In his book The Origin of Species, Darwin presents evidence in a sober manner for his “descent by modification” theory, which has come down to us as the theory of evolution, although Darwin avoided the term “evolution.” Essentially, Darwin suggested that random variations take place in living things and that some external agent in the environment selects those individuals better able to survive. The method of selecting individuals is known as natural selection. The selected individuals pass on their traits to their offspring, and the population continues to evolve.
Two essential points underlie natural selection. First, the genetic variations that take place in living things are random variations. Second, the genetic variations are small and cause little effect relative to a given population. Over time, these small genetic variations lead to the gradual development of a species rather than the sudden development of a species. Darwin proposed that variations appear without direction and without design. He assumed that among inherited traits, some traits were better than others. If an inherited trait provided an advantage over another, it would provide a reproductive advantage to the bearer of the trait. Thus, if long-necked giraffes could reach food better than short-necked giraffes, the long-necked giraffes would survive, reproduce, and yield a population consisting solely of long-necked giraffes.
As the central concept of Darwin's theory of evolution, natural selection implies that the fittest survive and spread their traits through a population. This concept is referred to as the survival of the fittest. The fitness implied is reproductive fitness, that is, the ability to survive in the environment and propagate the species. Natural selection serves as a sieve to remove the unfit from a population and allow the fittest to reproduce and continue the population. Today, scientists know that other factors also influence evolution.