You are a plant ecologist studying a plant in the genus \textit{Veronica. You notice that, at open rocky sites, \textit{Veronica} grows as a creeper spreading low to the ground, whereas in grasslands, the stem stands upright. You collect seeds from multiple populations in each habitat type and grow them under uniform conditions in a greenhouse. You find that all the plants grown in the greenhouse have stems that stand upright. Which one or more of the following explanations best support(s) your observations?}