And along with investigating how selection operates on organisms — Darwin’s unit of choice — scientists are also showing how it acts on groups, genes and behavior. Eldredge and Gould were familiar with the work of biologist Ernst Mayr and geneticist Theodosius Dobzhansky, who developed ideas on how species originate, laid out in Mayr’s 1942 book Systematics and the Origin of Species. While debate continues over where and how selection acts, many scientists advocate the “Russian nesting doll” approach that allows selection at numerous levels, including species, groups, individuals, cells and, of course, genes, as popularized by British evolutionary biologist Richard Dawkins.