Ernst Mayr, a biologist who expanded upon Darwin’s theory of evolution, died on February 3 at the age of 100. While he also earned acclaim as an ornithologist, naturalist, and historian of biology during his eight-decade career, Mayr will be best remembered as a champion of evolutionary theory.

Mayr’s major contribution came in 1942, when his book Systematics and the Origin of Species was published. Here, Mayr laid one of the cornerstones of the then new synthetic theory of evolution, which unified Charles Darwin’s theory of evolution by natural selection with Gregor Mendel’s theory of heredity. One of the shortcomings of Darwin’s theory was that it didn’t explain how new species appear. Geneticists who advanced Mendel’s theories on heredity, meanwhile, began to look for explanations of speciation at the level of the gene.

Mayr’s approach was first to define a species as an interbreeding population that is “reproductively isolated” from another interbreeding population. This improved upon the previous definition of a species as a population whose members share similar traits and characteristics. Mayr then argued that new species arise when a particular population is separated from the rest of its species; genetic mutations eventually make interbreeding with the original group impossible.

Born in 1904 in Kempten, Germany, Mayr enjoyed a career marked by extraordinary good fortune. In the tradition of a family with many doctors, he began studying to become a physician in 1923. But a childhood spent on nature outings with his father, Otto, had also instilled in Mayr a passion for bird-watching. On one solitary expedition near his home, Mayr spotted two rare ducks, red-crested pochards, that had not been seen in Europe in nearly 80 years.

Eager to officially confirm the sighting, Mayr was able through a chance acquaintance to meet Professor Erwin Stresemann, a distinguished ornithologist in Berlin. Stresemann was so impressed by Mayr’s intellect that he invited him to spend the summer working at Berlin’s Museum of Natural History and eventually convinced Mayr to abandon his medical studies and complete a doctorate at the University of Berlin. In exchange, Stresemann offered to send Mayr on an expedition to collect specimens, in the tradition of Darwin and other naturalists.

Mayr completed his doctorate in 1926, and in 1928, his formative expedition was arranged after he met with Lord Walter Rothschild, who was building a collection of bird skins at his private museum in Tring, England. Lord Rothschild, who was known to get around town in a zebra-drawn carriage, had recently suffered the death of his bird collector in New Guinea; Mayr leapt at the opportunity to succeed him, exploring six New Guinea mountain ranges, collecting 3,400 bird skins, and discovering 38 new species of orchids.

In 1931, Mayr was hired by the American Museum of Natural History in New York. He went on to curate the museum’s bird collection until 1953, when he was lured away by Harvard University to become a professor of zoology. Mayr remained at Harvard for the rest of his career, establishing his reputation as the preeminent neo-Darwinist and as a scientist of extraordinary intellectual range.

A prominent figure in the field of the philosophy of biology, Mayr criticized what he saw as the excesses of reductionism in biology, maintaining that natural selection acted on individual organisms rather than on individual genes. He formally retired in 1975 but continued his prodigious writing and research right up to the time of his death.