Natural selection — the process by which organisms with favorable traits are more likely to reproduce and pass on their genes (see Chapter 5 for more in-depth info) — acts on phenotype, not genotype. Think about it. No matter how great an organism's genes are, they can be reproduced in subsequent generations only if that organism reproduces — and that depends on outward characteristics (phenotypes).
In the earlier cheetah example, you expect that faster cheetahs will leave more descendents in future generations than the slow ones that couldn't catch dinner. Some phenotypic variation existed in the cheetah population, and as a result, some cheetahs did better than others. That's natural selection. But is it evolution? Depends.
Selection results in evolution only if there are genetic differences between the cheetahs of different speeds. The difference in speed has to be a heritable one — one that can be passed on genetically from parent to offspring. If the faster cheetahs have different genes than the slower cheetahs, then the next generation will have a higher proportion of those genes because the cheetahs that had them eat more antelope and made more baby cheetahs. But if the faster cheetahs aren't any different genetically than the slower ones, then there won't be any evolutionary change from one generation to the next.
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