Garrett Hardin was a biology professor who specialized in ecology. He was active in the 1960s environmental movement and was particularly concerned with human population. He wrote several texts laying out ideas to solve the overpopulation problems he believed were inevitable. Many of his ideas were controversial and were based on eugenics and social Darwinism.
Thomas Malthus, an 18th-century economist, predicted escalating population growth would lead to widespread famine. In his 1798 publication "An Essay on the Principle of Population," Malthus mathematically demonstrated food supply cannot keep up if the human population continues to double, which would result in widespread famine and deaths. Malthus noted food is not the only limiting factor. Population growth is also contained by destructive forces like disease, natural disasters, and war, as well as by preventive measures like celibacy.
Biologist Charles Darwin developed the theory of evolution and natural selection. His 1859 text On the Origin of Species theorized weak or unfit members of a species will die off. Those with an inheritable trait that helps them survive will live to have more offspring, which will then pass the trait to successive generations.