 Günter Theißen, a German professor of genetics, has classified homeotic mutants as hopeful monsters and documented many examples of animal and plant lineages that may have originated as hopeful monsters.American biologist Michael Freeling has proposed "balanced gene drive" as a saltational mechanism in the mutationist tradition, which could explain trends involving morphological complexity in plant and animal eukaryotic lineages.Examples of saltational evolution include cases of stabilized hybrids that can reproduce without crossing (such as allotetraploids) and cases of symbiogenesis.