According to Darwin's concept of natural selection, populations change over time because individuals with certain heritable traits leave more offspring than others. The budding of one or more new species from a parent species is referred to as branching evolution or cladogenesis . Although Darwin's theory of evolution by means of natural selection was a profound interpretation of the natural world, it was also a rather simple one. There is a high degree of heritable variation in most natural populations. That is, populations exhibit a substantial amount of variation in form and function and this variation is passed on through the generations.
Populations typically produce more offspring than can survive and reproduce successfully.
The survival and reproductive success of individuals in a population are not equal. If some offspring have traits that give them an advantage under a particular set of conditions those organisms will be more likely to survive and pass on those traits to future generations. As the differences accumulate from generation to generation, this will lead to gradual change in the population such that populations of organisms will diverge from their ancestors, becoming reproductively isolated and eventually forming new species. Behind Darwin's rather simple explanation for evolutionary change and organismal adaptations was a substantial amount of evidence supporting his claims. In the 140 year period since the publication of On the Origin of Species , evidence in support of Darwin's theory of evolution continues to accrue.