A Bird’s-Eye View of Evolution

1.

But for an errant pair of pochards, Europe might have made a fine physician out of Ernst Mayr. As it was, his sighting of the red-crested diving ducks in central Germany in 1923—the first in over seventy years—led to a career in ornithology and evolution that has seen him hailed as one of the world’s greatest biologists. Now in his ninety-eighth year, Mayr has published two major works that are the culmination of his lifelong scientific career. The Birds of Northern Melanesia (written with Jared Diamond) provides a detailed picture of one of Mayr’s principal fields of research—how birds develop into different species (“speciate”) on island archipelagos. The second, What Evolution Is, concerns the other grand theme of his researches—the elucidation of evolutionary theory—here presented in a form accessible to the general reader.

Ernst Mayr was born in 1904 in Kempten, Germany. From the earliest age he was a keen bird watcher, but for four generations the Mayrs had practiced as physicians, and family tradition was strong. Ernst was already enrolled in medical school when those distracting pochards appeared. He reported the sighting to the great Erwin Stresemann, then curator of birds at the Museum für Naturkunde in Berlin, who was so impressed with the enthusiastic youth that he encouraged Mayr to publish the observation in what was to become his first scientific paper.

By the time Mayr was twenty-one he had received both a degree in medicine and a Ph.D. for studies in avian biogeography. His medical diploma, however, would only gather dust, for by twenty-three he was already employed at the Museum für Naturkunde and about to embark on the greatest adventure of his life—a two-and-a-half-year-long expedition to Melanesia, initially as one of Lord Rothschild’s collectors.

Walter Rothschild is surely one of the most extraordinary figures of the early twentieth century. He is justly famous as the addressee of the Balfour Declaration, for he steadfastly believed in the “government dedicated to social and national justice” which that paragraph sought to establish in Palestine. His role in the family banking business, however, is less obvious. Between the ages of twenty-one and thirty-nine, at his father’s behest, Walter spent every working day at the bank’s headquarters in New Court, London. He arrived punctually at nine and departed on the stroke of five, yet there is not a scrap of evidence to indicate that he actually did anything there—no sign of a deal brokered or loan approved, nothing so much as a paper bearing his signature. The reason for this seems to be that all the time Walter was confined to that gilded cage, his mind and heart were soaring with the birds. Natural history, and ornithology in particular, was Rothschild’s lifelong passion and he devoted his entire income to it.

Between 1890 and 1931 Walter amassed, on the family estate at Tring in the Chiltern foothills, the greatest museum collection ever held in private hands. Among its treasures were two-and-a-quarter-million butterflies and moths, 300,000 stuffed birds, 200,000 bird eggs, and 144 giant tortoises. In stark contrast to the lack of tangible proof of his work at the bank, Rothschild and his staff at Tring left an enormous legacy, foremost of which are descriptions of 5,000 new species published in 1,200 scientific books and papers. The collectors Rothschild employed were largely responsible for this prodigious output, for time and again they obtained the first, last, or only known specimen of a particular species. It was Ernst Mayr’s good fortune to commence his ornithological career as one of this select and intrepid band.

Among Tring’s greatest rarities were the birds of paradise, and Rothschild gave Mayr the task of scouring northern New Guinea’s isolated mountain ranges for these feathered jewels. During his expedition Mayr would collect birds entirely new to science and live among people who had never seen a European. In the Wondiwoi Mountains of what was then Dutch New Guinea (now Indonesian West Papua) he would almost lose his life to tropical diseases; yet today virtually all that we know of the fauna of this remote and intriguing region was recorded by Mayr in 1928.

When the bounty of bird specimens and the bills for the expedition finally showed up at Tring, Rothschild proclaimed Mayr’s achievements a triumph, for they yielded 7,000 specimens for the modest sum of £1,017. Meanwhile, Mayr, who was still in New Guinea, made a significant decision. He would not return to Europe, but would instead join the American Museum of Natural History’s famous Whitney South Seas Expedition to survey the avifauna of the Solomon Islands.

Rothschild tried to tempt Mayr back by offering him the directorship of the Tring Museum, but a catastrophe was looming that would cut off both Mayr’s mooted directorship and Lord Rothschild’s joy at the expedition’s magnificent achievement for ornithology. This “catastrophe” took the form of a “charming, witty, aristocratic, ruthless” blackmailing peeress who would bleed dry one of the richest men in Britain. Forty years earlier the pair had enjoyed a brief affair—hardly the kind of peccadillo, one imagines, that could be worth millions to keep quiet. And indeed, as Miriam Rothschild relates in her tender biography of her uncle,^* the reason for the peeress’s firm grip is curious indeed.

Despite Walter’s vast bulk (he weighed over three hundred pounds), while at Tring he continued to sleep in his childhood room amid its scaled-down furniture. He might have moved out of the cramped abode at age thirty-nine, when his marriage to Nellie Goldsmith was mooted. His mother, however, fearing “insanity in their respective families,” forbade the match, and Walter obeyed her wishes without a peep. Indeed, with Walter it seems to have been a matter of either whispering or bellowing but nothing in between, for he was afflicted by an unusual speech impediment that left him chronically shy. What this awkward, kind-hearted man feared most, it seems, was that news of his indiscretion with the peeress might so shock his mother as to kill her. In fact she proved hardy, living to the advanced age of ninety-one, long enough for the blackmailer to ruin Walter almost entirely.

In order to meet his blackmailer’s insatiable demands, Rothschild decided to sell his beloved bird collection—all except the cassowaries and related flightless species, with which he simply could not bear to part. In October 1931 the collection was acquired by the American Museum of Natural History for the sum of $225,000, a little less than a dollar apiece and surely one of the greatest ornithological bargains of all time. The following year Ernst Mayr was appointed to curate the vast, newly combined Whitney-Rothschild bird collections. For over twenty years he immersed himself in the specimens and records in New York, describing nearly 450 new species and subspecies of birds—more than any other living researcher. Indeed it was this unparalleled richness of material that led Mayr to his greatest discoveries in evolutionary theory.

Mayr tells us that the ultimate origin of The Birds of Northern Melanesia lies in a letter written to him in 1929 by Ernest Hartert, then director of the Tring Museum. Here Hartert remarked, “There is no other place in the world more favorable for the study of speciation in birds than the Solomon Islands.” Thus the book is truly the culmination of a lifetime pursuit, with some chapters first having been drafted as long ago as the 1950s.

Northern Melanesia comprises the Bismarck Archipelago and Solomon Islands, which lie north and east of New Guinea in the Pacific Ocean. The birds inhabiting the region are a diverse lot—including Rothschild’s beloved cassowaries, and many different parrots, pigeons, honeyeaters, hawks, eagles, owls, and kingfishers. There are, however, some notable absences, including the birds of paradise and bowerbirds, both of which abound in adjacent New Guinea. These creatures, it seems, are so loath to fly across water that even the forty-eight kilometers separating the mainland of New Guinea from the northern Melanesian island of Umboi are an insurmountable barrier to them.

At its heart The Birds of Northern Melanesia is a retesting and recapitulation of Mayr’s concepts of species and speciation. These are of enormous importance in biology, for they were central to the establishment of the “modern evolutionary synthesis” that developed in the 1940s as Darwinian theory and modern genetics were reconciled, and which has been singularly influential in evolutionary thought ever since.

Northern Melanesia offers unparalleled advantages for research into bird speciation, principally because it is better studied than any comparably rich region, and because its islands vary greatly in size, isolation, and history, allowing for a complex series of potential interactions that illustrate every stage of evolution. Because bird speciation occurs over vast periods of time and thus cannot be studied directly, the book adopts a “snapshot” approach, assuming that the many gradations between various existing bird populations represent stages in the process by which species develop.

Speciation, Mayr tells us, means the multiplication of species. It is quite different from the process of slow evolution of one life form into another through natural selection that was envisaged by Darwin, for that process allows for no increase in the total number of species. Central to Mayr’s concept of speciation is the hypothesis that new species arise when members of an existing species become separated in space and time. Over time the separated populations can become genetically and ecologically so different as to constitute new species. While this sounds like common sense today, when Mayr first posited it, during his Jesup Lectures at Columbia University in 1941, it was an original insight.

Ernst Mayr is the originator of a second important idea that plays a large part in this book. This is the “biological species concept,” which uses reproduction as the yardstick of species distinction. The concept defines species as “groups of interbreeding natural populations that are reproductively isolated from other such groups.” This crisp definition is widely accepted, yet a fundamental problem exists in applying it, for the degree of reproductive isolation of biological populations is rarely known. In many cases we don’t have direct evidence that members of different species so defined would not in fact be able to reproduce with members of other species. Thus biologists must infer reproductive isolation from other factors, and in doing so they generate considerable, and often irreconcilable, differences of opinion about which populations constitute discrete species. In this new book a novel solution to this problem is proposed. Mayr and Diamond recognize as species only those populations that demonstrate their reproductive isolation and ecological viability in nature by coexisting with species that are close relatives (if they have them), but not interbreeding with them.

This cautious approach results in the recognition of a far smaller number of species—just 195 for all of northern Melanesia as opposed to the 251 traditionally recognized by many ornithologists for the region. The additional “species” recognized by others are considered by Mayr and Diamond to be “allospecies”—that is, populations for which reproductive isolation could be inferred, but for which there is no proof that they would persist in the presence of related allospecies. Specifically, they place allospecies at stage three of a five-stage schema of bird speciation that is drawn from their “snapshot” approach.

Mayr and Diamond’s five stages in the process of speciation are as follows: (1) species that are uniform throughout the region; (2) those exhibiting slight regional variation (variants designated as subspecies); (3) isolated populations different enough to infer reproductive isolation (allospecies); (4) differentiated populations that have come into contact and can survive; and (5) the “ancient” speciators—those birds belonging to species and genera unique to the region.

Only bird populations categorized as being in stages four and five are recognized as species. It seems to me that this concept represents a great advance, for its unambiguous definition of a species facilitates comparisons between the faunas of differing regions, and even of modern faunas with fossil faunas, in ways that were not possible previously. Yet so fraught is this problem that even Mayr and Diamond themselves cannot agree entirely on the matter; the sticking point between them being the curious case of “doublets.”

Imagine a series of bird populations, each differing somewhat from its neighbors, inhabiting an archipelago whose islands are spread out like numerals around a clock’s face, except that a conspicuous gap exists at about twelve o’clock. The birds inhabiting “one o’clock island” are white, but as one proceeds clockwise, more black is mixed in the plumage until the birds of “eleven o’clock island” are entirely black. Then, imagine that a cyclone blows some of these black birds to “one o’clock island,” where they coexist with the white birds, but do not interbreed. Such a pair of coexisting bird populations is called a “doublet,” and they are not infrequent in nature. Mayr considers the two populations that comprise the doublet to be distinct species, while Diamond concludes that despite their coexistence they are merely “allospecies.”

Mayr and Diamond provide abundant evidence for the fragility of the process that leads to new species. Most commonly, birds reach a new island but fail to flourish, perhaps because just a single bird arrives or there is no suitable breeding area. Even if they are fortunate enough to establish a new population and even if they begin to differentiate, they may still fail to establish a new species. This may be because they come into contact with a related population that absorbs them genetically, or because they establish full reproductive isolation but do not differ significantly in ecology from invading relatives, and so become extinct through competition. For each new species that is established, it seems, there are scores of failures.

For me the most intriguing northern Melanesian birds are the “ancient speciators”—particularly those birds of such distinctive mold that they are placed in their own genus. All five such creatures discussed by Mayr and Diamond are restricted to the Solomon Islands. The now extinct Choiseul ground pigeon was one such. Among the largest of all Solomon Islands birds, these flightless creatures were seen just once by a European, when six specimens and an egg were collected by Albert Meek in 1904. (The inappropriately named Meek was one of Rothschild’s most determined collectors. Upon learning that the Choiseul islanders had a fearsome reputation, he arranged for a rowboat full of armed men to track him along the shore as he stalked his birds.) Other ancient speciators include the large and mysterious fearful owl (which feeds on equally large and mysterious giant rats) and three intriguing kinds of honeyeaters.

The Birds of Northern Melanesia is the definitive study of speciation among island birds. It provides a cogent and clear hypothesis that doubtless will be the focus of research by molecular biologists and paleontologists for years to come. It has been constructed to allow for easy testing by other experts, who will appreciate its exhaustive appendices of tables, maps, and taxonomic lists. It must be said, however, that it gives us a distinctly “bird’s-eye” view of evolution, and whether the patterns seen in the feathered tribes holds true for other, less mobile creatures remains to be seen.

Despite its specialist focus, the book will, I think, prove attractive to many. Anyone who has wondered over the myriad birds of tropical isles will enjoy the color plates depicting jewel-like creatures, each one subtly different from its neighbor, as if in their creation Nature was ringing the changes with evolutionary bells.

2.

In 1953 Ernst Mayr was made Alexander Agassiz Professor of Zoology at Harvard’s Museum of Comparative Zoology, where he continues to work. With the move away from the great bird collections in New York came a shift in his research from avian systematics to evolutionary biology and the history and philosophy of biology, and the transformation from museum researcher to teaching professor at a leading university. The second production of Mayr’s tenth decade is very much the offspring of this phase of his career. With readers of all kinds in mind, he has written What Evolution Is, a clear and engagingly concise account of evolution as he sees it. It is Mayr’s hope that even creationists will consider it, if only to sharpen their arguments against evolution.

The book’s title strongly suggests that the author has firm views on how evolution proceeds, and this is indeed the case. In counterpoint to evolutionists such as Stephen Jay Gould, whose theories tend to limit the role of natural selection in evolution, Mayr is convinced of its efficacy in explaining much of what we see in nature. Mayr and Gould also hold differing views on the intriguing question of whether progress exists in evolution. While Gould seems to dismiss the possibility by explaining the evolution of complex organisms as a statistical phenomenon, Mayr favors Richard Dawkins’s thesis that progress in evolution is “a tendency of lineages to improve cumulatively their adaptive fit to their particular way of life, by increasing the number of features which combine together in adaptive complexes.” Mayr and Gould see eye to eye, however, in one significant respect: they share the belief that no grand, overarching evolutionary theory will be forthcoming because of the strong role played by contingent factors, such as extinction-generating asteroid strikes, in shaping life’s evolutionary pathways.

Following an introduction documenting the history of and evidence for evolution, Mayr leads us through a marvelous account of the rise of complex life. He explains how bacteria evolve in a fundamentally different way from higher organisms, in that they do not use sex to create genetic variety but instead share genes by a process known as “lateral transfer.” This occurs when one bacterium attaches itself to another, which may only be distantly related, and inserts some of its genes. Evidently only some kinds of genes are shared; otherwise all kinds of varying bacteria would eventually merge.

Mayr tells us that “after about 1,000 million years of exclusively bacterial life on earth, perhaps the most important and dramatic event in the his-tory of life took place.” This was the evolution of the eukaryotes—organisms whose cells possess a nucleus—which occurred around 2,700 million years ago. The first eukaryotes gave rise to an astonishing diversity of animals, plants, and fungi, which are classified into somewhere between thirty-six and eighty phyla, or major kinds of organisms, and which include, of course, ourselves. The origin of this successful group seems to lie in no ordinary Darwinian evolutionary event, but in the fusion of two more primitive, bacteria-like forms. Exactly how this may have happened is still debated. The classification of the eukaryotes seems likely to remain controversial and incomplete for some time because, as Mayr laments, many families and orders of living things lack even one living expert devoted to their study.

Mayr’s discussion of these myriad, poorly known, and often obscure organisms is admirably adroit. (Occasionally, though, in his enthusiasm he leaves us behind, such as when he informs us in passing that the sponges are descended from the otherwise mysterious “choanomonads.”) While at first glance such creatures may not seem interesting, this part of the book is filled with remarkable facts. Who would have guessed, for example, that the fungi, including all mushrooms, are “quite closely related to the Animalia” and only distantly to plants? And, once one had read it, who could ever forget the fate of the blastophore, a structure of the embryo, in the two major lineages of bilaterally symmetrical animals, the Protostomia and Deuterostomia? The lineage to which echinoderms and vertebrates (including humans) belong fashioned this structure into a mouth, whereas in the second lineage—which includes creatures such as cockroaches, flies, and worms—it became an anus. In Mayr’s fascinating account, such unexpected “end-around” evolution seems typical of life’s evolutionary complexity.

Mayr holds some intriguing, albeit nonorthodox views about the evolution of several vertebrate groups. Contrary to the widely accepted view that birds are close relatives of carnivorous dinosaurs such as Tyrannosaurus, for example, Mayr is of the opinion that they arose from more primitive reptiles. Despite the weight of evidence favoring a dinosaurian origin, it is impossible to dismiss Mayr’s seventy years of research into bird evolution—and thus his views here—lightly. His thoughts on hominid evolution too will raise some eyebrows, for he considers the bipedal australopithecines to be chimpanzees rather than relatives of humans, which in current thinking is novel to say the least.

Toward the end of What Evolution Is, Mayr tackles some questions of fundamental human importance and enduring interest. He notes that the human brain “seems not to have changed one single bit” in the last 150,000 years, yet over that time vast changes have occurred in the way that humans have lived. So what is the relationship between the brain and our social achievements? While Mayr does not explicitly answer this question, it is in his discussion of the evolution of human ethics that we begin to find insights. Mayr states that “altruism toward strangers is a behavior not supported by natural selection,” so why does it exist? For Mayr the answer is that some great and truly altruistic persons have lived, and that they taught us their kindness. If correct, then this is one of the greatest strokes of luck that humanity has ever had, for without kindness to strangers it is difficult to imagine a society that values basic human rights, or even democracy itself. Mayr’s intellectual investigation even extends to the question of whether there is intelligent life elsewhere in the universe. “For all practical purposes,” he informs us in the final pages of this wide-reaching and engaging book, “Man is alone.”

While this last remark may be true for our species as a whole, thankfully it is not so for us individually, for in these two books the extraordinary Ernst Mayr bestows a wonderful and enduring gift—a companionship through reading with one of the sharpest minds of our age.