Final answer:
Natural selection's ability to drive evolution is heavily dependent on the heritability of traits, where genetic differences in a population allow selection to increase the prevalence of beneficial traits over generations.
Step-by-step explanation:
The ability of natural selection to cause evolution is directly related to the heritability of traits. Heritability refers to the fraction of phenotype variation that can be attributed to genetic differences among individuals in a population. Natural selection acts on heritable traits -- those encoded in an organism's genetic code -- because these are the traits that can be passed down to offspring. For instance, if a trait such as a longer neck in giraffes, which allows them to reach leaves high in trees, has a genetic basis, it will be more likely passed on to subsequent generations if it proves advantageous.
Importantly, heritable traits provide the material upon which natural selection acts. Without genetic variance in these traits, natural selection would not have the means to sort and spread traits through populations over generations. Therefore, the greater the heritability of a trait, the more influence natural selection will have over that trait's prevalence in a population over time.