Natural Selection and Evidence for Evolution

Four conditions

Natural selection follows whenever these hold: variation exists in a population, that variation is heritable, more offspring are produced than can survive (overproduction), and some variants leave more offspring (differential reproductive success).

Note what the mechanism does not do: it does not create variation, and it does not act with a goal. Mutation supplies raw variation blindly; selection merely sorts it.

Evidence

• Homologous structures — same underlying plan, different function (whale flipper and human arm). Evidence of common ancestry.
• Analogous structures — same function, different origin (bird wing and insect wing). Evidence of convergent evolution, not relatedness.
• Molecular — shared genetic code and similar DNA sequences; the more similar, the more recent the common ancestor.