Final answer:
Heritability is the proportion of variation in a trait, across populations, that is due to genetic variation. It determines how much of a trait's variation is genetically based, in contrast to being influenced by the environment, and influences how traits can evolve under natural selection.
Step-by-step explanation:
Heritability is the proportion of variation in a trait, across populations, that is due to genetic variation between the populations. It represents the fraction of phenotype variation that can be attributed to genetic differences among individuals in a population. This concept is important as it determines how much of the variation seen in a trait is due to genetic factors as opposed to environmental factors. High heritability means that genetic differences cause most of the variation in a trait, which can then be acted upon by evolutionary forces like natural selection.
For instance, if a trait such as height has a high heritability in a population, this indicates that the differences in height that we see amongst individuals are largely due to their different genes rather than to differences in their environment. It's important to note, however, that a high heritability does not mean that the environment has no effect on the development of a trait, or that the trait cannot change over time.