E. Stresemann and B. Rensch, Mayr's mentors at the Museum of Natural History of Berlin, pursued their work as biologists rather than as species cataloguers. Systematics had become in their hands truly a branch of evolutionary biology. Other scientific staff members in Berlin were also excellent. Three of the entomologists more or less acknowledged and practiced new systematics: W. A. Ramme, worked on orthopterans and introduced Mayr to the concept of sibling species; E. M. Hering, with butterflies and mining insects, and H. Bischoff, who prepared a major book on the biology of hymenoptera. None of them was just an alpha taxonomist. When Mayr came to the United States, he was appalled at the typological spirit dominating taxonomy. Each species was thought to have an essential nature, and species were defined by their degree of difference from one another (Mayr 1999k: XIX). He therefore decided to include in his book (1942e) a complete presentation of the European approach, one whose spirit Julian Huxley (1940) had captured in the phrase "new systematics" (although that book itself contained very little of this new approach). Mayr's book (1942e) consists of two parts, (1) an introduction to the new systematics and (2) an analysis of species and speciation. Providing the first comprehensive treatment, after Rensch (1929,1934), of modern systematics at the species level in English, Systematics and the Origin of Species became highly popular among taxonomists. Whereas the English-American literature made not much of a contribution to the evolutionary synthesis, central European and also Scandinavian systematics contributed massively to the synthesis by stress on population thinking, the biological nature of species, the study of geographical variation, and the clear understanding of geographical speciation in a wide variety of animals (besides birds also in mammals, reptiles, amphibians, fish as well as invertebrates). One ofMayr's major achievements was to summarize and synthesize this scattered European literature which, for linguistic reasons, had remained unknown internationally. Among a total of 450 references, the bibliography includes 174 European, mainly German, titles. This integration of North American and European systematic research in clear and straightforward presentation, the interpretation of many facts in the light of modern systematics and population genetics soon extinguished neo-Lamarckian and typological views in contemporary biology. Allopatric spe-ciation, population thinking, i.e., an emphasis on the "horizontal" (geographical) componentofevolution, and thedemolitionofthe typologicalspecies conceptwere the general topics treated in detail. Additional aspects were geographically variable polymorphism, clinal geographical variation, population structure of species, sibling species, the biological species concept, and monotypic and polytypic species taxa (it should be noted that the distinction between category, concept, and taxon was not yet established at that time). Mayr promoted an adaptationist view of subspecies and species differences, but stressed "the point that not all geographic variation is adaptive" (1942e: 86). His volume includes a summary of his own 12 years of intensive taxonomic research in the spirit of new systematics, particularly in the island regions of Indo-Australasia.
In his first chapter Mayr contrasted old and new systematics like this:
"Old"—the species and purely morphological views occupy a central position; the individual, not the population, is the basic unit.
"New"—subdivisions of the species, subspecies, populations, individual and geographic variation are studied in detail; the population or the "series" of the museum worker has become the basic taxonomic unit. The morphological species definition has been replaced by a biological one which takes ecological, geographical, genetic, behavioral, physiological, biochemical, karyological and other factors into consideration. The new systematist approaches his material more as a biologist and less as a museum cataloguer, he attempts to formulate generalizations and syntheses. He inquires into the nature and the origin of the taxonomic units with which he works.
"The ornithologist knows his material so well that he can do what the geneticist also does that is to pick out one particular character and study its fate under the influence of geographical variation, and in the phylogenetic series. In the less worked groups the taxonomist is forced to consider his taxonomic units as complete entities and all he does is to arrange them in the most natural order. I think I shall add that among the differences between the old and the new systematics" (Ernst Mayr to Edgar Anderson, when preparing his Jesup Lectures, 28 January 1941; HUGFP 14.7, Box 2).
Mayr also referred to the procedures of the species systematist (collecting and analyses of the material, naming of populations and the selection of a type specimen) and the rules of nomenclature. When, in early 1946, he sent copies of his publications during war time to Erwin Stresemann in Berlin, he stated:
"A book came out here last year entitled The Reader over your Shoulder [by R. Graves and A. Hodge]. It was a treatise on the technique of writing and on the good usage of English. It emphasized that you should always write as if somebody was looking over your shoulder reading what you put down. In a somewhat different way you were always the reader over my shoulder. Time after time, when writing my Systematics and the Origin of Species (1942) or my Timor report (1944e), it was you with whom I really discussed my problems while I put them down on paper. Perhaps you can read this between the lines of some of the passages" (24 January 1946).
Stresemann praised Mayr's Systematics and the Origin of Species as "a synthesis of taxonomic, genetic, and biological ways of viewing evolution, ... [which] will long remain a reliable guide for systematists working in the complicated labyrinth of phenomena through which his predecessors had tried vainly to find their way during the past 150 years" (1951: 281,1975: 277-278).
The new systematics had a significant impact on the development of population genetics, and genetics in turn profoundly influenced the ideas of new systematists. Mayr (1948c) reviewed this progress in the field of systematics for the benefit of geneticists (polytypic species and trinominal nomenclature, population thinking, biological species concept). A majority of scientists now agreed on the following seven statements (Mayr 1948c): (1) Animal species taxa have reality in nature and are well delimited except in borderline cases; (2) The species concept is defined biologically, using reproductive isolation as a criterion; (3) Species are composed of distinct populations (subspecies or geographical races); (4) All characters (morphological, ecological, physiological) are subject to geographical variation; (5) Geographical separation results in genetic differentiation which varies among species; (6) Isolating mechanisms will inhibit the interbreeding of the daughter populations when they come in secondary contact; (7) Except in borderline cases, "bridgeless gaps" in morphological and other phenotypical features separate sym-patric species. These considerations changed the entire field of animal taxonomy (Huxley 1940, 1942; Mayr 1942e). A revolution had occurred—a change from the static species concept of Linnaeus to the dynamic species concept of modern systematics.
In the years since 1942 many North American systematists have told Mayr that his book had entirely changed their professional lives. They had been conventional taxonomists, had described new species and made generic revisions, but had never risen above the essentially descriptive level. It was Mayr's book that showed them the immense potential of systematics and demonstrated to them that systemat-ics at the species level occupied an important realm that was inaccessible either to the geneticist dealing with the gene level or the paleontologist dealing with phyletic lines and higher taxa. The volume was a real revelation to the traditional taxonomists. One of them was Ralph Chermock, an assistant professor at the University of Alabama and E. O. Wilson's teacher in 1947. As the latter wrote in his autobiography:
"The prophets of the Chermock circle were the architects of the Modern Synthesis of evolutionary theory. [.] The sacred text of the Chermock circle was Ernst Mayr's 1942 work Systematics and the Origin of Species. Mayr was the curator of birds at the American Museum, but his training had been in Germany, a source of added cachet. The revolution in systematics and biogeography that Mayr promulgated was spreading world-wide but especially in England and the United States. [.] The Modern Synthesis reconciled the originally differing world-views of the geneticists and naturalists. [.] The naturalists were given a hunting license, and for the Chermock circle Mayr's Systematics and the Origin of Species, following upon Dobzhansky's book, was the hunter's vade mecum. From Mayr we learned how to define species as biological units. With the help of his written word we pondered the exceptions to be expected and the processes by which races evolved into species. We acquired a clearer, more logical way to think about classification by using the phylogenetic method" (Wilson 1994: 110-112; see also Wilson 1998: 4).
Working on a revision of his successful 1942 book, Mayr soon decided to separate methodological aspects of systematic zoology from the science of species and evolutionary biology. In this way originated his textbook, Methods and Principles of Systematic Zoology (1953a, with Linsley and Usinger) which covered methodological aspects (p. 321) and his masterly monograph on Animal Species and Evolution (1963b) treating the species problem and aspects of evolutionary biology.
In his article, "Trends in avian systematics" Mayr (1959e) emphasized two main topics: (1) Population systematics and (2) macrotaxonomy. Subspecies are unsuitable for describing the population structure of a species. This can be done in terms of three major population phenomena: (a) Geographical isolates which are particularly common near the periphery of the species range and which represent important units of evolution; (b) The population continuum, a continuous series of populations which make up the main body of the species, and (3) Zones of secondary intergradation between a former geographical isolate (which did not yet attain full species rank) and the main body of the species. To make such an analysis of all species of a family, and to record the relative frequency of the three elements, is one of the steps leading to "comparative systematics." Recording the position of belts of secondary hybridization permits the reconstruction of formerly existing barriers and of the location of "refuges" caused by drought conditions in low latitudes during the Pleistocene. With respect to macrotaxonomy, Mayr proposed to utilize various new sources of information like promising new character complexes of behavior and biochemistry to determine the phylogenetic relationships between genera, families and orders.
Behavioral characters are completely equivalent to morphological characters (Mayr 1958g). If there is a conflict between the evidence provided by morphological characters and that of behavior, taxonomists are increasingly inclined to give greater weight to the ethological evidence, e.g., in the case of grasshoppers, in certain swallows (p. 327), ducks (p. 327), and finches (p. 329). Based on behavioral characteristics the weaver finches (Estrildidae) and weaverbirds (Ploceidae) are not as closely related as previously thought. Movements often precede special morphological features that make them particularly conspicuous. Comparative studies of behavior permit statements about trends in the evolution of behavior, as shown, e.g., by the work of Whitman on pigeons, Tinbergen on gulls, Heinroth and Lorenz on ducks, Meyerriecks on herons, Hinde on finches, Morris on weaver finches. Other groups studied in this way include grasshoppers, Drosophila, hymenopterans, and cichlid fishes.
In 1964(n), Mayr again summarized the then "new systematics" mentioning that certain aspects can be traced back to some authors of the mid-19th century who collected "series" of specimens (population samples). A more general change in the working methods of systematists occurred during the 1930s instigated in Europe by Rensch (1929,1934). "New systematics" is a viewpoint, an attitude toward taxonomic work. The emphasis is on non-morphological characteristics derived from behavior and ecology, bioacoustics, biochemistry, individual and geographical variation, weighting of characters and computer analysis. Basic taxonomic description and cataloguing is still necessary. Genetic research had shown that the phenotype is the product of the total genotype: Many genes shape a character (polygeny), and a single gene contributes to the expression of many characters (pleiotropy). This had an impact on the thinking of systematists. Other improvements were, e.g., the recognition of sibling species which seem nearly identical morphologically, the biological definition of the species concept, the new evalua-tionof manygeographicalisolatespreviously considered tobe speciesassubspecies of polytypic species. The taxonomist has to (1) identify and define populations, subspecies and species; (2) assemble the units into aggregates, groups, taxa, and (3) assign such taxa to taxonomic categories (ranking).
The 1930s and 1940s saw a preoccupation with the species and subspecies, that is microtaxonomy, whereas in the 1950s and 1960s (Simpson, Cain, Rensch) the discussion shifted to higher taxa, that is macrotaxonomy. Mayr (1971g) returned to methods and strategies of taxonomic research, some of which he had reviewed in his textbook (1969b). With this article he intended to stimulate curatorial practices and taxonomic publications. Owing to major ecological projects and rescue operations in areas where the habitat is being destroyed, the burden on the taxonomists has increased, although the number of positions for taxonomists has not increased anywhere near as much. This requires streamlining of taxonomic operations. Mayr felt that at least half of a curator's time should be devoted to research and inspired by projects not restricted to the study of local faunas and floras. Advice is here also given on illustrations, the preparation of a manuscript for publication, the cataloguing of collections and the (questionable) maintenance of filing and cross filing systems. "Even though a classical branch of biology, taxonomy does not need to be old-fashioned."
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