If we have the population illustrated below, with a particular mean (T), and we select a group of individuals which have a mean of Ts, then the
Can you see how these concepts again describe the concept of heritability. What proportion of the change is heritable in a manner we can predict --- the proportion of the variation that is due to additive genetic variance. The selection response is the additive genetic component of all the variation in portion of the population selected and then intermated. Thus the ratio of the selection response to selection differential is indeed the narrow sense heritability.