"when physical characteristics, such as early onset of menstruation or increased height and weight (due to better nutrition), change over the course of several generations, this is an example of a

This is a secular trend, which is a pattern of change over several generations. It is a variation of the expression of the organism based on the environment. An example of a secular trend is that puberty is now happening earlier on average, than it has in the past, for both boys and girls.