BIO 102 - Dr. Tom Wilson notices that there is an equal

In early spring, Dr. Tom Wilson notices that there is an equal distribution of long and short stemmed buttercups in the vacant lot across the street from his house as well as in his own backyard. In late summer he notices that there are mostly long stemmed buttercups in the lot, but mostly short stemmed buttercups in his yard. These observations are examples of what biological principle? Develop a hypothesis to explain Dr. Wilson's observations.