Genes determine single characteristics in a species.
A species is often defined as the largest group of organisms. Any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. A species is a group of organisms that share a genetic heritage, are able to interbreed, and create offspring that are also fertile. Charles Darwin and Alfred Wallace separately conceived of the mechanism that creates multiple species from a single species. This process of natural selection applies to the adversity of different forms that organisms must overcome to reproduce.