ERNST MAYR devoted eight decades to the systematic study of the natural world that he learnt to love as a boy growing up in Bavaria before the First World War. He began as an ornithologist, became an evolutionist, and in later life developed his interests in the philosophy of biology. His longevity and his appetite for work enabled him both to establish a prominent place in the history of 20th-century evolutionary biology, and to write that history himself.

In an essay marking his 100th birthday, which fell last July, Mayr described himself as “the last survivor of the golden age of the evolutionary synthesis”. That process, which occupied the middle decades of the past century, integrated two theories which had originated in the middle of the century before: Darwin’s theory of evolution by natural selection, and Mendel’s theory of heredity.

Much of the credit for it has conventionally been awarded to geneticists, whose methods were mathematical, but Mayr challenged their significance. A naturalist and a confirmed non-mathematician, he insisted that the geneticists had only addressed the question of how organisms become adapted to their conditions of life. For the synthesis to be complete, it needed to incorporate the question of how new species are formed. It was this dimension of the synthesis which Mayr helped to clarify, most notably in his landmark book Systematics and the Origin of Species (1942).

In particular, Mayr explained the “biological species concept”, a view of species not as forms defined by a collection of attributes but as populations of interbreeding organisms. He emphasised the role of geographical isolation in the formation of new species: when parts of a population became physically separated, they would take different evolutionary paths, and would eventually become incapable of interbreeding. In this he followed his mentor, Erwin Stresemann; both in the concept itself, which he considered to have been held by Stresemann and other German ornithologists, and in his conviction — not typical of the age in which he was formed — that biology should be a theoretical discipline as well as a project to accumulate facts.

Ernst Walter Mayr was born in Kempten, Bavaria, in 1904. His love of nature was inculcated by his father Otto, a judge, on Sunday natural history excursions, and later his family’s tradition of medical practice guided him to medical studies — though his choice of university, Greifswald, was influenced by the likelihood that the local bird life would be interesting. He made contact with Erwin Stresemann, who was curator of birds at the University of Berlin Museum of Natural History, to report a sighting of rare ducks, and in 1925 moved to Berlin, where Stresemann supervised his PhD. Mayr completed his doctoral thesis the following summer, at the age of 21, and became an assistant at the museum.

He followed the tradition of the great Victorian naturalists by going on formative expeditions in his youth, followed by long years studying specimens. In 1928 he went to New Guinea to collect for Lord Walter Rothschild’s museum at Tring, Hertfordshire, and the American Museum of Natural History, then joined the museum’s Whitney expedition further east in the Solomon Islands. Surviving tropical diseases and other hazards — a highly premature report of his death claimed that he had met his end at native hands — he returned to Berlin in 1930. In 1931 he sailed to the US, to take up a post at the museum in New York. Rothschild’s collection of 280,000 bird specimens followed him not long after — Rothschild sold it to the museum in the face of blackmail demands arising from an affair. Curating this collection was Mayr’s main responsibility for the next 20 years.

In 1935, Mayr married Margarete (Gretel) Simon. Their marriage lasted 55 years, until her death in 1990.

Mayr left the natural history museum in 1953 to become Alexander Agassiz Professor of Zoology at the Harvard University Museum of Comparative Zoology. He stayed at Harvard for the rest of his life, serving as the director of the MCZ from 1961 to 1970, and becoming Professor Emeritus in 1975.

He projected his vision of biology and his role in its development in the historical and philosophical ventures he undertook from the 1970s onwards, from the historical and biographical essays in The Evolutionary Synthesis, which he co-edited with the historian of science Will Provine, through the 900 pages of his 1982 book The Growth of Biological Thought, to the articles and books he wrote in his nineties.

These later works summarise the persistent themes in his thought, expressed with characteristic directness and conviction. He asserted the importance of naturalists such as himself in the development of evolutionary theory. Naturalists, he considered, were disposed to recognise that natural selection acts on individual organisms rather than individual genes. Mayr’s view of the evolutionary geneticists’ project, in which they used mathematics to treat individual genes in isolation, was expressed in the phrase “beanbag genetics”. Swimming against one of the most powerful currents in modern biology, much of which now deals with molecules rather than organisms, he rejected reductionism wholesale.

In this, as in his vision of biology overall, he was a self-consciously continental thinker, holistic in inclination and schooled in the importance of systematic classification. His historical commentaries asserted the significance of scientists from a continental European background, most of all the Russian-born Theodosius Dobzhansky, whose 1933 paper on variation in beetles provoked Mayr to exclaim: “Here is finally a geneticist who understands us taxonomists!”

In relocating across the Atlantic, however, Mayr left behind the Central European philosophical tradition of idealism, whose notion of ideal types he saw as an obstacle to the concept of species as populations of varying individuals. He maintained a commitment to German natural history, and acquired a following in the country of his birth.

After the war, he and his wife Gretel supported his colleagues in Germany by sending them journals as well as food parcels. Meanwhile, in his adopted country (he and Gretel became US citizens in 1950) he founded the journal Evolution in 1947.

As a professor he was American in his informality, in striking contrast to the strict hierarchies that still endured back in Europe. One of his former students recalls how, on a visit to France, Mayr deliberately contrived to shock his hosts by engaging in debate with his junior colleague on first-name terms.

Over the course of his career Mayr supported and guided many younger colleagues — most dramatically to New Guinea, in the case of Jared Diamond, whom he met when Diamond was a teenager, and Mayr worked with his father on the role of natural selection in the evolution of blood groups. In 1971, they began a project which eventually resulted in their book The Birds of Northern Melanesia (2001).

Ernst Mayr remained an ornithologist and a systematist even after he had turned to history and philosophy. All these fields were unified by his understanding of evolution.

Mayr’s many honours include the Japan Prize, the Balzan Prize and the Crafoord Prize, one of the most prestigious international awards open to biologists. His tally of productivity includes the description of 26 species and 473 subspecies of birds previously unknown to science, and 25 books, the last of which (What Makes Biology Unique) he completed in time for his centenary, 81 years after his first publication. He is survived by his two daughters.

Ernst Mayr, biologist, was born on July 5, 1904. He died on January 3, 2005, aged 100.