The so-called rediscovery of Gregor Mendel‘s laws of inheritance in 1900 ignited a fierce debate between two camps of biologists. In one camp were the Mendelians, who were focused on discrete variations and the laws of inheritance. Their opponents were the biometricians, who were interested in the continuous variation of characteristics within populations. The biometricians rejected Mendelian genetics on the basis that discrete units of heredity, such as genes, could not explain the continuous range of variation seen in real populations.
When T. H. Morgan began experimenting with breeding the fruit fly Drosophila melanogaster, he was a saltationist who hoped to demonstrate that a new species could be created in the lab by mutation alone. Instead, the work at his lab between 1910 and 1915 reconfirmed Mendelian genetics and provided solid experimental evidence linking it to chromosomal inheritance. His work also demonstrated that most mutations had relatively small effects, such as a change in eye colour, and that rather than creating a new species in a single step, mutations served to increase variation within the existing population.