Human beings have a need to categorize the living world in order to define and delineate it within their own environment. This classification activity makes up the field of taxonomy. Species can be defined according to various concepts. According to the biological concept of a species, which prevails among evolutionary biologists, species are genetically isolated from each other and are not capable of crossbreeding. There are indeed various natural reproductive barriers that prevent the exchange of genes between species.
When hybridization occurs, however, there is crossbreeding between two distinct species, producing hybrid individuals with a mix of features that are characteristic of either parent entity. Does hybridization invalidate the concept of a biological species? Would it otherwise have to be an extremely rare occurrence?
This was the subject of an evening talk I gave before a live audience in the series NatureTalks. The evening was hosted by Fabienne L’Abbé.
Staunch defenders of the biological-species concept, particularly zoologists, consider hybridization as a defect or glitch in the mechanisms that isolate species for the purposes of reproduction.
In fact, hybridization is a natural process. Genetic analyses show that polar bears (Ursus maritimus) have been crossbreeding with grizzlies (Ursus arctos) for 120 000 years. Similar cases can be found in the animal world.
In the animal kingdom, the average rate of hybridization is around 10%. In fact, some taxonomic groups have much higher averages. Seventy-six percent of duck species practice crossbreeding. In the Phasianidae family, which includes turkeys, partridges, pheasants and chickens, the hybridization rate is just as high, if not higher. The numbers for butterflies range between 6% and 35%, depending on the genus.
On an individual basis, however, hybridization appears to be a much rarer phenomenon. When hunting of fin whales was permitted, 0.1% to 0.2% of individuals captured were hybrids produced by crossbreeding with blue whales. The hybridization rate on an individual basis typically ranges from 0.01% to 0.1%.
Even though hybridization is natural, human beings are often responsible for breaking down reproductive barriers, thus creating conditions conducive to hybridization. This phenomenon has completely changed the natural distribution range for 65% of the 822 species of North American fish, leading to a significant increase in crossbreeding between native and non-native species.
Of course, natural hybridization can be beneficial both for the species and biodiversity. It encourages the creation of new species; it also makes hybrid individuals more robust. But it can also have deleterious effects.
Hybrid individuals are often unviable and sterile and have a poor ability to survive in natural conditions as a result of hybridization. Gametes and energy are squandered in this delicate effort to reproduce, especially when it comes to endangered species, possibly leading to extinction in some cases.
Hybridization has greatly contributed to the decline of 38% of North American fish that have recently become extinct. The large-scale decline of Golden-winged Warblers may be partly due to its crossbreeding with the Blue-winged Warbler, which is growing in numbers. Do we need to feel concerned about this? This probably requires a case-by-case approach. In any case, we need to completely re-examine our relationship to the natural world.
What do you think?
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Check out the comments from some of the people who participated in the informal discussion on this topic, at our NatureTalks evening event.
Translated from French.