The evolutionary biologist Ernst Mayr died on 3 February at the age of 100, after a short illness. A hugely prolific writer and researcher, he was instrumental in developing modern ideas in evolutionary theory.As an ornithologist, Mayr classified many birds, most notably risking the
hostile terrain of New Guinea to catalogue the region's birds of paradise. But he will arguably be best remembered for formulating the concept of species that students still use today.It was Mayr who defined a species as a group of individuals that are capable of breeding with one another, but not with others outside the group. This led to the idea that new species can arise when an existing species becomes separated into two populations that gradually become too distinct tointerbreed; it was an answer to a biological conundrum that had eluded Charles Darwin.

Born in Bavaria, Germany, on 5 July 1904, the young Mayr was fascinated with wildlife but, at 20, was set to enter the medical profession. When offered the chance to visit the tropics to study birds, he completed a PhD in just 16 months before taking up a position at the Berlin Museum in 1926, from which sprang his work in New Guinea.Mayr's later career was spent in the United States, first at the American Museum of Natural History in New York and, from 1953, at Harvard University in Massachusetts. In 1961 he was appointed director of the university's Museum of Comparative Zoology.

Walter Bock, a student of Mayr's in the 1950s and now an evolutionary biologist at
Columbia University in New York, places Mayr's work on a par with two other great biologists, Theodosius Dobzhansky and George Gaylord Simpson. The three were, he says, architects of the 'evolutionary synthesis', the reconciliation of evolutionary theories with the processes of genetic inheritance.

Mayr's contribution was to define the species, which he did through his 25 books,
including his first, Systematics and the Origin of Species, in 1942. "His books pointed out what was going on with the whole notion of the species concept," Bock says.We may never again see someone so influential, in this era of large research groups and even larger databases, Bock adds. "Things have changed," he says. "You can't look at single people any more."