One key question about Natural Selection is the potential scope of change it may cause.
It is relatively uncontroversial that selection processes can change organism within their basic form - so that different breeds and sub-species of dogs, cats or sheep - with different appearances and functional characteristics - can be bred by selection.
And such selection could be human or natural, as when sheep which bred on Northern Hills for many generations will be hardier than those bred in Southern Valleys.
What is uncertain is whether Natural Selection can create new forms.
I know that it is assumed that Natural Selection can lead to new forms, but this is an assumption; especially so because there is no answer in modern biology as to where forms come from in the first place.
Once there are forms, then selection can potentially change them by selection, absorption and combination of forms (for example) - but evolutionary theory typically does not recognise forms as qualitatively distinct - yet it is the reality of form which is assumed whenever we assert that evolution by natural selection has occurred.
Such forms might be a species, or a functional organ such as the eye, or something which is regarded as homologous across evolutionary history - such as the human anterior pituitary gland having evolved (by many steps) from a chemo-sensitive 'nose' in primitive chordates.
Natural selection has to assume the reality of such forms before it can theorise about them evolving - therefore it lacks the basis for explaining how forms arise and in what order and how to identify forms.
On this basis, it seems that it may be coherent to use natural selection only as an explanation of modification within the scope of a form which has already been assumed - and to look elsewhere for explanations of the creation of qualitatively new forms.
So, the scope of natural selection is therefore restricted to suggested explanations for modification of existing forms.
Of course, this leaves open arguments about when a form is indeed novel: for example is the bird a qualitatively new form compared with the reptile or merely a quantitative modification?
Is the chimpanzee a new form compared with the amphibian or merely a quantitative modification?
Is the human a new form compared with the amoeba or merely a quantitative modification?
Is there just one species modified tens of millions of times (hence tens of millions of forms), or tens of millions of qualitatively distinct species, each with a different form?
Forms are therefore assumed, not explained, by Natural Selection; and selection ought no to be used to explain novel forms.