In 1980 Raven continued discussing problems associated with defining species in plants. He discussed the widespread ability of plant species to hybridize, especially in perennial plants, and the historical observations of such back to 1717. He used as examples of perennial plants in the genera Epilobium, Scaevola, Bidens, and Ceanothus as examples of plants that appeared to use hybridization as a means to adapting to new environments. He stated "If the hybrids are particularly favored in specific ecological situations, asexual reproduction, polyploidy, or simply autogamy may favor the perpetuation of specific genotypes through a narrowing of the spectrum of genetic recombination characteristic of the population. No general conclusions about the most appropriate way to treat these populations taxonomically appear to be possible." In annual plants, using examples from Clarkia, he asserted that several species of "Clarkia" often occur sympatrically, yet hybrids are very rare in the wild," and that much of the sterility is due to chromosomal repatterning between species."