevolution of thermoregulatory characteristics in Colias is constrained by the need to avoid high body temperatures. The differences in mean body temperatures which follow from this constraint and the elevational differences in meteorological variation may be a major factor in the elevational patterns of daily flight activity time observed in previous studies. By documenting the biological importance of thermal stress and flight activity for Colias, we can develop optimality models for thermoregulatory strategy. To our knowledge, this is the first comprehensive demonstration of quantitative differences in environmental variability and their consequences for differences in adaptive characteristics among animal populations. Results are discussed in relation to strategies of insect thermoregulation, the structure of the micromete-orological environment, and general principles of adaptive design in variable environments.

Population structure of pierid butterflies IV. Genetic and physiological investment in offspring by male Colias

SummaryPopulation structure encompasses all the rules by which a populations gametes come together, including genetic and physiological investment in offspring. We document female use of nutrients donated by males at mating, and complete sperm precedence, in Colias eurytheme Boisduval. The effect of these phenomena on the population structure of this species is discussed.

Nectar resource use by Colias butterflies

SummaryNectar foraging preferences of Colias butterflies in two different mountain ecosystems are examined with respect to plant distribution, nectar quantity, carbohydrate (and amino acid) content of nectar, and visual pattern of the plants utilized and avoided. Colias, and apparently numerous other small, ectothermic, low-energy-demand pollinators, “patronize” plants producing relatively dilute nectars containing a high proportion of monosaccharide sugars and significant amounts of polar, nitrogen-rich amino acids. These plants also converge on a common “target” flower pattern in ultraviolet and human-visible light. High-energy demand, endothermic pollinators, by contrast, appear to require higher concentration nectars and/or higher proportions of di- and oligosaccharide sugars. These results are discussed in the light of water balance and energy budget demands of different pollinator classes. Questions are also raised concerning behavioral aspects of pollinator search for resources and the pertinence of these data to the concept of floral mimicry.

The thermal ecology of someColias butterfly larvae

SummaryThe thermal ecology ofColias butterfly larvae has been studied, using simple modifications of previous thermistor implantation technology. Like their adults, these larvae rely on a repertoire of thermoregulatory behavior to control body temperature in relation to external heat sources and sinks. They neither heat nor cool by metabolic means. They display narrow, well-marked body temperature ranges for their major activity, feeding. These are 10–15 °C lower than the maximum activity temperatures of the adults. Also in contrast to the adults, the locations of the larval activity maxima differ by several degrees C between the taxa studied. In each taxon studied the rate of feeding reaches a maximum in a body temperature range corresponding roughly to the temperature range maximizing the occurrence of feeding. The overall larval growth rate is maximized under constant temperature regimes corresponding to the maximum feeding range. A qualitative model for larval activity in the field in relation to daily temperature changes is constructed and apparently supported in its essentials. These results are discussed in relation to other aspects of larval ecology, notably predator pressure, and some speculation on their meaning for larval metabolic organization is raised.

Adaptation at specific loci. VII. Natural selection, dispersal and the diversity of molecular–functional variation patterns among butterfly species complexes (Colias: Lepidoptera, Pieridae)

Natural genetic variants at the phosphoglucose isomerase, PGI, gene differ in spatial patterning of their polymorphism among species complexes of Colias butterflies in North America. In both lowland and alpine complexes, molecular–functional properties of the polymorphic genotypes can be used to predict genotype‐specific adult flight performances and resulting large genotypic differences in adult fitness components. In the lowland species complex, there is striking uniformity of PGI polymorph frequencies at a number of sites across the American West; this fits with earlier findings of strong, similar differences in fitness components over this range. In an alpine complex, Colias meadii shows similar uniformity of PGI frequencies within habitat types, either montane steppe or alpine tundra, over several hundred kilometres in the absence of dispersal. At the same time, large shifts (10–20%) in frequency of the most common alleles occur between steppe and tundra populations, whether these are isolated or, as in some cases, are in contact and exchange many dispersing adults each generation. Data on male mating success of common C. meadii PGI genotypes in steppe and tundra show heterozygote advantage in both habitat types, with shifts in relative homozygote disadvantage between habitats which are consistent with observed frequency differences. Nonadaptive explanations for this situation are rejected, and alternative, thermal‐ecology‐based adaptive hypotheses are proposed for later experimental test. These findings show that strong local selection may dominate dispersal as an evolutionary agent, whether or not dispersal is present, and that selection may often be the major force promoting ‘cohesion’ of species over long distances. This case offers new opportunities for integrating studies of molecular structure and function with ecological aspects of natural selection in the wild, both within and among species.

Thermal physiological ecology of Colias butterflies in flight

SummaryAs a comparison to the many studies of larger flying insects, we carried out an initial study of heat balance and thermal dependence of flight of a small butterfly (Colias) in a wind tunnel and in the wild.Unlike many larger, or facultatively endothermic insects, Colias do not regulate heat loss by altering hemolymph circulation between thorax and abdomen as a function of body temperature. During flight, thermal excess of the abdomen above ambient temperature is weakly but consistently coupled to that of the thorax. Total heat loss is best expressed as the sum of heat loss from the head and thorex combined plus heat loss from the abdomen because the whole body is not isothermal. Convective cooling is a simple linear function of the square root of air speed from 0.2 to 2.0 m/s in the wind tunnel. Solar heat flux is the main source of heat gain in flight, just as it is the exclusive source for warmup at rest. The balance of heat gain from sunlight versus heat loss from convection and radiation does not appear to change by more than a few percent between the wings-closed basking posture and the variable opening of wings in flight, although several aspects require further study. Heat generation by action of the flight muscles is small (on the order of 100 m W/g tissue) compared to values reported for other strongly flying insects. Colias appears to have only very limited capacity to modulate flight performance. Wing beat frequency varies from 12–19 Hz depending on body mass, air speed, and thoracic temperature. At suboptimal flight temperatures, wing beat frequency increases significantly with thoracic temperature and body mass but is independent of air speed. Within the reported thermal optimum of 35–39°C, wing beat frequency is negatively dependent on air speed at values above 1.5 m/s, but independent of mass and body temperature. Flight preference of butterflies in the wind tunnel is for air speeds of 0.5–1.5 m/s, and no flight occurs at or above 2.5 m/s. Voluntary flight initiation in the wild occurs only at air speeds ≦1.4 m/s.In the field, Colias fly just above the vegetation at body temperatures of 1–2°C greater than when basking at the top of the vegetation. These measurements are consistent with our findings on low heat gain from muscular activity during flight. Basking temperatures of butterflies sheltered from the wind within the vegetation were 1–2°C greater than flight temperatures at vegetation height.

Avoiding Paradigm-Based Limits to Knowledge of Evolution

Since Darwin (1859) first proposed that evolution proceeds by natural selection, we have learned much about it. The founding of population genetic theory (summaries: Fisher, 1958; Haldane, 1932; Wright, 1931) showed the genetic feasibility of natural selection, removing a major objection to Darwin’s theory (Provine, 1971), and led to extended study of population genetic phenomena (e.g., Nei, 1987; Hartl and Clark, 1989). The “Modern Synthesis” (Jepsen et al., 1949; Mayr and Provine, 1980) brought paleontology and systematics together with population genetics to endorse Darwin’s insights and, many thought, to lay the foundation of steady progress in understanding.

Emergence of Complex Haplotypes from Microevolutionary Variation in Sequence and Structure of Colias Phosphoglucose Isomerase

A molecular evolutionary explanation of natural genetic variation requires analysis of specific variants’ evolutionary dynamics. To pursue this for phosphoglucose isomerase (PGI) of Colias butterflies, whose polymorphism is maintained by strong natural selection, we assembled a large data set of wild haplotypes, highly variable at the amino acid and DNA levels. The most common electrophoretic, i.e., charge macrostate, allele class, 3, is conserved in its pattern of charged amino acid residues. The next most common macrostate, 4, has multiple patterns of charge, i.e., microstates, while less common (1, 2, 5, 6) macrostates are very diverse. Macrostate 4 shows significant linkage disequilibrium (LD) among its variants, especially for two groups of five haplotypes each. We find extensive intragenic recombination among all haplotypes except the two high-LD groups of macrostate 4, which display none. Phyletic relations among haplotypes are largely reticulate, again except for the high-LD groups of macrostate 4, which form clades with strong bootstrap support. Charge-changing and linked charge-neutral amino acid variants occur in diverse parts of PGI’s sequence. Homology-based modeling of PGI’s structure shows that these regions are related spatially in ways suggesting functional interaction. The high-LD groups of macrostate 4 display parallel amino acid variation in these regions. This pattern of haplotype clades with high LD among multiple varying sites, emerging from chaotically recombining variation, may be a “signature” of refinement of complex adaptive sequences by recombination and selection. It should be tested further in this study system and others as a possibly general feature of the evolution of living complexity.

Specific‐gene studies of evolutionary mechanisms in an age of genome‐wide surveying

The molecular tools of genomics have great power to reveal patterns of genetic difference within or among species, but must be complemented by the mechanistic study of the genetic variants found if these variants’ evolutionary meaning is to be well understood. Central to this purpose is knowledge of the organisms’ genotype–phenotype–environment interactions, which embody biological adaptation and constraint and thus drive natural selection. The history of this approach is briefly reviewed. Strategies embracing the complementarity of genomics and specific‐gene studies in evolution are considered. Implementation of these strategies, and examples showing their feasibility and power, are discussed. Initial generalizations emphasize: (1) reproducibility of adaptive mechanisms; (2) evolutionary co‐importance of variation in protein sequences and expression; (3) refinement of rudimentary molecular functions as an origin of evolutionary innovations; (4) identification of specific‐gene mechanisms as underpinnings of genomic or quantitative genetic variation; and (5) multiple forms of adaptive or constraining epistasis among genes. Progress along these lines will advance understanding of evolution and support its use in addressing urgent medical and environmental applications.