The Origin, Expansion, and Demise of Plant Species, p. 126 (2000)
Donald A. Levin

Selection for reproductive isolation between two populations of maize, Zea mays L.

Selection against hybridization between white flint maize and yellow sweet maize was conducted in field plots (Paterniani, 1969). After five generations of selection, the percentage of hybrids produced by white flint declined from 36% in the original population to 5%, and the percentage produced by yellow sweet declined from 47% to 3%. The strains diverged in flowering time, although they had similar phenologies to begin with. The selected white flint began to flower five days earlier than the original population and the selected yellow sweet two days later.

Paterniani also found that a crossing barrier had arisen in the white flint selection line. In the original populations, 1:1 pollen mixtures of the two strains, produced close to 50% hybrids in both strains. This is to be expected with complete cross-compatibility. However, when white flint of the fourth selection generation received 1:1 pollen mixtures of various types, less than 30% of the progeny were hybrids (table 6.2). Yellow sweet pollen was at a competitive disadvantage to white flint pollen in white flint stigmas and styles. Oddly enough the selected yellow sweet line did not become more discriminating when used as the female parent.