Natural Selection Acts on Existing Variation Steps:

• Define natural selection as the differential survival and reproduction of individuals due to differences in phenotype.
• Determine if it causes mutations: no, mutations are random genetic changes independent of selection.
• Assess if it leads to new species: yes, through gradual adaptation and reproductive isolation over generations.
• Evaluate if it results in extinction: yes, as poorly adapted populations decline and disappear.

Why D is correct:

• Darwin's theory states natural selection favors existing advantageous traits from prior mutations, promoting speciation while eliminating unfit lineages via extinction.

Why the others are wrong:

• A: Incorrectly attributes causation of mutations to selection and denies extinction of maladapted species.
• B: Falsely links selection to causing mutations and rejects its role in forming new species.
• C: Wrongly denies that natural selection drives extinction of existing species unable to adapt.

Final answer: D