Hybridization: “Genetic pollution” or a natural process?

The San Francisco Chronicle’s gossip columnist, Leah Garchik, recently published a story about California poppies in the Presidio.  Apparently, someone planted the “wrong” poppy, or it migrated there.  The poppy that is native to the Presidio is small and yellow.  This “alien” poppy is the large orange poppy that most of us consider the classic California poppy.  The historical record indicates that this classic poppy grew elsewhere in San Francisco, but since it didn’t grow in the Presidio it must be removed because the Presidio’s Vegetation Management Plan “contains the requirement to remove any plants that could jeopardize the integrity of the genetics of native plants in the Presidio.” 

For the benefit of our readers who aren’t gardeners, we should explain what Jake Sigg and the Presidio are worried about.  In a word, they are worried about hybridization, defined as “to breed or cause the production of a hybrid,” which is defined as “the offspring of two animals or plants of different breeds, varieties, or species.”  Hybridization is as likely to occur between two native plants as two non-native plants, but native plant advocates are concerned about the possibility of a native and a non-native plant producing a hybrid variety that is distinct from the native plant.  Hybridization is not inevitable, but it does occur naturally as well as through human manipulation.  The “From the Thicket” blog recently told the fascinating story of the development of a valuable garden cultivar variety of a favorite California native, ceanothus or California Lilac. 

Aside from the unpleasant association with eugenics, Mr. Sigg’s advice raises several practical questions.  How is the gardener supposed to know exactly which variety of a native plant “belongs” in San Francisco or even in a specific neighborhood within San Francisco, such as the Presidio?  And, in the unlikely event that gardeners might have such esoteric knowledge, where would they get the seeds of this specific variety?  Jake Sigg acknowledges this practical obstacle, but advises gardeners to get their seeds and plants only from the annual plant sale of the local chapter of the California Native Plant Society.  One wonders how many gardeners will follow this rather restrictive advice.

However, the more important question is the scientific question.  Is hybridization an inherently harmful process that always reduces species diversity?    We turn to Mark Davis for a less gloomy view of hybridization.  Like most scientific questions, there is evidence of both positive and negative effects of hybridization on species diversity.  Since you’ve heard the negative view from Mr. Sigg, we’ll let Mark Davis speak for the positive view.  Professor Davis reports in his book Invasion Biology* that “the fossil record generally shows that following the invasion of new species, the number of species resulting from adaptive radiations and evolutionary diversification exceeds the number of extinctions.”   And he concludes his discussion of hybridization and evolution by saying, “…a fair appraisal must also acknowledge that species introductions can enhance diversity as well, through hybridization, and the creation of new genotypes.”

The native plant movement has a narrow view of nature, which we do not share.  Their ideology is based on dire predictions of ecological disaster if we don’t follow their restrictive advice.  And when the managers of public lands choose to follow their advice, the consequences are usually the destruction of plants and animals that we value, in this case a field of California poppies.