The Hybrid Mystique
Harvard University * Arnold Arboretum
Hybrids, the results of successful breeding between two parents of different species, occur frequently in nature, yet are perhaps most familiar to us when they result from human intervention. We encounter in our intentionally cultivated hybrids the utility of the mule, the stateliness of the London plane tree, and the sensuous smells and tastes of myriad vegetables and fruits, including broccolini and the tangelo. These remarkable examples are of our own making, but hybridization between closely related species is perhaps the norm rather than the exception in nature. Though hybrid offspring are sometimes sterile and can be visually distinct from their parents (like mules), they are just as likely to be fertile and to pass unnoticed by us. These cryptic hybrids, diagnosable only through genetic testing, breed with each other or with individuals of their parent species (in a process called backcrossing), giving rise to new hybrid progeny. Over generations, such interbreeding consolidates novel hybrid traits, sometimes leading to the formation of new species. Because what counts as a species is, after all, merely conventional, it could be said that we humans, the descendants of interbreeding between Neanderthals and early Homo sapiens, are just as much hybrids as the most luscious of tangelos. Perhaps foremost among the natural world’s “hopeful monsters”—a term that geneticist Richard Goldschmidt coined in 1940 for evolutionary transformations that occur through rare but large-scale mutation—hybrids often captivate and delight human observers.
Grossman, J. J. 2019. The hybrid mystique. Arnoldia, 76(3): 2–13.