Genotype × <math xmlns="http://www.w3.org/1998/Math/MathML" altimg="urn:x-wiley:00029122:media:ajb216091:ajb216091-math-0001" wiley:location="equation/ajb216091-math-0001.png"><mrow><mrow><mo>\unicode{x000D7}</mo></mrow></mrow></math> environment interaction obscures genetic sources of variation in seed size in Dithyrea californica but provides the opportunity for selection on phenotypic plasticity