"Heteroecy appears to be a consequence of the seasonal polymorphism of aphids, which causes some morphs (the fundatrices) to be more evolutionarily constrained than others (the summer females) in their abilities to acquire hosts. Some aphid lineages have escaped this constraint by replacing the ancestral fundatrix morph and remaining all year on former secondary hosts, thus becoming secondarily autoecious. Most of the large and species-rich groups of Aphidinae on herbaceous angiosperms probably are derived from ancestors showing such life-cycle reduction."