Michael Hopkin in Nature:
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 to interbreed; it was an answer to a biological conundrum that had eluded Charles Darwin.
More here. And there is an obituary in the New York Times here.