The greatest change in the species concept was made by Charles Darwin, for evolution meant that hard-and-fast lines could not be drawn between species. Another shift came when Ernst Mayr proposed the biological species concept, which emphasized the interbreeding population as the heart of the species concept. This meant that, in his view, species were objective, that is not just the subjective opinion of a taxonomist. His explanation of the process of speciation was geographical isolation between populations which had once been interbreeding. Today, emphasis has moved back closer to Darwin's ideas.