evolutionary (or modern) synthesis of Mendelian genetics and Darwinian evolution, died on February 4 at the age of 100. His work led to the development of the biological species concept and his theory of peripatric speciation is the basis of the theory of punctuated equilibrium. A mover and shaker in the philosophy of biology, Mayr was a proponent of macroevolutionary theory--invesigations into how some species survive better than others and how likely or unlikely they are to give rise to other species.
Carol Kaesuk Yoon has written an obit for the New York Times. Here's Mayr's Wikipedia entry.
Also check out Edge.org on Mayr and Nature.