Explain that mutations are changes in DNA sequence that can be neutral, harmful, or beneficial, and that mutations are the ultimate source of new genetic variation.
A mutation is a change in an organism's DNA sequence. Mutations can happen from random copying errors when a cell divides, or from exposure to things like UV radiation or certain chemicals. Once a mutation happens, it can be passed to offspring if it occurs in the cells that produce eggs or sperm. Mutations are the ultimate source of brand-new alleles -- without them, a population would never gain any genetic variation beyond what it already has.
Most mutations are neutral: the changed DNA does not noticeably change how a protein works, so there is no visible effect on the organism. Some are harmful, disrupting a protein's normal shape or function -- the sickle cell allele, for example, changes the shape of red blood cells, causing health problems, though carriers of a single copy also happen to have some added resistance to malaria in regions where that disease is common. A small number of mutations are beneficial: in bacteria, a mutation that changes a surface protein can sometimes block a particular antibiotic from working, giving that bacterium, and its offspring, a survival advantage whenever that antibiotic is present.
Mastery looks like: They can classify example mutations as neutral, harmful, or beneficial and explain why mutations are the ultimate source of new genetic variation.
Common stumbles: Believing mutations happen in response to what an organism needs; assuming "mutation" always means something dramatic or harmful.