Darwin brought back many samples from the Galapagos, and thought that the finches found across the different islands making up the group, were all different species. Then he made a remarkable discovery.
The industrial revolution across Europe and North America, and the burning of coal, led to every surface, in certain areas, being covered with black soot. So animals evolved to get camouflage from the contamination.
Long before DNA was known, Frederick Griffith, established that there was a transforming principle in bacterial genetics in a ground-breaking experiment. He postulated that information could somehow be transferred between different strains of bacteria.
Identical twins studies has been used for a long time, to study the effects of environment and genetics on human development. Genetics and environmental factors contribute to intelligence, aggression or substance addictions are some subjects.