Question

You may know that, on average, individuals were shorter 400 years ago compared to today. the fact that modern-day humans are now taller is likely a reflection of:

Answer 1

This most likely a reflection of both environmental and genetic variation.

Environmental variation indicates that they live in an environment where it is easier for them to obtain proper nutrition that helps to sustain their growth.
The genetic Variation indicates that the taller individuals has the higher probability of surviving and passing their genes.

Answer 2

On average, individuals were shorter 400 years ago compared to today. the fact that modern-day humans are now taller is likely a reflection of both environmental and genetic variation. With the passage of time, there are environmental changes in the world. Also with time the species have undergone a genetic change and as a result, the humans are getting more taller as compared to previous generations.