Ernst Mayr, the ornithologist.
Animal Species and Evolution, of course, was just one of his many influential volumes. Averaging nearly ten papers a year for 70 years, with still more to come, Ernst Mayr's prodigious productivity includes an amazing mixture of empirical and conceptual papers, essays and reviews, and influential books. He
certainly has contributed more than his fair share to the evolutionary synthesis, speciation theory, the history of biology, and philosophy of science; some contributions have been fulfilling retirement projects. But, first and foremost, Ernst Mayr is an ornithologist. He has described more new valid species of birds (26) than any other living avian systematist. He served as President of the American Ornithologists' Union (1957-1959) and the Thirteenth International Ornithological Congress (1958-1962). Three of his first four books dealt strictly with birds (Mayr 1941 a, 1945; Delacour and Mayr 1946). Beyond these, Ernst Mayr's wide-ranging contributions to ornithology comprise a major body of scholarly taxonomic and distributional papers, editorial responsibility for the completion of Peters' Check-list of the Birds of the World, and pioneering syntheses on the historical biogeography of the avifaunas of the Australo-Papuan region and the Americas. Overlapping these accomplishments were his roles in the evolutionary synthesis, which affected all of ornithology and much of systematic biology. Mayr's forceful and articulate leadership, plus his ability to package a broad range of taxonomic examples into coherent syntheses, elevated the discipline of ornithology to high profile. For years, often to the resentment of other "ologists," ornithology under Mayr's leadership was a dominant force in the evolutionary synthesis. "Well, that may be true for birds, but it sure doesn't apply to my group," was a common, reactionary refrain--sometimes true, sometimes not.
I am pleased and honored to have this moment to honor Ernst Mayr in a short essay on his ornithological career. As one of those impressionable graduate students in the 1960s, my interests in avian speciation, biogeography, and island birds stem from reading Ernst Mayr's publications, from hearing his lectures, and from debating with him personally. My admiration of his diverse contributions to ornithology widened even more as I wrote my own textbook (Gill 1990). Aiding this pleasant writing exercise has been my access to Waiter Bock's (1994) thorough manuscript "Ernst Mayr, naturalist: his contributions to systematics and evolution." I deeply appreciate Bock's willingness to share with me this manuscript, as well as his bibliography of Mayr's scholarly publications.
Ernst Mayr, the ornithologist, has authored or coauthored (to date) 287 primary titles specifically about birds. Papers on avian taxonomy make up more than half of these titles, complemented by assorted topics of "natural history," plus a steady stream of papers on speciation and biogeography. These ornithological publications, which exceed in number the lifetime contributions of most ornithologists, constitute a mere 42% of his brimming bibliography, which comprises at least 684 titles published from 1923 to 1993.
As an exercise of both admiration and curiosity, I sorted Mayr's bibliography into three arenas: Ornithology, Interdisciplinary Topics, and "Other," the latter comprising 144 assorted reviews, obituaries, interviews, and peripheral essays, which, by themselves, provide insights into a superior, fertile mind. I then segregated the 540 ornithological and interdisciplinary titles into five successive 14-year periods. In the first 27 years of his career, 183 (92%) of Mayr's 199 primary papers and books were oriented to the ornithological community. Starting particularly with Systematics and the Origin of Species (1942), Mayr's taxonomic and conceptual interests broadened rapidly, causing ornithological titles to drop to about 25% of his publications.
Mayr's initial, consuming ornithological interests built sturdy foundations for his subsequent conceptual and interdisciplinary works. With the publication of Systematics and the Origin of Species in 1942, Mayr's intellectual menu diversified steadily. Ornithological papers dropped from an exclusive preoccupation to about 25% of his steady production. Illustrating the diversity of writings are the 22 titles he published in 1986, 14 years after he officially retired from Harvard University. These titles included a note on the correct name of Rallus hodgeni on the bird journal Notornis, revisions of six bird families (Maluridae, Acanthizidae, Monarchidae, and Eopsaltriidae, and parts of the huge families Sylviidae, and Muscicapidae) for the Check-list of Birds of the World, a paper on Pleistocene biogeography with J. O'Hara in Evolution, an essay in Nature on "Uncertainty in science: Is the giant panda a bear or a raccoon?," four articles on systematics and Darwinism, plus two book forwards, five reviews, and two memorials.
To look more closely at Ernst Mayr, the ornithologist, I selected material from three overlapping phases of his comprehensive interest in avian biology: Ernst Mayr, the bird watcher, the museum curator, and the concept advocate.
Ernst Mayr, the Bird Watcher
Many ornithologists start their careers early; Ernst Mayr was no exception. Encouraged as a young boy by his naturalist father, Ernst became a skilled birdwatcher and general naturalist. A career in ornithology followed only accidentally, however, because Mayr first went to medical school to continue a long-standing family tradition. Just before he left home to start medical school in the spring of 1923, Ernst Mayr, the birdwatcher, discovered a pair of rare diving ducks, red-crested pochards (Netta rufina), in Moritzburg, Sachsen. This pair proved to be the vanguard of a recolonization by pochards of the southern coast of the Baltic Sea. On his way to the University of Greifswald, Mayr stopped to introduce himself to Erwin Stresemann at the Berlin Natural History Museum and to report his carefully documented observations of the pochards. Stresemann was impressed by young Mayr's potential, offered to publish his first paper, on the pochards, in the journal Ornithologische Monatsberichte (Mayr 1923), and invited him to work in the bird collections at the Berlin Museum during his school holidays. Two years later, Mayr switched from medicine to ornithology. Throughout his career as a museum curator, Mayr continued avidly to observe live birds, as well as to examine specimens, thereby sustaining his vital understanding of avian ecology and behavior and, hence the forces of adaptation and speciation.
Mayr's ornithological bibliography starts with reports of his important bird sightings and observations on the natural history of local bird species. While still in medical school, Ernst Mayr took an interest in the European serin (Girlitz) (Serinus serinus) in northern Germany, which led to his dissertation (completed in only 1 1/2 years), on the range expansion and biogeography of this species (Mayr 1926). In his earliest publications, Mayr brewed general concepts from sightings of rare birds, concepts and contexts that were to be continuing themes for the rest of his career, particularly the biogeographical dynamics of dispersal and gene flow and the relationships among populations. His interests in dispersal as a function of local population density and breeding-range expansion also led directly to the theoretical bases of intercontinental avian migration (Mayr and Meise 1930). Following the completion of his Ph.D. in 1926, Mayr then spent more than 2 years (February 1928-April 1930) in the South Pacific collecting and observing birds. This field experience, principally in the Solomon Islands and New Guinea, became a primary organizing force of his ornithological career. The fieldwork provided first-hand expertise and a formative interest in tropical Old World birds and island avifaunas, which later permeated both his empirical and conceptual contributions. Now, with more than 60 years accumulated perspective, he has produced a new book manuscript (coauthored with Jared Diamond) on bird speciation in northern Melanesia.
Ernst Mayr, the Curator
Ernst Mayr, the ornithologist, has been devoted to the curation and study of museum collections throughout his career. All his formal academic positions have been in great natural history museums: the Berlin Museum (1926-1931), the American Museum of Natural History (AMNH) (1931-1953), and the Museum of Comparative Zoology (1953 to the present). The majority of his empirical research is based on the ornithological collections of these three institutions. As the curator at AMNH, he was consumed by unpacking, curating, and studying the vast Whitney-Rothschild Collections. As curator, Mayr not only reorganized the ornithological research collections to incorporate the hundreds of thousands of new specimens for which he was responsible, but he also designed the progressive public education exhibits on avian biology that graced the new Whitney Wing. In addition, he participated in a formidable era of global inventory of birds, in which he and his colleagues virtually completed this primary effort ahead of any other taxonomic discipline.
Ernst Mayr, the curator, concentrated particularly on issues of microtaxonomy; patterns of variation and distribution. He grappled daily with the practical questions of species recognition, trying to achieve a rational balance between recognizing as separate species all distinguishable allopatric populations versus more encompassing units that realistically incorporated complicated patterns of geographical variation. His taxonomic works include not only the description of more than two dozen valid bird species, more than by any other living ornithologist, but also the description of 410 subspecies and several new genera. His species descriptions were conservative, lasting ones, and his nomenclature was scholarly. As a practicing microtaxonomist, Mayr was not nearly as subjective and arbitrary as some have charged. Always a creative scholar, "he used the degree of distinctiveness of closely related sympatric species as the basic measure of whether to recognize allopatric populations as species or subspecies" (Bock 1994)--for example, the sibling species of Pericrocotus (Mayr 1940), the Australian continental complex of Neositta species (Mayr 1950), and the highly variable Eurasian Motacilla flava (Mayr 1956). Mayr's careful taxonomic and curatorial efforts inevitably led to the formulation of the polytypic species concept, which, of course, is under increasing attack by lineage-oriented cladists. Mayr's approach, however, set new standards that have prevailed in ornithology for half a century, despite continuous challenges.
Ernst Mayr's contributions to the macrotaxonomy of birds--primarily generic and familial revisions--numbered fewer than his contributions to microtaxonomy, but they were nevertheless influential and foresightful. Whereas most of Mayr's museum colleagues were experts on the birds of the Americas, Mayr brought to American ornithology a rare, strong complementary understanding of the birds of the Old World. His Classification of Recent Birds (Mayr and Amadon 1951), for example, included the suggestion that some of the Australo-Papuan songbirds, which had been subsumed in major Eurasian families, constituted a major endemic radiation of their own. Relatively recently, Sibley and Ahlquist (1985) amplified this hypothesis with their highly publicized DNA-DNA hybridization studies. After he moved to Harvard University, Mayr assumed editorial responsibilities for completion of Peters' Check-list of Birds of the World (1920-1986). Over the years, Mayr himself contributed lasting revisions on 19 families or subfamilies of Old World birds to this essential reference. Among the families he revised was the Zosteropidae, or white-eyes, a notoriously challenging taxonomic group of island specialists. This work became my personal taxonomic bible when white-eyes were the focus of my graduate research (Gill 1971, 1973).
Mayr's career as a museum ornithologist, interested particularly in problems of microtaxonomy of birds, led inevitably to scholarly concerns about the rules of zoological nomenclature, particularly the role of priority in choosing among synonyms. He argued forcefully in favor of common sense in scientific communication and in favor of conservation of well-established names. As with all topics that interested him, Mayr assumed a leadership role in defining the rules for the practice of taxonomy, which he laid out as a coauthor of Methods and Principles of Systematic Zoology (1953, with Linsley and Usinger) and in later revised editions of this work (Mayr 1969; Mayr and Ashlock 1991).
One of the distinctions of Ernst Mayr's contributions to avian macrotaxonomy was his pioneering incorporation of life-history and behavioral information into phylogenetic reconstruction. His first two revisions of avian families, the swallows (Hirundinidae) (Mayr and Bond 1943) and the waterfowl (Anatidae) (Delacour and Mayr 1945), for example, used nesting habits and courtship behavior, respectively. The use of what were assumed to be plastic, adaptive behaviors devoid of historical information was greeted with skepticism for years, but as Prum (1990) showed for manakins (Pipridae), the fundamental elements of avian courtship displays can be surprisingly conservative; cladograms based on behavior are congruent with cladograms based on anatomical traits. Such discoveries of highly conserved behavior reaffirm early premises of ethology that behavioral characters carry phylogenetic information, as Delacour and Mayr had proposed. The Delacour-Mayr classification of waterfowl pioneered subfamilial and tribal groupings, which have prevailed, with some modifications, for four decades. Livezey's (1986) classification based on parsimony analysis of 120 anatomical characters followed by Sibley and Monroe's (1990) classification based on DNA comparisons have challenged some traditions, especially with respect to the assignment of aberrant taxa such as the magpie goose (Anseranas semipalmata), or of enigmatic taxa such as the perching ducks (Carinini).
Mayr's approach to macrotaxonomy employing life-history characters has also received modern support in the case of the swallows, which construct nests in more diverse ways than any other family of songbirds. Some species burrow into hillsides, others adopt treehole cavities, and others build mud nests on cliffs. Winkler and Sheldon (1993) reconstructed a phylogenetic history of 17 swallow species using DNA-hybridization methods, and then asked whether related species constructed the same kinds of nests. They do; nest-construction habits reflect the evolutionary history of species inferred from genetic comparisons. Whereas Mayr and Bond (1943) suggested that cavity adoption was the most primitive state of nest construction among swallows, the DNA studies suggest instead that burrowing into the soil is the primitive mode of nesting. Burrowing ancestors gave rise first to a diverse clade, including the tree swallow and purple martin, that adopt natural cavities, and to another major clade of species, including the cliff swallow, that use mud-construction techniques.
While a curator of ornithology at the AMNH, Mayr lacked opportunities to train graduate students, which was one of the reasons he moved to Harvard. There, in reflection of his broad interests, he mentored biogeographers (A. J. Keast, F. Vuillemier), systematists (W. Bock, R. Thorington), and behaviorists (W. J. Smith, M. R. Lein, and A. J. Meyerriecks), among others. One can only imagine what the current composition of museum curators would be like if AMNH had had in the 1940s what it does now, a graduate training program linked to City University of New York.
Ernst Mayr, the Concept Advocate
Mayr's era as a curator at AMNH was a formidable one in many respects, but most of all because of his pioneering syntheses of taxonomic information in the broader contexts of biogeography, community ecology, and speciation theory, heralding brilliantly what was to follow in later eras. While conducting first-class systematic research, Mayr probed deeply into the theoretical foundations of his empirical work. His scholarly commitment to detail deepened his understanding of birds as organisms as well as his understanding of variation, populations, and evolutionary patterns.
As a biogeographer, for example, Mayr appreciated the dynamics of dispersal, colonization, extinction, and turnover in island avifaunas (Mayr 1941b) and clearly developed the foundations of island biogeography that MacArthur and Wilson (1967) later formalized. His detailed studies of geographic variation in molts and plumages and sexual dimorphism in birds of the South Pacific led to a novel appreciation of the interactions between genetic and hormonal control of plumage coloration. In his papers on Neolalage and Petroica (Mayr 1933, 1934), which went essentially unnoticed for 50 years, Mayr proposed that the known physiological mechanisms responsible for seasonal changes in plumage coloration were insufficient to explain the patterns of geographic variation in sexual dimorphism and subadult plumages that he observed in South Pacific birds. Beyond the proximate effects of sex hormones were genetic systems subject to evolutionary change and sexual selection. With this hypothesis, Mayr previewed brilliantly modern research questions about the evolution of delayed plumage-color maturation (Thompson 1991).
Such conceptual visions, however, paled beside Mayr's advocational role in the rejection of typological thinking in systematics and the conversion of his colleagues to population thinking for the analysis of biodiversity. We now accept the genetic divergence of allopatric populations as the primary, if not exclusive, mode of speciation in birds; the original arguments of Ernst Mayr for geographical speciation in birds clearly prevail after 50 years of study. His polytypic species concept imposed conceptual order on disparate patterns and degrees of geographical variation, and led to analyzing the process of speciation in terms of the transformation of geographically isolated populations. Most of the key published papers in the recent rush toward the Phylogenetic Species Concept (PSC) tend to be reactions--sometimes superficial and often inconsistent--to the same problems that Ernst Mayr addressed deeply 50 years ago. Few advocates of the PSC have the perspective and practical experience with variability among allopatric populations that Mayr developed from his primary taxonomic studies of South Pacific birds.
Granted, there are deeply opposing philosophies as to how to do science generally, systematics in particular, and how to treat "species." Inevitably, however, the species concept problem boils down to one of semantics and practical scale (O'Hara 1993). The Biological Species Concept, which Mayr advocated and promoted so effectively, but did not originate (Mayr 1993), has been faulted principally for the problems associated with the term "reproductive isolation," which sets up nontestable inferences. It has been much easier to criticize the BSC than to improve on it (Coyne et al. 1988; Coyne 1994). Mayr recognized that reproductive isolation is simply the flip side of "cohesion," which PSC advocates accept as a given, starting with Hennig (1966) himself. Cohesion, and its sister concepts of reticulation and gene flow, violate Hennig's basic principle of hierarchical descent, on which phylogenetic analysis rests. The reasons for this lie simply in the limits of phylogenetic character analysis of lineages: the species concept begins where parsimony analysis ends (Hennig 1966; DeQueiroz and Donaghue 1990). Interbreeding or reticulation, which ultimately defines the taxa of sexually reproducing organisms we call species, produces character networks that cannot be reduced to simple branching diagrams. As PSC advocates themselves grapple with real populations rather than ideological logic, their practical proposals converge naturally toward the Biological Species concept. Consider these two quotations, taken only slightly out of context, from adherents to the PSC:
"[S]ome individuals, moreover, may exhibit variation that obscures recognition of diagnostic characters. It is because of situations such as these that understanding the reproductive relationships of individuals within populations is often critical for delineating species correctly." (Cracraft 1989, p. 36.)
"But unless a complete pedigree of all sexual organisms becomes available, empirical delimitation of the widespread population systems we call phylogenetic species will remain a matter of analyzing distribution patterns of observable attributes within and among local populations" (Davis and Nixon 1992, p. 430).
In the most practical sense, the definition of cohesive, terminal taxonomic units called species, by autapomorphies or any other means, requires rigorous analysis of patterns of spatial variation and arbitrary definition of the limits of cohesion. This is the problem that Ernst Mayr solved mostly with the principles of the Biological Species Concept, and this is the practical problem that will ultimately route PSC advocates back towards a modified version of the BSC.
Ernst Mayr's never-ending study of birds in the field and in museum collections was his primary source of biological education and inspiration. Evident from his overwhelming lifetime bibliography is his ability to combine relentlessly an undiminished devotion to ornithological detail with ever increasing conceptual leadership. Each year of his impressive productivity included a mix of empirical and conceptual papers. Throughout his career, Ernst Mayr has been rooted deeply as an ornithological scholar and naturalist, who understands variation, ecological interactions among organisms, and the real-life operation of natural selection. From these foundations, he was able to construct profound, lasting conceptual syntheses.
Bock, W. J. 1994. Ernst Mayr, naturalist: his contributions to systematics and evolution. Biology and Philosophy 9:1-61.
Coyne, J. A. 1994. Ernst Mayr, speciation and the modern synthesis. Evolution 48:19-30.
Coyne, J. A., H. A. Orr, and D. J. Futuyma. 1988. Do we need a new species concept? Systematic Zoology 37:190-200.
Cracraft, J. 1989. Speciation and its ontology: the empirical consequences of alternative species concepts for understanding patterns and processes of differentiation. Pp. 28-59 in D. Otte and J. A. Endler, eds. Speciation and its consequences. Sinauer, Sunderland, Mass.
Davis, J. I., and K. C. Nixon. 1992. Populations, genetic variation, and the delimitation of phylogenetic species. Systematic Biology 41:421-435.
Delacour, J., and E. Mayr. 1945. The family Anatidae. Wilson Bulletin 58:104-110.
-----. 1946. Birds of the Philippines. Macmillan, New York.
De Queiroz, K., and M. J. Donoghue. 1990. Phylogenetic systematics and species revisited. Cladistics 6:83-90.
Gill, F. B. 1971. Ecology and evolution of the sympatric Mascarene white-eyes, Zosterops borbonica and Zosterops olivacea. Auk 88:35-60.
-----. 1973. Intra-island variation in the Mascarene White-eye, Zosterops borbonica. Ornithological Monographs 12. Allen Press, Lawrence, Kans.
-----. 1990. Ornithology. W. H. Freeman, New York.
Hennig, W. 1966. Phylogenetic systematics. (Transl. by D. D. Davis and R. Zangerl.) University of Illinois Press, Urbana.
Livezey, B. C. 1986. A phylogenetic analysis of recent Anseriform genera using morphological characters. Auk 103:737-754.
MacArthur, R. H., and E. O. Wilson. 1967. The theory of island biogeography. Princeton University Press, Princeton, N.J.
Mayr, E. 1923. Die Kolbenente (Nyroca rufina) auf dem Durchzuge in Sachsen. Ornithologische Monatsberichte 31:135-136.
-----. 1926. Die Ausbreitung des Girlitz (Serinus canaria serinus L.). Journal fur Ornithologie 74:571-671.
-----. 1933. Birds collected during the Whitney South Sea Expedition. XXVII. Notes on the variation of immature and adult plumages in birds and a physiological explanation of abnormal plumages. American Museum Novitates 666:1-10.
-----. 1934. Birds collected during the Whitney South Sea Expedition. XXIX. Notes on the genus Petroica. American Museum Novitates 714:1-19.
-----. 1940. Pericrocotus brevirostris and its double. Ibis 4 (Series 14):712-722.
-----. 1941a. List of New Guinea birds. A systematic and faunal list of the birds of New Guinea and adjacent islands. American Museum of Natural History, New York.
-----. 1941b. The origin and the history of the bird fauna of Polynesia. Proceedings 6th Pacific Science Congress 4:197-216.
-----. 1942. Systematics and the origin of species. Columbia University Press, New York.
-----. 1945. Birds of the southwest Pacific. Macmillan, New York.
-----. 1950. Taxonomic notes on the genus Neositta. Emu 49:282-291.
-----. 1956. The interpretation of variation among the yellow wagtails. British Birds 49:115-119.
-----. 1969. Principles of systematic zoology. McGraw-Hill, New York.
-----. 1993. Fifty years of progress in research on species and speciation. Proceedings of the California Academy of Science 48:131-140.
Mayr, E., and D. Amadon. 1951. A classification of Recent birds. American Museum Novitates 1496:1-42.
Mayr, E., and P. Ashlock. 1991. Principles of systematic zoology, (revised edition). McGraw-Hill, New York.
Mayr, E., and J. Bond. 1943. Notes on the generic classification of the swallows, Hirundinidae. Ibis 85:334-341.
Mayr, E., and W. Meise. 1930. Theoretisches zur Geschichte des Vogelzuges. Der Vogelzug 1:149-172.
Mayr, E., E. G. Linsley, and R. L. Usinger. 1953. Methods and principles of systematic zoology. McGraw-Hill, New York.
O'Hara, R. J. 1993. Systematic generalization, historical fate and the species problem. Systematic Biology 42:231-246.
Prum, R. 1990. Phylogenetic analysis of the evolution of display behavior in the Neotropical manakins (Aves: Pipridae). Ethology 84:202-231.
Sibley, C. G., and J. E. Ahlquist. 1985. The phylogeny and classification of the Australo-Papuan passerine birds. Emu 85:1-13.
Sibley, C. G., and B. L. Monroe, Jr. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, Conn.
Thompson, C. W. 1991. The sequence of molts and plumages in painted buntings and implications for theories of delayed plumage maturation. Condor 93:209-235.
Winkler, D. W., and F. H. Sheldon. 1993. Evolution of nest construction in swallows (Hirundinidae): A molecular phylogenetic perspective. Proceedings of the National Academy Sciences, USA 90:5705-5707.