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182 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES Series 4, Volume 60, No. 10 Daly (2004): bioreactive natural products that are found in the sea rarely occur in non-marine habitats and, conversely those from non-marine sources are rarely found in the sea. Although classes of similar chemicals occur in both habitats, the structures are usually quite different. This rule may seem a bit less odd when we consider the taxonomic groups that are numerically dominant in the sea, in fresh water, and on land. The dominant terrestrial organisms that have been rich sources of bioactive chemicals, namely, vascular plants and insects, occur around the periphery of the sea, where they are replaced by algae and crustaceans. The main groups of marine animals that are sources of such compounds are ones that are restricted to the sea or have just a few representatives in fresh water: sponges, soft corals, tunicates, echinoderms, and opisthobranch gastropods. Tetrodotoxin and related compounds are largely associated with tropical to semitropical aquatic or semiaquatic vertebrates. The great attention that was devoted to marine drugs began in the 1960s, and was well funded thanks in large measure to lavish spending by United States governmental agencies and also by Canada, Europe, and Japan. After several decades of concentrated effort only a few such drugs have become commercially successful. A synthetic peptide identical to one that occurs in snails of the genus Conus is now available as ziconotide (Prialt®) and used as a pain-killer. Quite a number of compounds, such as kahalalide F (Atlas 652), have been found to be active against tumors, but they are still undergoing clinical tests (Faircloth & Cuevas, 2006). The sophisticated approach now being applied involves synthesizing variants upon molecules that have been found to have promising biological activity and also understanding the chemical control of the relevant biological processes. The interested reader may wish to consult some of the following recent review articles: Newman and Cragg (2004); Newman, Cragg and Snader (2000, 2003); Paterson and Anderson (2005); Li and Vederas (2009); Molinski, Dalisay, Lievens and Saludes (2009). Before he died at the age of sixty, D. John Faulkner published splendid annual reviews summarizing research on marine natural products chemistry in the journal Natural Product Reports (Faulkner, 1984a, 1984b, 1986, 1987, 1988, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000a, 2001b, 2002). These reviews are still exceedingly useful. The series has been continued by other authors (Blunt, Copp, Munro, Northcote & Prinsep, 2004, 2005, 2006; Blunt, Copp, Hu, Munro, Northcote & Prinsep, 2007, 2008, 2009). Other valuable review articles have also been published in the same journal and we refer to some of these in the text. A massive Dictionary of Marine Natural Products has been edited by Blunt and Munro (2008). The chemical literature presents something of a bibliographical barrier to the outsider because of such practices as the incomplete citation of references, leaving out titles and providing only the initial page. Herein the references are more nearly complete. Early in the nineteenth century two unfortunate terms were introduced into the scientific literature: “organic chemistry” and “invertebrate zoology.” Organic chemistry, introduced early in the nineteenth century by the Swedish chemist Jöns Jakob Berzelius, reflected the ancient doctrine of vitalism, according to which living organisms possessed a “vital force” that endowed them with capacities that transcended the laws of inert matter and could not be synthesized in the laboratory. The refutation of vitalism began with Friedrich Wöhler’s synthesis of urea, performed in 1824 and made public in 1828. Subsequently the term “organic chemistry” was redefined so as to make it the chemistry of carbon compounds and the vitalism has become a dead metaphor. Yet as Hendrickson (1965) points out, during the early part of the nineteenth century organic chemistry was almost entirely concerned with the chemistry of natural products. Organic chemists continued to study them, but as a specialized branch of a larger discipline. The chemicals of interest to natural products chemists are mainly what are called “secondary metabolites” to distinguish them from the “primary metabolites,” which are involved in the major biochemical pathways of the organism.
CIMINO & GHISELIN: CHEMICAL DEFENSE AND EVOLUTION OF GASTROPODS 183 Secondary metabolites are secondary in the sense that they are made from the primary metabolites. Although secondary in that sense, they may be of crucial significance in the lives of the organisms in which they occur. It was Jean-Baptiste Lamarck, the most influential pre-Darwinian evolutionist, who introduced the term “invertebrate” into the scientific literature. Invertebrates are animals without backbones. The sub-phylum of chordates to which we belong, Vertebrata, is a legitimate assemblage of animals all of which do in fact have backbones, and all of which are descended from a common ancestor. But invertebrates have nothing in common. They are just the animal kingdom minus one of its many branches. Dividing the animals in that way is like giving equal importance to British and non-British history. Lamarck believed that progress is a law, and thought that he could arrange all of the animals in a single series from lower to higher, culminating in Man. Simple animals, he thought, are continually being generated spontaneously and then giving rise to increasingly complex ones. He did suggest that adaptation to particular circumstances has led some animals to deviate somewhat from the general tendency for progressive change. But for him branching was unimportant. His classifications, because they arranged the materials in series, or “grades” proceeding from lower to higher, did not reflect the underlying pattern of diversification through time. Darwin, on the other hand, recognized the importance of common ancestry and evolutionary divergence. Arranging the materials in terms of recency of common ancestry meant that classification could be truly historical. The different groups of organisms could now be seen as diversifying, and adapting to ever-changing circumstances. The groups would come to occupy an increasing variety of ecological niches, or, as he put it, “places in the economy of nature.” The natural economy that he envisaged was a competitive one, in which the resources necessary for survival and reproduction were of crucial significance. The features of organisms could be explained as furthering the ability of their possessors to survive and reproduce—in other words, as adaptations in the sense of the products of an evolutionary process. Darwin’s theory of natural selection suggested that a much closer fit exists between organisms and their environments than had previously been realized. Impressive evidence for natural selection came in the form of discoveries about defensive adaptations that were made by three of Darwin’s followers. Camouflage, which conceals an insect from a predator, has an obvious advantage in the struggle for existence, but what about the insects that are conspicuously colored? Darwin put that question to the co-discoverer of natural selection, Alfred Russel Wallace, who came up with the answer: warning coloration. The conspicuous animal is in effect advertising the fact that it is distasteful or otherwise not appropriate as food. Henry Walter Bates, who accompanied Wallace to South America, discovered what is called “Batesian mimicry” in his honor. Distasteful butterflies often live together with butterflies of other species that are not distasteful but look like them (respectively the model and the mimic). Bates was able to show that often the mimic and the model are quite distantly related, and that a butterfly species may mimic different models in different parts of its geographical ranges. Another of Darwin’s early supporters, Fritz Müller, then a political refugee living in South America, discovered what is called “Müllerian mimicry,” in which butterflies of several different species, all of them distasteful, resemble one another. The predators, mainly birds, are more apt to leave a butterfly alone if all the distasteful ones look the same. Darwin (1862) himself wrote a book on orchids, in which he explained how these flowering plants reproduce with the aid of symbiotic insects. One of his points was that what might seem to be an unimportant structural detail often is of crucial importance in accomplishing fertilization. He also suggested that the structural arrangements are largely the result of historical accidents and are not the sort of thing that an intelligent creator would have produced. While he showed that natural
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