On the Origin of Species for Existence" Darwin includes "slow-breeding man" among other examples of Malthusian population growth. In his discussions on morphology, Darwin compares and comments on bone structures that are homologous between humans and other mammals. Darwin's early notebooks discussed how non-adaptive characteristics could be selected when animals or humans chose mates, with races of humans differing over ideas of beauty. In his 1856 notes responding to Robert Knox's "The Races of Man: A Fragment", he called this effect sexual selection. He added notes on sexual selection to his "big book on species", and in mid-1857 he added a section heading; "Theory applied
Genetics and the Origin of Species variation would allow populations to evolve rapidly as environmental conditions change. This book was a landmark in the evolutionary synthesis, as it presented the union of Mendelian genetics and Darwinian theory. In "Genetics and the Origin of Species", polyploidy is considered as a type of mutation. Polyploid cells have a chromosome number that is more than twice the haploid number. The effects of polyploidy between two different species causes hybridization and even greater evolution. Natural selection in an environment produces reproductive success, which benefits the species. Speciation is a process of evolution through which new biological species are formed. Dobzhansky