Some people, including some biologists, seem to be unclear about what the theory/Law of evolution by natural selection actually is. Below is a diagram which explains it. The theory was first proposed in Darwin and Wallace's joint paper published by the Linnean Society of London in August 1858 (see HERE). It explains adaptive evolutionary change within a lineage over time i.e. anagenesis. Anagenesis is the gradual evolution of a species that continues to exist as an interbreeding population. This contrasts with cladogenesis, which occurs when there is branching or splitting, leading to two or more lineages and resulting in separate species. Additional 'add on' theories are required to explain cladogenesis (sympatric, parapatric or allopatric speciation). Natural selection can act at various levels (genes, individuals, or groups of individuals), but this is an additional component - not one which is intrinsically part of the theory of natural selection. In their 1858 paper, both Darwin and Wallace proposed that selection acted on individuals (see HERE), although Darwin later suggested it could also act at the group level.
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