This prosperous, cultivated family, like many others throughout the German Empire, was ruined by the First World War and the subsequent collapse of the German economy. Mayr's father died during the war, and Ernst, along with his mother and brothers, went to live in Dresden, far from his Bavarian home.
As a teenager, he spotted a red-billed duck, rarely seen north of the Alps, and took his information to the country's leading ornithologist, Erwin Stresemann in Berlin. Stresemann was impressed with the youngster's learning and his powers of observation and urged him to consider a career in zoology. True to his family's medical tradition, Mayr instead began medical studies at the University of Greifswald, but the attraction of wildlife was irresistible and on completing pre-clinical studies, he transferred to the University of Berlin to study zoology. Under Stresemann's guidance, he completed his Ph.D. by age 21.
Although he won appointment as an assistant at the Museum of the University of Berlin, Mayr was eager to undertake an expedition to a remote, exotic place, in the manner of the great 19th century naturalists. Stresemann recommended Mayr to Lord Walter Rothschild, a banker and naturalist who possessed the world's largest private collection of bird specimens, housed at his own museum in Tring, England. Rothschild paid for Mayr to undertake a collecting expedition in the Dutch East Indies, now Indonesia.
On this expedition, Mayr found numerous plant and animal species previously unknown to western science. Over the course of his career, he would name 26 new species of birds and 38 new species of orchids. While in New Guinea, he encountered a team of naturalists from the American Museum of Natural History, who invited him to join them on a collecting expedition in the Solomon Islands.
In 1931, Mayr accepted an invitation to join the staff of the American Museum in New York. Lord Rothschild decided to sell his treasured bird collection, and Mayr arranged for the collection to be sold to the American Museum, where it became an essential component of the collection that Mayr tended for the next 20 years.
Grateful to be safe in New York as the Nazis took power in his native Germany, Mayr settled permanently in the United States. In 1935, he married Margarete Simon. Their marriage was to last more than 55 years, until her death in 1990.
The father of evolutionary theory, Charles Darwin, described the process of evolution within a given species by means of natural selection, but had never been able to explain the process through which descendants of a common ancestor can diverge widely enough to become different species. The rediscovery, early in the twentieth century, of the work of Gregor Mendel, supplied a new piece of the puzzle. Mendel, a Moravian monk, had lived and died in obscurity in the mid-19th century, but his discovery of the mechanism of heredity had laid the groundwork for the modern study of genetics.
In the 1940s, genetic scientists studying evolution relied heavily on mathematical models; few of them had the field experience of observing diverse species in the wild. Mayr brought just such experience to the problem of the formation of species. Mayr found a more constructive approach in the work of Russian geneticist Theodosius Dobzhansky. At the time, one of the greatest difficulties in evolutionary theory was the lack of a clear definition of what actually constitutes a species. It was widely assumed that a species consisted of a number of creatures sharing certain identifiable characteristics, but exactly which characteristics were definitive was much in dispute. Without a clear definition of what a species is, it was impossible to answer the question of how species diverge.
In his landmark 1942 book, Systematics and the Origin of Species, Mayr outlined the concept of "biological species." In the past, scientists had tried to define a species as a number of individual creatures with common features. Drawing on the work of Dobzhansky, among others, Mayr changed the paradigm permanently by defining a species as a group whose members can interbreed only with one another. This biological species concept has become so widely accepted that most laymen now take it for granted.
Mayr described the process of allopatric speciation, in which an isolated population of a given species experiences different environmental factors than creatures of the same species in another location. The isolated population also offers a different set of random mutations for natural selection to act upon, until this isolated population grows so different that it is no longer compatible with other populations descended from the same ancestors. Mayr has never contended that this process accounts for all speciation, but his work established allopatric speciation as the prevalent model of evolution among more complex life forms.
In 1953 Mayr left the American Museum to accept an appointment as Louis Alexander Agassiz Professor of Zoology at Harvard University. He was to remain at Harvard for the rest of his long, productive life. From 1961 to 1970, he also served as Director of the Harvard University Museum of Comparative Zoology.
Mayr continued to write on evolution, in books such as Animal Species and Evolution, and found himself in frequent conflict with the approach of some mathematical geneticists, whom he once dismissed as practicing "bean bag genetics." He experienced a similar disagreement with some molecular geneticists, whose views he saw as reductionist, focusing on individual genes, or even molecules, without taking into account the environmental factors that act on whole organisms, populations and species.
In the 1970s, Mayr's work turned more and more to defining a philosophy of biology, in the larger context of the philosophy of science. He wrote prolifically on the history of biology, especially on the history of evolutionary theory, in works such as The Growth of Biological Thought (1982), One Long Argument: Charles Darwin and the Genesis of Modern Evolutionary Thought (1991) and This Is Biology (1997). As Stresemann had once served Mayr as an invaluable mentor, Mayr in turn found a promising protégé in the young zoologist Jared Diamond. Their decades of work culminated in the publication of The Birds of Northern Melanesia in 2001.
Over the course of his lifetime, Ernst Mayr received every award possible for a scientist in his field, including the National Medal of Science, the International Prize and the Balzan Prize. There is no Nobel Prize for evolutionary biology. Mayr once observed that Charles Darwin himself would not have been eligible for a Nobel Prize. Mayr officially retired in 1975, but remained at Harvard as Professor Emeritus. He published more than 200 articles after he retired, more than most scientists publish in a lifetime. He was still publishing when he died at the age of 100 in 2005.