What is a Hybrid Zone?

When two closely related animal or plant species breed together, the resulting offspring are called hybrids. You may have heard of a hybrid called a liger, a cross between a lion and a tiger. However, ligers do not occur in the wild—they have only been bred with human help in zoos. Hybridization can also occur in nature. For instance, the two largest animals on Earth, the blue whale and the fin whale, occasionally produce hybrids [1]. But how come the species that hybridize do not blend into a single species?

Hybrids typically have lower fitness than their parents—they are less healthy and produce fewer babies. That is because the genes of the two species that produced them have trouble working together. For example, hybrids between horses and donkeys cannot have babies themselves. As a consequence, the two parent species do not merge into a new species, instead they remain distinct. Still, if these species continue to mix, a narrow band will remain in between their distribution ranges, where hybrids keep being produced (Figure 1A). This region is called a hybrid zone [2].

Can Hybrid Zones Move?

Traditionally, researchers thought hybrid zones should pretty much stay in the region where the parent species first met and started hybridizing. But what if one of the two parent species is better adapted to the local environment than the other, at the location where they first meet? Or what if the environment changes in favor of one of the parent species, because the environment gets warmer or wetter, for example? Under such circumstances, you would expect one of the two parent species to outcompete the other one. The better-adapted species expands its range at the expense of the less-adapted one, which in turn is forced to surrender part of its range (Figure 1B).

As a result, the hybrid zone in between the ranges of the two species moves [3]. This movement continues until the hybrid zone reaches a point where both species are equally well-adapted to the environment, or until one species has completely replaced the other. This all seems to make sense, but is there any evidence for such hybrid zone movement? Recent studies say yes!

Direct Observations of Hybrid Zone Movement

Researchers have been studying certain hybrid zones for decades now. And for some of these hybrid zones, they have observed movement “live”. A particularly well-studied case is that of chickadees, small birds that occur in North America. Warming temperatures have caused the hybrid zone between black-capped chickadees and Carolina chickadees to shift northward by over 10 km in a decade [4].

Researchers mapped the distribution of the two chickadee species each year, based on sightings of birdwatchers reported in the citizen science database eBird. By comparing the distribution over the years, the researchers revealed the gradual movement of the chickadee hybrid zone, with the Carolina chickadee pushing out the black-capped chickadee (Figure 2). Examples such as this provide undeniable evidence that hybrid zones can and do move, at least over relatively short periods.

Long-Term Hybrid Zone Movement

Many hybrid zones are already thousands of years old. This means that they may have moved over huge distances (many hundreds of kilometers). However, nobody was around to see it. Still, we can figure out if these hybrid zones have moved by studying the genes of the hybridizing species. Some genes of the species whose range was pushed back are expected to be left behind in the species that is expanding its range. This tell-tale pattern of left-behind genes has been called a genomic footprint of hybrid zone movement [5]. A good example is provided by crested newts in Turkey (Figure 3). Here, genes of the retreating Balkan crested newt can be found in the advancing Anatolian crested newt. Such a genomic footprint allows us to determine that a hybrid zone has moved, even many centuries after it happened. This exchange of genes between species that allows us to see the genomic footprint is called introgression. Introgression also applies to humans, as many of us possess a bit of Neanderthal DNA resulting from past hybridization between Neanderthals and early modern humans. The direction of introgression can reveal past hybrid zone movement.

Why is Hybrid Zone Movement Interesting?

That species regularly need to change their ranges in response to one another is an important realization in biogeography (the study of life’s distribution across the Earth). Nowadays, humans are causing hybrid zone movement. By transporting species all over the world, we bring species together that did not previously meet in nature, and potentially create new hybrid zones. When the newly introduced species subsequently outcompetes the native one, the hybrid zone moves. This is again illustrated by crested newts: in western Europe the Italian crested newt has been regularly introduced inside the range of the northern crested newt—a species of conservation concern. Because the two crested newt species can hybridize, we now observe hybrid zones that are expanding outwards from the original introduction sites.

Furthermore, by modifying species’ habitats and contributing to climate change, we are shifting the balance of which member of a hybridizing species pair is most successful [6]. While these man-made changes happened only recently compared to the natural processes that shape hybrid zones, we can expect hybrid zones to form—and move—in response. You could even consider hybrid zones as an indicator of global change [7].

Conclusion

Hybrid zone movement seems to be much more common than previously assumed. Perhaps it should even be considered the rule rather than the exception? Hybrid zones have been moving in response to past global changes and therefore can be expected to move in response to future global changes—including those caused by us. In fact, studying hybrid zone movement helps us to anticipate how species might respond to ways that humans are changing the planet.

Glossary

Fitness: ↑ How successful an individual is in producing offspring compared to other members of its species.

Gene: ↑ Segments of DNA that contain information about traits, like eye color, that can be passed on from parents to their offspring.

Distribution Range: ↑ The geographical area where a species can be found.

Hybrid Zone: ↑ The area where two species meet, mate, and produce offspring.

Adaptation: ↑ A trait that makes an individual better suited to deal with a particular environment and that can be inherited by that individual’s offspring.

Outcompete: ↑ To be more successful than another species in obtaining resources (such as food or living space), so that the more successful species replaces the less successful one.

Citizen Science: ↑ A practice in which regular citizens participate in and help with scientific research by, for example, monitoring birds or other animals.

Introgression: ↑ The exchange of genes between two species following hybridization.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank Michael Fahrbach for allowing us to use his newt pictures in the figures. This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie grant agreement No. 655487.