In the late 1970s, Janzen and Mary Willson, noting that male flowers are often larger than female flowers, expanded the field of sexual selection into plants.
Ademais, a finais da década de 1970 Janzen e Mary Willson, decatándose de que as flores masculinas son xeralmente maiores que as femininas (en especies con flores unisexuais), ampliaron o campo da selección sexual ás plantas.