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Neither Darwin nor anyone else in his time knew the answer to the species problem: how multiple species could evolve from a single common ancestor. Ernst Mayr approached the problem with a new definition for the concept species. In his book Systematics and the Origin of Species (1942) he wrote that a species is not just a group of morphologically similar individuals, but a group that can breed only among themselves, excluding all others. When populations of organisms get isolated, the sub-populations will start to differ by genetic drift and natural selection over a period of time, and thereby evolve into new species. The most significant and rapid genetic reorganization occurs in extremely small populations that have been isolated (as on islands).
His theory of peripatric speciation (a more precise form of allopatric speciation which he advanced) based on his work on birds, is still considered a leading mode of speciation, and was the theoretical underpinning for the theory of punctuated equilibrium. Mayr is generally credited with inventing the modern philosophy of biology, particularly of evolutionary biology, which he distinguished from physics, for its introduction of (natural) history into science.
Mayr was the second son of Helene Pusinelli and Dr. Otto Mayr. His father was a jurist (District Prosecuting Attorney at Würzburg) but took an interest in natural history and took the children out on field trips. He learnt all the local birds in Würzburg from his elder brother Otto. He also had access to a natural history magazine for amateurs, Kosmos. His father died just before he was thirteen. The family then moved to Dresden and he studied at the Staatsgymnasium (“Royal Gymnasium” until 1918) in Dresden-Neustadt and completed his high school education there. In April 1922, while still in high school, he joined the newly founded Saxony Ornithologists’ Association. Here he met Rudolf Zimmermann who was his ornithological mentor during this time. He passed his high school examination (Abitur), in February 1923, and Mayr's mother rewarded him with a pair of binoculars. On 23 March 1923 on the lakes of Moritzburg, the Frauenteich, he spotted what he identified as a Red-crested Pochard. The species had not been seen in Saxony since 1845 and the local club argued about the identity. Raimund Schelcher (1891–1979) of the club then suggested that Mayr visit his classmate Erwin Stresemann on his way to Greifswald where Mayr was to begin his medical studies. After a tough interrogation, Stresemann accepted and published the sighting as authentic. Stresemann was very impressed and suggested that Mayr could work as a volunteer between semesters in the ornithological section of the museum. Mayr wrote about this event It was as if someone had given me the key to heaven. He entered the University of Greifswald in 1923 and, according to Mayr himself, "took the medical curriculum (to satisfy a family tradition)." Mayr was endlessly interested in ornithology and "chose Greifswald at the Baltic for my studies for no other reason than that...it was situated in the ornithologically most interesting area." Although he ostensibly planned to become a physician, he was "first and foremost an ornithologist." During the first semester break Stresemann gave him a test to identify treecreepers and Mayr was able to identify most of the specimens correctly. Stresemann declared that Mayr 'was a born systematist'. In 1925 Stresemann suggested that he give up his medical studies and join the Berlin Museum with the prospect of bird-collecting trips to the tropics on the condition that he completed his doctoral studies in 16 months. Mayr completed his doctorate in ornithology at the University of Berlin under Dr. Carl Zimmer, who was a full professor (Ordentlicher Professor), on 24 June 1926 at the age of 21. On 1 July he accepted the position offered to him at the Museum for a monthly salary of 330.54 Reichsmark.
At the International Zoological Congress at Budapest in 1927, Mayr was introduced by Stresemann to banker and naturalist Walter Rothschild, who asked him to undertake an expedition to New Guinea on behalf of himself and the American Museum of Natural History in New York. In New Guinea Mayr collected several thousands bird skins (he named 26 new bird species during his lifetime) and, in the process also named 38 new orchid species. During his stay in New Guinea, he was invited to accompany the Whitney South Seas Expedition to the Solomon Islands.
He returned to Germany in 1930 and in 1931 he accepted a curatorial position at the American Museum of Natural History, where he played the important role of brokering and acquiring the Walter Rothschild collection of bird skins, which was being sold in order to pay off a blackmailer. During his time at the museum he produced numerous publications on bird taxonomy, and in 1942 his first book, Systematics and the Origin of Species, which completed the evolutionary synthesis started by Darwin.
After Mayr was appointed at the American Museum of Natural History, he influenced American ornithological research by cultivating mentoring relationships with young birdwatchers. Mayr organized a monthly seminar under the auspices of the Linnean Society of New York. This society, under the influence of J. A. Allen, Frank Chapman and Jonathan Dwight concentrated on taxonomy and later became a clearing house for bird banding and sight records. There were a group of eight young birdwatchers from the Bronx and later became the Bronx County Bird Club and they were led by Ludlow Griscom. Mayr was surprised at the differences between American and German Birding Societies. He noted that the German society was "far more scientific, far more interested in life histories and breeding bird species, as well as in reports on recent literature." Mayr also encouraged his Linnaean Society seminar participants to take up a specific research project of their own. "Everyone should have a problem" was the way one Bronx County Bird Club member recalled Mayr's refrain. One of Mayr's seminar participants was Joseph Hickey and under Mayr's influence went on to write A Guide to Birdwatching (1943). Hickey remembered later –"Mayr was our age and invited on all our field trips. The heckling of this German foreigner was tremendous, but he gave tit for tat, and any modern picture of Dr E. Mayr as a very formal person does not square with my memory of the 1930s. He held his own." Mayr's said of his own involvement with the local birdwatchers: "In those early years in New York when I was a stranger in a big city, it was the companionship and later friendship which I was offered in the Linnean Society that was the most important thing in my life."
Another person that Mayr greatly influenced was Margaret Morse Nice. Mayr encouraged her to correspond with the European ornithologists of the time, and helped her in her landmark study on Song Sparrows. Nice wrote to Joseph Grinnell in 1932 trying to get foreign literature reviewed in the Condor: "Too many American ornithologists have despised the study of the living bird; the magazines and books that deal with the subject abound in careless statements, anthropomorphic interpretations, repetition of ancient errors, and sweeping conclusions from a pitiful array of facts. ... in Europe the study of the living bird is taken seriously. We could learn a great deal from their writing." Mayr ensured that Nice could publish her two volume Studies in the Life History of the Song Sparrow, finding her a publisher, and her book was reviewed by Aldo Leopold, Grinnell, Jean Delacour. Nice dedicated her book to "My Friend Ernst Mayr."
Mayr joined the faculty of Harvard University in 1953, where he also served as director of the Museum of Comparative Zoology from 1961 to 1970. He retired in 1975 as emeritus professor of zoology, showered with honors. Following his retirement, he went on to publish more than 200 articles, in a variety of journals—more than some reputable scientists publish in their entire careers; 14 of his 25 books were published after he was 65. Even as a centenarian, he continued to write books. On his 100th birthday, he was interviewed by Scientific American magazine.
He received awards including the National Medal of Science, the Balzan Prize, the Sarton Medal of the History of Science Society, the International Prize for Biology, the Loye and Alden Miller Research Award, and the Lewis Thomas Prize for Writing about Science. In 1939 he was elected a Corresponding Member of the Royal Australasian Ornithologists Union. He was never awarded a Nobel Prize, but he noted that there is no Prize for evolutionary biology, and that Darwin would not have received one, either. Mayr did win a 1999 Crafoord Prize. That prize honors basic research in fields that do not qualify for Nobel Prizes and is administered by the same organization as the Nobel Prize.
Mayr was co-author of six global reviews of bird species new to science (listed below).
He was also an atheist, stating "there is nothing that supports the idea of a personal God".
In many of his writings, Mayr rejected reductionism in evolutionary biology, arguing that evolutionary pressures act on the whole organism, not on single genes, and that genes can have different effects depending on the other genes present. He advocated a study of the whole genome rather than of isolated genes only. Current molecular studies in evolution and speciation indicate that although allopatric speciation seems to be the norm in groups (possibly those with greater mobility) such as the birds, there are numerous cases of sympatric speciation in many invertebrates (especially in the insects).
After articulating the biological species concept in 1942, Mayr played a central role in the species problem debate over what was the best species concept. He staunchly defended the biological species concept against the many definitions of "species" that others proposed.
Mayr was an outspoken defender of the scientific method, and one known to sharply critique science on the edge. As a notable recent example, he criticized the search for aliens as conducted by fellow Harvard professor Paul Horowitz as being a waste of university and student resources, for its inability to address and answer a scientific question.
“The idea that a few people have about the gene being the target of selection is completely impractical; a gene is never visible to natural selection, and in the genotype, it is always in the context with other genes, and the interaction with those other genes make a particular gene either more favorable or less favorable. In fact, Dobzhanksy, for instance, worked quite a bit on so-called lethal chromosomes which are highly successful in one combination, and lethal in another. Therefore people like Dawkins in England who still think the gene is the target of selection are evidently wrong. In the 30's and 40's, it was widely accepted that genes were the target of selection, because that was the only way they could be made accessible to mathematics, but now we know that it is really the whole genotype of the individual, not the gene. Except for that slight revision, the basic Darwinian theory hasn't changed in the last 50 years.”