ENVIRONMENTAL CONTROL OF SEASONAL VARIATION IN THE BUTTERFLY COLIAS EURYTHEME. I. ADAPTIVE ASPECTS OF A PHOTOPERIODIC RESPONSE

Organisms respond in a variety of ways to changing environmental conditions. They can maintain a single, broadly adapted phenotype in the face of environmental fluctuations with the expectation that, on the average, at least some members of a population will survive most conditions. Or, a population can maintain a variety of genetically determined phenotypes in anticipation of environmental changes. Finally, many organisms have evolved means of varying their phenotypes on cue, according to the environmental conditions they face. Frequently this is a reversible process, as is temperature acclimation (see Kinne, 1970). If environmental conditions cycle in a regular fashion as do the seasons in temperate zones, it may be advantageous for phenotypes to vary in anticipation of forthcoming changes. For example, insects avoid harsh winter (or summer) conditions by entering diapause; this may be determined long in advance of the onset of environmental adversity, and frequently long before the physiological modifications involved occur (Danilevsky et al., 1970; Levins, 1968). While animals could respond directly to such environmental parameters as temperature, such a response allows them no reliable way of predicting a change in living conditions before it actually occurs. The single most reliable indicator of seasonal change is photoperiod. (Photoperiodism in insects has been thoroughly reviewed: Beck, 1968; Danilevskii, 1965; Danilevsky et al., 1970; Lees, 1968; and de Wilde, 1962.) Multivoltine insects are faced with anticipating changes of the seasons and adjusting development accordingly for maximal fitness during each portion of the year. Although an indirect index of thermal change, photoperiod is a noise-free signal, completely unaffected by short-term meteorological changes, and, indeed, it determines long-term climatological change of interest to the organism. Diapause is a common, but not the only, photoperiodically determined response in the insects. Many also exhibit seasonal morphological variation. The changes in morphology may be as subtle as changes in aedeagus (penis) morphology in the leaf hopper, Euscelis plebejus (Muller, 1957), or they may be so striking that the seasonal morphs were at first mistaken for separate species, as with the butterfly Araschnia levana (Danilevskii, 1965). Many pierid butterflies also exhibit seasonal variation that involves the melanic and pteridine pigmentation of the wing surfaces. For instance, Pieris rapae, the European cabbage butterfly now widespread in North America, is rather dark in coloration in the spring and light in the summer (Kolyer, 1966, 1969; Hoffmann, pers. obs.). The alfalfa butterfly Colias eurytheme Boisduval (a pierid), together with its near relatives, C. philodice Latreille and C. eriphyle Edwards, is highly multivoltine and widely distributed over most of the North American continent. It has been known for some time to exhibit phenotypic variation, depending upon the season of collection. The "cold weather" and "warm weather" forms, which differ most strikingly in the extent of dark melanin pigment on the hindwing undersides, have been accorded the varietal names "ariadne" and "amphidusa," respectively (Klots,