IV.1 At the very least, natural selection comprises environmental selection and sexual selection
Darwin introduced the expression natural selection as an analogy or rather as an antithesis of the term artificial selection,i.e. selection performed by humans.Later, it was found that natural selection consists of at least two components, of selection performed by the environment, i.e. environmental selection, and of selection that occurs through the competition of members of the same sex for partners for reproduction, i.e. sexual selection (Darwin 1909).However, Darwin did not explicitly introduce the term environmental selection as supplementary to the term sexual selection and used the expression natural selection in both a broader and a narrower sense.
This inadequacy in the professional terminology leads, for example, to frequent misunderstanding in relation to the position of sexual selection.One faction of biologists considers this to be part of natural selection, while another faction considers it, as Darwin did, to be the biological process itself, operating independently of environmental selection and acting in the same population, frequently in the opposite direction to environmental selection. It is apparent that the proponents of the former concept understand natural selection in the broader sense of selection that is natural, not artificial, while the proponents of the latter concept understand it in a narrower sense, i.e. in the sense of selection performed by the environment (Flegr 1996).
The category of natural selection can also include parental selection, i.e. selection performed by parents amongst their progeny.For example, it is assumed for altricial birds that the brightly coloured lining of their beaks and mouths emerged because parents preferentially fed young with the most obvious, i.e. most brightly coloured beak linings (Lyon, Eadie, & Hamilton 1994).With certain reservations the category of natural selection can also be considered to include subconscious selection performed by humans.It is quite possible, for example, that the phenotype of house cats (appearance, behaviour) was created through just this mechanism, through a form of unconscious domestication.People preferentially kept in their houses individuals that exhibited traits reminiscent of human young and that were also clean and affectionate.