Eight Little Piggies

  When I first quoted this line in my Ph.D. thesis of 1969, I did so with derision (and as a firm adaptationist). Twenty years later, I am not so sure that Gulick was wrong in his implication. I still feel that his personal religious motive has no place in science, but people often reach correct answers for wrong or illogical reasons. The contingency of history (both for life in general and for the cultures of Homo sapiens) and human free will (in the factual rather than theological sense) are conjoined concepts, and no better evidence can be provided than the “experimental” production of markedly different solutions in identical environments.

  In any case, Gulick’s conclusions drew a storm of protest from Darwinians. Alfred Russel Wallace, most committed of strict adaptationists, retorted (and not without justice) that Gulick’s supposedly “identical” environments might only seem so to humans, but would appear markedly different to snails:

  It is an error to assume that what seem to us identical conditions are really identical to such small and delicate organisms as these land molluscs of whose needs…we are so profoundly ignorant. The exact proportions of the various species of plants, the numbers of each kind of insect or of bird, the peculiarities of more or less exposure to sunshine or to wind at certain critical epochs, and other slight differences which to us are absolutely immaterial and unrecognizable, may be of the highest significance to these humble creatures, and be quite sufficient to require some slight adjustments of size, form, or color, which natural selection will bring about.

  In 1906, after reading Gulick’s monograph, Henry Edward Crampton decided to enter the fray and to devote the remaining fifty years of his career to an immense study of Partula on Tahiti, Moorea, and surrounding islands. Crampton (1875–1956) had done excellent work in experimental embryology and studies of natural selection. This earlier effort led to a slight preference for adaptation, but Crampton maintained an open mind and was prepared to support Gulick’s “internal factors” against Wallace’s shaping by environment should the evidence warrant. Crampton made twelve expeditions to the Pacific and published three magnificent monographs—probably the finest work ever done on the evolution of land snails—collectively entitled Studies on the Variation, Distribution, and Evolution of the Genus Partula (Tahiti in 1917, other islands in 1925, and Moorea in 1932).

  In short, and to summarize a half century of effort in a sentence, Crampton came down firmly on Gulick’s side. He could find no evidence that the forms and colors of Partula could be predicted from surrounding environments. Identical climatic conditions seemed to evoke different solutions time after time.

  Crampton interpreted the differences between snails in adjacent valleys as results of three major causes—isolation, mutation (“congenital factors” in his terminology), and adaptation by natural selection—with only a minor role for Darwin’s favorite mechanism. He viewed the first factor, isolation, as a disposing precondition rather than an actual cause: Geographic separation produces nothing directly but establishes an independent population in which new features may spread. He saw the third factor, natural selection, as primarily negative. Once new features arise by some other mechanism, natural selection may eliminate them if they prove unworkable—but the source of creative change must lie elsewhere. Crampton, who was one of the first American biologists to recognize the importance of Mendel’s work, located this source of creativity in his second factor of mutation, or “internally generated” change by congenital factors. In any environment, hundreds of possible anatomies might work—and the forms and colors of this particular population in that specific valley are fortuitous consequences of the largely nonadaptive mutations that happened to arise and spread in an isolated population.

  The resulting pattern of differences among valleys is largely nonadaptive. Every local race must avoid elimination by natural selection (and is fit in this negative sense), but its particular features represent only one in a myriad of workable possibilities, and any particular solution arises by the happenstance of mutation in an isolated population, not by natural selection. Crampton contrasted the greater importance of mutation over selection in writing about Partula on Tahiti:

  The role of the environment is to set the limits to the habitable areas or to bring about the elimination of individuals whose qualities are otherwise determined, that is, by congenital factors.

  How can we assess the importance of Crampton’s work sixty years after his last great monograph on Partula from Moorea (1932)? I am biased to be sure, for snail men (I am one) revere Crampton as a kind of patron saint, but I rank Crampton’s Partula studies among the most important in the history of evolutionary biology for three major reasons. First, he was probably right in his central claim about the nonadaptive nature of most small-scale differences in form and color among snails of adjacent valleys. Evolutionary biology went through a phase of strong belief in strict adaptationism in the generation just following Crampton, and his works did suffer a temporary eclipse. But his three great monographs are winning new respect and attention in our current, more pluralistic climate of opinion.

  Second, Crampton must gain our highest admiration, verging on appropriate awe, for the sheer dedication and effort of his immense labors. I spent only a day in a rented car on Moorea, and scarcely ventured out of the shade or off the paths—but I still nearly passed out from sunstroke. Crampton spent months on twelve separate expeditions, all in an age of ships and horses (not to mention shank’s mare), rather than airplanes and rent-a-cars. In the charming understatement of conventional “objective” scientific prose, Crampton wrote but one small comment on working conditions:

  Field-work in such a region of Polynesia presents difficulties that are common to most tropical areas…. Steamship lines ply between only the principal ports, from which excursions to neighboring islands must be made by cutter, whaleboat, or canoe…. At times it is possible to procure horses. Almost without exception, however, the exploration of a valley can be accomplished only on foot, owing to the steep declivities to be traversed, the deep streams to be forded, and the absence of any trails whatsoever in the thick forest and undergrowth of the areas inhabited by Partulae.

  But Crampton also recorded the countervailing pleasures that keep us all going:

  The experiences incidental to the active life necessitated by such work were many, varied, and interesting; but the present monograph is not the place for a description of the beautiful islands or of their delightful inhabitants. Suffice to say that the days and nights of arduous and sometimes dangerous effort included hours of keen enjoyment, for the island of Tahiti, especially, is of matchless beauty, while the chiefs and their families offered abundant hospitalities which it was a privilege to enjoy at the time as it is now a pleasure to acknowledge them.

  Moreover, Crampton’s labor only began with collecting. He then spent years measuring his snails (some 80,000 for the Tahiti monograph, and a whopping 116,000 for the Moorea work) and calculating statistics—all of which, incredibly (even for his day), he did personally and by hand! (No computers, no hand-held calculators; when Crampton speaks of “calculating machines,” he means those old mechanical jobbies that performed division by successive subtraction and clanked away for minutes to perform simple operations.) Again, he wrote in understatement:

  The author is personally responsible for every direct measurement and for every detail of classification; hence the personal coefficient is uniform throughout the entire research…. In computing the standard deviations fractions were carried out to eight decimal places…. The length of time required for such quantitative analysis can be estimated only by those who themselves have engaged in such work…. These figures, together with a single line of text, may be all that represents two to eight weeks of mathematical drudgery…. Yet the employment of such methods is justified in the final results.

  Third, and most important, ultimate judgment must reside in a criterion of utility. All good science is accumulative; no one can get everything right the first time. If Cra
mpton’s monographs were only monuments to past effort and ideas, they might still be admired, but only as items of human paleontology. They are, in fact, precious mines for continuing revision and extension. I know this in the most personal way, for I have used Crampton’s tables, the product of his years of “mathematical drudgery,” in at least three of my technical papers.

  A figure from Crampton’s monograph illustrating a Partula shell and some of the measurements that he made on each specimen. Carnegie Institution of Washington.

  To put this crucial point in another and stronger way, Crampton spent fifty years documenting the current geographic distribution and variation of Partula on Tahiti, Moorea, and nearby islands. This work has great and permanent value as a frozen snapshot, but Crampton’s half century should be but a transient moment in the future history of Partula. Crampton devoted this lifetime of effort in order to establish a baseline for future work. Partula would continue to evolve rapidly, and Crampton’s baseline would become a waystation of inestimable value. No scientist could view such dedication in any other light. Future changes have much more value than current impressions.

  One of nearly a hundred tables, most of comparable length and equally chock full of numbers, from Crampton’s monograph on the Partula of Moorea. Each number in the chart is a calculated average based on many specimens, not simply a measurement. Carnegie institution of Washington.

  And Crampton’s plan paid off—or so it seemed at first. Three of the world’s finest biologists of land snails took up the study of Partula in the next generation, building explicitly on Crampton’s work—Bryan Clarke of the University of Nottingham, Jim Murray of the University of Virginia, and Mike Johnson of the University of Western Australia. They have published numerous papers, in varying combinations of authorship, from the mid-1960s to the present day. Working primarily on everyone’s favorite island of Moorea, they have made important revisions to Crampton’s conclusions and have added great sophistication in mathematical procedures (now computerized) and genetical methods not available to Crampton. In 1980, Murray and Clarke ended an important paper, “The genus Partula on Moorea: Speciation in progress,” with these words:

  Although we cannot yet reconstruct exactly the evolutionary history of the Moorean taxa, they have already revealed in exceptional detail the pattern of interactions between incipient species, and have presented some fascinating paradoxes. They offer both a museum and a laboratory of speciation.

  Add snails to Burns’ litany about the best laid plans of mice and men. Great expectations die quickly on the bonfires of human vanity. We are only a decade from these brave words of 1980, but Moorea is no longer a laboratory for studying active speciation in Partula. It has become a mausoleum.

  Think of all the metaphors you know for little things made worse by attempted solutions that cascade to even greater problems, for you need this apparatus to grasp the extirpation of Partula on Moorea. Think of Pandora’s box. Think of the old woman who swallowed a fly in the folk song. (She then swallowed a spider to catch the fly, a bird to catch the spider, a cat to catch the bird…and up the size range of the animal kingdom. Each successive verse gets longer as singers run through the full range of ingestions, but the last is stunningly brief: “There was an old lady who swallowed a horse. She died, of course.”)

  Partula eats fungi growing upon dead vegetation and poses no threat whatever to agriculture. Its only, and slight, impact upon the native economy is entirely positive, as women string the shells together to make leis for the tourist trade. But animals introduced onto isolated islands often play havoc both with native organisms and with agriculture, witness the rabbits of Australia and, to cite the most dangerous creature of all, the humans who wiped out so many species of moas on New Zealand. An introduced snail began the sad chain of destruction on Moorea.

  In sharp contrast with the benign Partula, African tree snails of the genus Achatina are, in almost all cases, unmitigated disasters. First of all, they are gigantic (as snails go); second, they are voracious herbivores of living plants, including many agriculturally important species. With their clear record of destruction on island after island, I am amazed that people still introduce them purposefully. (They are brought in for food, for I’m told that they are succulent, and you do get a lot of meat per creature.) Achatina was first imported to the Indo-Pacific realm in 1803 by the governor of Réunion who brought them from Madagascar so that a lady friend could continue to enjoy snail soup. They escaped from his garden and devastated the island. By 1847, they had reached India. In the 1930s, they began to spread into South Pacific islands, usually by purposeful introduction for food.

  Achatina fulica reached Tahiti in 1967 and soon spread to neighboring islands. By the mid-1970s, the infestation had become particularly serious on Moorea. The snails even invaded human dwellings; one report tells of a farmer who removed two wheelbarrow-loads of Achatina from the walls of his house. Admittedly, something had to be done. But que faire, as they say in this very French land?

  The attempted solution, like the horse ingested to catch the fly, created greater havoc than the original problem. Biological control is a good idea in principle—better a natural predator than a chemical poison. But predators, particularly when introduced from alien places and ecosystems, may engender greater problems than the creature that inspired their introduction. How can you know that the new predator will eat only your problem animal? Suppose it prefers other creatures that are benign or useful? Suppose, in particular, that it attacks endemic species (often so vulnerable for lack of evolved defenses in the absence of native predators)?

  Biological control should therefore be attempted only with the utmost caution. But, speaking of folk songs and citing a more recent composition than the old lady and the fly, “When will they ever learn?” In my personal pantheon of animals to hate and fear, no creature ranks higher than Euglandina, the “killer” or “cannibal” snail of Florida. Euglandina eats other snails—with utmost efficiency and voraciousness. It senses slime trails, locks onto them, and follows the path to a quarry then quickly devoured.

  Euglandina has therefore developed a worldwide reputation as a potential agent of biological control for other snails. Yet, despite a few equivocal successes, most attempts have failed, often with disastrous and unintended side-effects, as Euglandina leaves the intended enemy alone and turns its attention to a harmless victim.

  Forgive my prejudices, but I know what Euglandina can do in the most personal way (biologists can get quite emotional about the subjects of their own research). I spent the first big chunk of my career, including my Ph.D. dissertation, working on a remarkable Bermudian land snail named Poecilozonites. (This Darwin’s finch among mollusks is the only large land snail that reached Bermuda. It radiated into a score of species in a great range of sizes and shapes. The fossil record is particularly rich, but at least three species survived and were thriving in Bermuda when I began my research in 1963.) Euglandina had been introduced in 1958 to control Otala, an imported edible snail that escaped from a garden and spread throughout the island as an agricultural pest (same story as Achatina and Partula on Moorea). I don’t think that Euglandina has even dented Otala, but it devastated the native Poecilozonites. I used to find them by thousands throughout the island. When I returned in 1973 to locate some populations for a student who wanted to investigate their genetics, I could not find a single animal alive. (Last year, I relocated one species, the smallest and most cryptic, but the large Poecilozonites bermudensis, major subject of my research, is probably extinct.)

  Thus, I feel the pain of Jim Murray, Bryan Clarke, and Mike Johnson. They had published papers on Moorean Partula since the mid-1960s. They never expected that their last pair of articles would be a wake.

  Euglandina was introduced to Moorea on March 16, 1977, with the official advice and approval of the Service de l’Economie Rurale and the Division de Recherche Agronomique—despite easily available knowledge of its failures and devastations elsewhe
re.* Euglandina ignored Achatina and began a blitzkrieg, against Partula—more thorough, rapid, and efficient than anything that Hitler’s armies ever accomplished. When my colleagues wrote their first article about this disaster in 1984 (see bibliography), Euglandina had already wiped out one of the seven Partula species on Moorea, and was spreading across the island at a rate of 1.2 km per year. Moorea is about 12 km across at the widest, and you quickly run out of island at that rate. My colleagues made the grim prediction that Partula would be completely gone by 1986.

  One hates to be right about certain things. In 1988, Jim, Bryan, and Mike published another note with a brief and final title: “The extinction of Partula on Moorea.” Partula is gone. My colleagues managed to collect six of the seven species before the end, and they have established captive breeding programs in zoos and biological research stations in several nations. Perhaps, one day, Partula can be reintroduced into Moorea. But Euglandina must be eliminated first, and no one knows how this can be done. Deep grafts, whether physical or emotional, are hard to extirpate—as Mary Martin discovered in her unsuccessful attempt to wash that man right out of her hair. Hope remains in Pandora’s box, but how do you reenclose the bad guys?

  Moorea may be the Bali Ha’i of our dreams, but life for Partula has become an unenchanted evening. Now night has fallen.

  The story would be sad enough if only Moorea (and Bermuda) had fallen victim. But Euglandina is spreading just as rapidly on the larger, adjacent Tahiti, and Partula now survives in only two valleys. The even more diverse Achatinella is gone (or nearly so) on Oahu, largely for the same reason, although the spread of Honolulu hasn’t helped either. More than half the species of bulimulids are extinct on the Galápagos.