Butterflies and Plants: a Study in Coevolution Paul R. Ehrlich; Peter
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Butterflies and Plants: A Study in Coevolution Paul R. Ehrlich; Peter H. Raven Evolution, Vol. 18, No. 4. (Dec., 1964), pp. 586-608. Stable URL: http://links.jstor.org/sici?sici=0014-3820%28196412%2918%3A4%3C586%3ABAPASI%3E2.0.CO%3B2-8 Evolution is currently published by Society for the Study of Evolution. Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/ssevol.html. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is an independent not-for-profit organization dedicated to and preserving a digital archive of scholarly journals. For more information regarding JSTOR, please contact [email protected]. http://www.jstor.org Wed Apr 18 12:34:09 2007 BUTTERFLIES AND PLANTS: A STUDY IN COEVOLUTIOK1 PAULR. EHRLICHAND PETERH. RAVEN Depovtnlei?t of Biological Sciei?ce~,Stan fovd Unive~sity,Stat? fovd, Califori?ia Accepted June 15, 1964 One of the least understood aspects of plants with the hope of answering the fol- population biology is community evolu- lowing general questions: tion-the evolutionary interactions found 1. Without recourse to long-term esperi- among different liinds or organisms where mentation on single systems, what can be exchange of genetic information among the learned about the coevolutionary responses kinds is assumed to be minimal or absent. of ecologically intimate organisms? Studies of community evolution have, in general, tended to be narrow in scope and 2. Are predictive generalities about com- to ignore the reciprocal aspects of these munity evolution attainable? interactions. Indeed, one group of orga- 3. In the absence of a fossil record can nisms is all too often viewed'as a kind of the patterns discovered aid in separating physical constant. In an extreme example the rate and time components of evolution- a parasitologist might not consider the ary change in either or both groups? evolutionary history and responses of hosts, 4. Do studies of coevolution provide a while a specialist in vertebrates might as- reasonable starting point for the under- sume species of vertebrate parasites to be standing of community evolution in general? invariate entities. This view~ointis one factor in the general lack of progress toward FACT^^^ DET~R~ININGFOODcHOICE the understanding of organic diversifica- tion. Before proceeding to a consideration of One approach to what we like to the relationships between butterfly groups call coevolution is the examination of pat- and their food plants throughout the world, terns of interaction between two major it is necessary briefly to consider some of groups of organisms with a close and evi- the factors that determine the choice of dent ecological relationship, such as plants food plants in this group and in ph~toph- and herbivores. The considerable amount agous insects in general. Any group of of information available about butterflies phytophagous animals must draw its food and their food plants make them particu- supply from those plants that are available larly suitable for these investigations. in its iFographica1 and ecological range Further, recent detailed investigations have (Dethier, 1954). For instance, the butter- provided a relatively firm basis for state- flies are primarily a tropical group, and merits about the phenetic relationships of therefore there is a relatively greater utili- the various higher groups of Papilionoidea zation of primarily tropical than of tern- (Ehrlich, 1958, and unpubl.). It should, Perate families of plants. The choice of however, be remembered that we are con- oviposition site by the imago is also im- sidering the butterflies as a model. They portant. Man~.adult butterflies and moths are only one of the many groups of herbiv- lay their eggs on certain food plants with orous organisms coevolving with plants. In great precision as stressed by Merz (1959), this paper, we shall investigate the relation- but On the hand, urnis- ship between butterflies and their food takes" have been recorded (e.g., Reming- ton, 1952; Dethier, 1959). In such cases, larvae have either to find an appropriate lThis work has been supported in part by ~ ~ science~ ~ i~ ~ ~G~~~~~ ~ ~d GB-~~~~ ~ plantt li or perish.~ ~ There is an obvious selec- (Ehrlich) and GB-141 (Raven). tive advantage in oviposition on suitable EVOLUTION18: 586-608. December, 1964 586 BUTTERFLIES AND PLANTS 587 plants, but inappropriate choices can be These substances are repellent to most overcome by movement of the larvae. Fur- insects and may often be decisive in pat- thermore, larvae feeding on herbs often terns of food plant selection (Thorsteinson, consume the entire plant, and then must 1960). It has further been demonstrated move even if the adult originally made an that the chemical compounds that repel appropriate choice. most animals can serve as trigger sub- Larval choice therefore plays an impor- stances that induce the uptake of nutrients tant role in food plant relationships. An by members of certain oligophagous groups excellent review of a long series of experi- (Dethier, 1941, 1954; Thorsteinson, 1953, ments pertinent to this subject has recently 1960). Presence of such repellent com- been presented by Merz (1959); much of pounds may be correlated with the presence the following is based on his account. The of the nutrients. Both odor and taste seem condition of a given larva often has an to be important. effect on what foods it will or will not The chemical composition of plants often accept. In addition, many structural and changes with age, exposure to sunlight, or mechanical characteristics of plants modify other environmental factors (Merz, 1959; these relationships, mostly by limiting the Fliick, 1963), and this may be critical for acceptability of those plants in which they phytophagous insects (Dethier, 1954). For occur. For example, Merz (1959) found example, insects that feed on Umbelliferae that larvae of Lasiocampa quercus, a moth prefer the old leaves, which appear to us that normally feeds along the edge of less odorous than the young ones. Some leaves, could not eat the sharply toothed insects that feed on alkaloid-rich species of leaves of holly (Ilex, Aquifoliaceae). When Papaver (Papaveraceae) prefer the young these same leaves were cut so that untoothed leaves, which are relatively poor in alka- margins were presented, the larvae ate loids. Diurnal chemical cycles, influenced them voraciously. In other cases, larvae by exposure of the plant to sunlight, may eat the young, soft leaves of plants but not be of prime importance in determining the the old, tough leaves of the same plants. habits of night-feeding groups, such as Many Lycaenidae feed on flowers, and Argynnini. these butterflies may be unable to utilize Merz (1959, p. 159) has given a particu- the tough foliage of the same plants. larly interesting case of chemical repellents iXumerous similar examples could be given, at the specific level. The larvae of the but it must be borne in mind that chemical moth Euchelia jacobaeae feed on many factors are operative in the same plants species of Senecio (Compositae), but not that present mechanical difficulties to lar- on the densely glandular-hairy S. viscosus. vae (Thorsteinson, 1960), and actually When the glandular substance was dis- may be more important. solved in methyl alcohol, the larvae ate S. Chemical factors are of great general viscosus. When the same substance was importance in determining larval food painted on the leaves of other normally choice. In the first place, potential food acceptable species of Senecio, these were sources are probably all nutritionally un- refused. In an extensive study of the food balanced to some extent (Gordon, 1961). plants of Plebejus icarioides (Lycaeninae) , The exploitation of a particular plant as Downey (1961, 1962) showed that larvae a source of food thus involves metabolic would feed on any species of Lupinus adjustments on the part of an insect. These (Leguminosae) in captivity, but popula- render the insect relatively inefficient in tions in the held normally utilized only one utilizing other sources of food and tend to or a few of the possible range of Lupinus restrict its choice of food plants. Secondly, species growing locally. This work suggests many plants are characterized by the the subtle interaction of ecological, chemi- presence of secondary metabolic substances. cal, and mechanical factors that doubtless 588 PAUL R. EHRLICH AND PETER H. RAVEN characterizes most natural situations. Re- Lycaenidae about tied for third. Difficult lationships with predators (Brower, 1958), as this diversity is to estimate, it is clear parasites (Downey, 1962), or, at least that Papilionidae are a more heterogeneous in the case of Lycaenidae, ants (Downey, group of organisms than any of the other 1962), may further modify patterns of families, whereas, considering the number food plant choice. of species and genera included, Lycaenidae Despite all of these modifying factors, are remarkably uniform. A rough idea of there is a general and long-recognized pat- the phenetic relationships of the major tern running through the food plants of groups of butterflies is given by Ehrlich various groups of butterflies, and it is this (1958). pattern with which we shall be concerned. With food plant records from between 46 It certainly should not be inferred from and 60% of all butterfly genera (table I), anything that follows that all members of it seems highly unlikely that future dis- a family or genus of plants are equally coveries will necessitate extensive revisions acceptable to a given butterfly (for ex- of the conclusions drawn in this paper.