 On Darwin’s original formulation, natural selection acted on individual organisms (entries:  Darwinism;  natural selection). As a population of organisms struggled to survive in some particular environment, some of the organisms had traits that made them slightly fitter in that environment, and this meant they were more likely to survive and reproduce; the offspring of those organisms in turn inherited the traits of their parents, and so the character of the population changed over time, eventually forming a new or different species if the process went on long enough (entries:  evolution;  fitness). For example, in an environment that is growing increasingly cold, animals with thicker fur might have an advantage over the animals with less fur because of their ability to tolerate the temperature; these thicker-furred animals would be more likely to survive in that cold environment and also more likely to reproduce offspring that inherit their parents’ thicker coats. Eventually, the entire population of those animals may display this thicker fur (entry:  adaptationism).  