Ernst Mayr

Ernst Walter Mayr (July 5, 1904, Kempten, Germany – February 3, 2005, Bedford, Massachusetts), was a German American scientist. He was one of the 20th century's leading evolutionary biologists. He was also a well-known taxonomist, tropical explorer, ornithologist, historian of science, and naturalist. He was a leading contributor to the modern evolutionary synthesis. He was especially interested in how new species formed.
Mayr joined the faculty of Harvard University in 1953, where he also served as director of the Museum of Comparative Zoology from 1961 to 1970. He retired in 1975 as emeritus professor of zoology, showered with honors.
After his retirement, he went on to publish more than 200 articles, in a variety of journals—more than some reputable scientists publish in their entire careers; 14 of his 25 books were published after he was 65. Even as a centenarian, he continued to write books.
Mayr was awarded the Linnean Society's prestigious Darwin-Wallace Medal in 1958. He was never awarded a Nobel Prize, because there is no such prize for evolutionary biology. He commented that Darwin would not have received one, either. Mayr did win a 1999 Crafoord Prize. That prize honors basic research in fields that do not qualify for Nobel awards, and is administered by the same organization as the Nobel Prize.
Speciation.
Neither Darwin nor anyone else in his time knew the answer to the "species problem": how species could evolve from a single common ancestor.
Ernst Mayr approached the problem with a definition for the concept species. He wrote that a species is not just a group of individuals that look similar, but a group that can breed only among themselves.
When populations of organisms get isolated, the sub-populations will start to differ as time goes by. This way, they will evolve into new species. The most rapid genetic reorganization occurs in small populations which have been isolated. This happens if a species gets trapped on an island, for example. Or, perhaps, if the species lives in more than one environment.
Today, it is accepted that reproductive isolation is by far the most frequent cause of species splitting, and that geographical separation is the most frequent cause of this isolation. This was Mayr's most characteristic idea. Debate continues over the extent to which speciation occurs when a population is not so isolated.