Ironically, one great unsolved problem in Darwin's master work, On the Origin of Species, was just that: How and why do species originate? Darwin and his later followers were faced with a seeming paradox. They described evolution as a continuous, gradual change over time, but species are distinct from each other, suggesting that some process has created a discontinuity, or gap, between them.
Credit for doing the most to crack this puzzle goes to Ernst Mayr, perhaps the greatest evolutionary scientist of the twentieth century. Along with Theodosius Dobzhansky, George Gaylord Simpson, and others, Mayr achieved the "modern synthesis" in the 1930s and 1940s that integrated Mendel's theory of heredity with Darwin's theory of evolution and natural selection
Born in 1904 in Germany, Mayr trained as a medical student but realized he had a greater passion for studying birds and biology. Emigrating to the United States, he became a curator at the American Museum of Natural History, working on bird classification while formulating his key ideas about evolution. In 1942 he published his most important work, Systematics and the Origin of Species. Mayr moved to Harvard University in 1953 and served as director of the school's Museum of Comparative Zoology from 1961 to 1970. Since then, he has published a number of books and chapters and received the prestigious Japan Prize for Biology in 1983.
In his landmark 1942 book, Mayr proposed that Darwin's theory of natural selection could explain all of evolution, including why genes evolve at the molecular level. On the stubborn question of how species originate, Mayr proposed that when a population of organisms becomes separated from the main group by time or geography, they eventually evolve different traits and can no longer interbreed.
It's this isolation or separation that creates new species, said Mayr. The traits that evolve during the period of isolation are called "isolating mechanisms," and they discourage the two populations from interbreeding.
Moreover, Mayr declared that the development of many new species is what leads to evolutionary progress. "Without speciation, there would be no diversification of the organic world, no adaptive radiation, and very little evolutionary progress. The species, then, is the keystone of evolution."