Essentially, Darwin’s theory of speciation via descent from a common ancestor, and without the connotation that it is a progressive process as, for example, birds with flight can evolve into a flightless species. For him, and for the Modern synthesis, speciation is gradual, rather than saltatory, process arising from the accumulation of point mutations. The tempo of speciation continues to be a controversial topic in evolutionary biology.
See Biological evolution, Darwinism, Macroevolution and microevolution, Phyletic gradualism and punctuated equilibrium, Speciation, Theory of natural selection