Hybridization, a post script

We recently published an article in defense of hybridization, inter-breeding of two different species.  Conservation Sense and Nonsense defends hybridization because it is under fire from the native plant movement.  Many projects that needlessly destroy non-native plants (or one locally perceived as such) do so to prevent them from hybridizing with a native plant, which has the potential to cause a localized loss of a variant of a  native plant species.

This classic California poppy is eradicated in the Presidio in San Francisco because of fear it could hybridize with a sub-species of poppy that is considered “native” to the Presidio.

We are revisiting the topic because The Economist magazine recently published a comprehensive article about recent discoveries of the prevalence of hybrids among both plants and animals.  Until the advent of DNA analysis in the 1970s, the extent to which plant and animal species were the result of inter-breeding was largely unknown.  Also, conventional wisdom was that such inter-breeding was usually an evolutionary dead-end because offspring were often sterile, as exemplified by mules, the offspring of horses and donkeys.  In general, the consequences of hybridization were assumed to be negative.

Recent advances in DNA analysis have largely disproved these assumptions.  Hybridization is not only common, it can result in the creation of new species more rapidly than other forces of evolution, such as mutation and natural selection:  “Hybridisation also offers shortcuts on the long march to speciation that do not depend on natural selection at all.” (1)

Both the positive and negative effects of hybridization are real. In plants, the effects of hybridization are often beneficial because of plants’ unusually flexible genetics.  Plants, for instance, are frequently polyploid—meaning that each nucleus contains genomic copies in greater multiples than those of animals.  Polyploidy provides spare copies of genes for natural selection to work on, providing additional possibilities for selection.

Polyploidy confers another advantage. It creates a barrier to breeding with either parent species. That gives a new, emerging species a chance to establish itself without being reabsorbed into one of the parental populations. Recent evidence suggests that hybridization between two plant species in the distant past, followed by a simple doubling of the number of chromosomes in their offspring, may be responsible for much of the diversity in flowering plants that is seen today.

Plants seem to benefit from hybridization more often than animals. “For many animals, however—and for mammals in particular—extra chromosomes serve not to enhance things, but to disrupt them. Why, is not completely clear. Cell division in animals seems more easily confounded by superfluous chromosomes than it is in plants, so this may be a factor. Plants also have simpler cells, which are more able to accommodate extra chromosomes. Whatever the details, animal hybrids appear to feel the effects of genetic incompatibility far more acutely than do plants.” (1)

The Economist provides many important examples of hybridization among animal species, most notably the history of hybridization of our species, Homo sapiens.  We are now the sole surviving species of genus Homo.  Our genome contains the relicts of the genes of other members of our genus that are now extinct, which indicates hybridization with other hominoid species.  The modern human genome contains 1-4% of Neanderthal genes. 

The Economist article concludes, “This is a more complex conception of evolutionary history, but also a richer one. Few things in life are simple—why should life itself be?”   Keep your eyes and your mind open to new scientific knowledge that improves our understanding of life.

The bottom line

Biodiversity is the mantra of the native plant movement.  Native plant advocates claim that the primary purpose of saving native plants is preserving biodiversity.  But is it?  When non-native plants are eradicated, aren’t we depriving native plants of the opportunity to breed with a hardy new comer?  Are we preventing the creation of a new species by eliminating potential mates?  Are we dooming the native plant that is not adapted to survive the changing climate by depriving it of the opportunity to improve its survivability?


  1. The Economist, “Match and mix, hybrids and evolution,” October 3-9, 2020, page 67-70.  Available here:  Economist – hybridization and evolution

 

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