Every middle-school student learns the dogma: a species is defined as a group of organisms that interbreed and produce fertile young. When individual plants and animals can’t, we call them different species. Sometimes it’s a little confusing to imagine exactly how that might work between Great Danes and Chihuahuas, which are both Canis familiaris, but for the most part it’s a comforting way to make sense of the biological diversity around us. Except that it’s wrong.
Older genetic sequencing techniques meant that until recently, scientists could examine only a small percentage of an organism’s DNA, providing us a reasonable proxy for the heritage of the organism as a whole—whether the creature came from an interbreeding event, for instance. We used these DNA snippets to infer how two creatures were related. But now, with the ability to sequence entire genomes quickly and accurately, we can examine most of the millions of pieces of DNA contained in every living thing. And we are discovering that “species” aren’t so clear-cut.
Hybrids, or organisms that contain DNA from more than one species, were supposed to be the unlucky reproductive dead ends of parents from different species that lacked some Darwinian common sense; instead, new genomic data give us rampant evidence that species, even ones that aren’t very closely related, have been swapping genes in an act of evolutionary defiance that isn’t so vanishingly rare.
And as a parade of droughts, floods and heat waves replaces the relative ecological stability on which many species depend, these hybrid species may be better equipped to handle climate change than the parent species from which they came.
Typically, each species has to evolve its own beneficial genes, a laborious process of small mutations that can be hit or miss when it comes to conferring a benefit. This process takes many generations. But when different species hybridize, beneficial genes that have evolved in one species can, through mating with the hybrid, migrate to the other species in the blink of an evolutionary eye. This shuffle of DNA, when beneficial, could confer serious benefits, like disease resistance or adaptations to new environments.
Hybridization gives species a way to quickly swap their best genes as if they were like trading cards. Rather than being detrimental, liaisons between two species serve as a mechanism for rapid genetic upgrades and might set up advantageous events for many kinds of animals and plants. So, while many or even most individual interspecific hybrids might not always be reproductively successful, genomics clearly indicates hybrids are sometimes, or even frequently, able to find a partner and reproduce, eventually blending their novel combination of genes with one of the parent lineages.
What does this have to do with surviving climate change?
Hybridization has profound implications for weathering climate change—Homo sapiens is a great example of this. Scientists have long thought that when humans and Neandertals were on the planet at the same time, interbreeding snagged for our species important survival genes that Neandertals had honed over millennia of living in Eurasia. These could include adaptations for an improved immune response and better UV absorption—helpful as we migrated out of sunny Africa, encountered new pathogens, and had to adjust to dark and cold Eurasian climates. Thanks to genomic technology, we know that between 2 per cent and 4 per cent of most Eurasian people’s DNA is directly traced to Neandertals, and the genomes of most of us are equipped with a package of non–Homo sapiens genes because they confer clear benefits to our species.
Rather than just tolerating such hybrid flexibility, evolution conceivably could be driving it; those species that are able to reap the benefits of hybridization and skip ahead in the race to adapt have a better chance at long-term survival, and they may cope better with the new environments and the intensifying instability they now face.
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The recent news of a climate bill in Washington which will significantly reduce US carbon emissions over the coming decades is a small step in the right direction, though it hasn’t come in time for many of the species threatened by climate change. Indeed, last fall the US Fish and Wildlife Service declared another 23 species of plant and animal extinct. These lists will grow longer as the weather becomes more erratic. Climate change has gone from an abstract prediction to a hard-hitting, intensifying, global catastrophe whose effects on biodiversity are dire.
Yet, there is a small sliver of hope for the resilience of nature.
Those species that hybridize may be those most likely to weather the storm we’ve created, if we leave enough wildlands intact for species to persist and come into contact with each other in the first place. Rampant habitat destruction may have already sealed the fate of far too many species, and hybridization isn’t magic; the beneficial genes have to exist in the first place and be successfully transferred between the species. It is a pathway to survival, not a guarantee. Unfortunately, most species won’t have the time or opportunity to benefit from hybridization given the pace of climate change and development. But it isn’t a lost cause; anything we can do to slow climate change and preserve natural areas will give biodiversity a chance to adapt. Our actions will continue to cause extinctions across the tree of life, but hopefully an old, previously unrecognized, habit of exchanging genes will provide an unexpected way for some species to dodge extinction.
(This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.)