Hybrids between two species can produce “swarms” that flourish

There are no wild ligers. Indeed, hybrids were once thought to be rare in nature—and of little consequence in an evolutionary sense. But now we know they can play an important role in speciation—the creation of new, genetically distinct populations.

As it turns out, hybridization in nature is quite common. Some 25 percent of plant species hybridize and some 10 percent of animals do the same.

“Hybridization as an event is rare,” said Jeremie Fant, a conservation scientist with the Chicago Botanic Garden who has worked on plant hybridization. “But in evolutionary history, it's been very common. Hybrids in the plant kingdom are everywhere. They are scattered through most lineages. When hybridization does occur, it can have important evolutionary impacts.”

Often, crosses between two species are evolutionary dead ends. They may be infertile, or they may simply be absorbed into populations of one of the parent species, leaving only a few spare genes from their oddball parent drifting in the gene pool. But in a number of rare but significant cases, hybridization events can significantly alter the trajectory of evolution.

When two related species overlap geographically, they may form what are called “hybrid zones.” Some of the most obvious hybrid zones occur at the boundaries of divergent ecosystems. A plant species adapted to one soil type may exchange genes with a related plant adapted to another, and their offspring thus develop a population that thrives in an intermediate area with characteristics of both soil types.

These hybrid zones are often quite stable over time, with insignificant introgression, or breeding back, to the parent populations. That’s because the genes that serve the organisms in the hybrid zone may not be particularly useful to those outside of it, so they do not spread more widely.

Sometimes, however, hybridization events become something more. They turn into swarms. The first instance of the term “hybrid swarm” occurred in 1926 in a Nature article about New Zealand flora.

“As far as biologically defining the difference between that zone and a swarm, I've been struggling to find a nice, clean definition,” Fant said.

“A hybrid swarm is the ultimate erosion of two species into some other thing that's a combination of both,” suggested Scott A. Taylor, an associate professor at the University of Colorado who has worked on hybridization in chickadees.

Defining a swarm is a challenge because the definition of a species is itself contested within the scientific community. A species is crudely defined as a group of organisms that can interbreed, but plenty of organisms that are considered separate species are capable of interbreeding—take the lion and the tiger, for example.

So, the definition of a hybrid swarm is malleable—it’s applied to situations in which distinct populations of two or more species merge, situations in which all members of two or more species merge, and even in situations when subspecies or regional variations among species merge.

It might be best conceived as a working definition of the ways in which two or more genetically distinct populations encounter each other, breed, and become an entirely new group comprising genes from all of the parent species. These swarms are often variable in their genotypic and phenotypic compositions—meaning that both their genetics and physical characteristics are intermediate between the parent species.

Sometimes, these crosses go in only one direction. That is, the initial hybrids may produce viable offspring by mating back to one of the parent species but not the other. The resulting mixtures of genes may introduce new combinations that are better adapted to the environment shared by the parent species and the hybrids.

Unlike the hybrids that form in hybrid zones, swarms are highly unstable. They may fizzle out, or they may dominate and eventually erase the species from which they derive. The formation of swarms, even unsuccessful ones, is a rarity.

“In a lot of cases in nature, hybrid swarms aren't formed,” Taylor said. “Hybrids are formed, but for whatever reason, they don't do as well as either parent species.”

But when they do, they can constitute a powerful ecological force.

Mechanisms of swarm formation

Hybrid swarms may form due to any number of factors, from climatic changes that influence the ranges of the parent species to human introductions of invasive species. Their effects may be minute and local, or, if we view our global spread as a human/Neanderthal/Denisovan swarm, positively earthshaking.

In humans, the swarm likely developed due to the itinerant and adaptable nature of our ancestors. We simply traveled more than other species and thus came into contact with other human species more frequently.

The mechanisms behind the formation of other naturally occurring swarms are less well understood. While stable hybrid zones may occur as the result of hybrids between two species adapting to ecological conditions that were inhospitable to either parent species, the reasons that sometimes lead to hybrids subsuming their parent species entirely are mysterious.

In one fascinating recent case, it was discovered that the brooding brittle star (Amphipholis squamata) was likely the result of multiple hybridization events that resulted in a potentially asexual form of reproduction. The species is nearly circumglobal in distribution and is absent only from deep sea abysses and polar regions.

Hybridization may lead to the presence of polyploid genomes, in which there are more than two copies of each chromosome. Polyploidy appears to be linked to reversion to asexual means of reproduction, even in organisms whose parent species originally reproduced sexually. (This does not always occur, of course—mammals and birds, for example, cannot reproduce asexually.) The brittle star’s clonal reproduction strategy is effective because it allows the swarm to colonize any new area where it arrives, drifting on clumps of debris. Its diverse genome may allow it to be more resilient to differing ecological demands—including parasites that are unique to certain environments.

Hybrid swarms created by human interventions are better understood, mainly because we can more easily ascertain how species come into contact. This may be due to species purposely or accidentally introduced, or it may be due to our disruptions of ecosystem barriers that kept species separate.

In one remarkable instance, the construction of a dam trapped two herring species, alewives (Alosa pseudoharengus) and blueback herring (A. aestivalis), in the Kerr Reservoir on the border between North Carolina and Virginia. These fish, which typically spawn in freshwater and live in saltwater as adults, had different breeding habits under natural conditions. However, when prevented from engaging in their natural reproductive cycles, they interbred and eventually became a hybrid swarm. The entire population comprised individuals of hybrid ancestry, with multiple backcrosses among the hybrids.

A similar situation occurred in fish breeding ponds in Croatia. Four different species of water frog (genus Pelophylax) ended up in the same ponds, some native and some introduced. Five different hybrid forms were discovered, potentially interbreeding with each other as well, suggesting the formation of a hybrid swarm.

Accidental human interventions in natural ecosystems have created similar results. Lakes in Switzerland have become eutrophic—poor in oxygen due to excessive algal growth facilitated by agricultural runoff. These ecosystem changes have disrupted the natural patterns of feeding and breeding that once kept multiple species of whitefish separate. Lack of oxygen has forced them into more frequent encounters because only certain zones of the lakes are habitable. They have begun interbreeding as a result, thus creating hybrid swarms and collapsing the boundaries that once differentiated them as species.

The swarms we know

When Homo sapiens trooped out of Africa some 50,000–60,000 years ago, we could not have known that our distant relatives awaited on other continents—and were viable mates. But around 10,000 years later, we encountered Neanderthals, who had been inhabiting Europe and Asia for more than 400,000 years.

Another 10,000 years later, we met the Denisovans, another little-understood lineage that had colonized Asia. We shared a common ancestor with both groups and retained sufficient compatibility to mate and produce viable offspring. Some scientists argue that this ancestor was likely Homo heidelbergensis, though other candidate species have been considered as well. The smaller populations of Neanderthals and Denisovans, as well as those of other yet unidentified hominins, were swallowed by the swarm and contributed genetic variants that are present in many populations today.

Both groups appear to have offered non-African humans a number of genetic advantages, including tolerance to colder temperatures and resistance to certain diseases, enabling our species to achieve its current global dominance, stretching to the hottest and coldest portions of Earth. Neanderthals may have even enhanced fertility, making women less likely to miscarry—an advantage carried by women with Neanderthal DNA to this day.

Conversely, they passed along some deleterious mutations, including increased susceptibility to certain viruses—COVID among them. People with Neanderthal DNA also appear to be more inclined toward addiction.

Our spectacular swarm has since begotten additional swarms. Among the most headline-grabbing are killer bees, so named for their heightened aggression. Scientists typically refer to them as Africanized bees because the swarm was initiated by Apis mellifera scutellata, an African subspecies of the European honeybee imported to Brazil from South Africa and Tanzania in 1956 to introduce their heat tolerance genes. Typical European honeybee subspecies had floundered in the steamy Brazilian climate, producing negligible amounts of honey.

The African bees proved far more adaptable than anticipated—they soon escaped cultivation and moved northward at a rapid pace. Along the way, they hybridized with other populations of feral honeybees. By 1990, the swarm had reached the southern border of the United States.

While in some ways the hybrid worker bees are not as fit as workers from European subspecies, they seem to be more resistant to the parasitic mite Varroa destructor—thought to be a contributor to colony collapse disorder affecting honey bees throughout North America due to its capacity to transmit disease. This advantage may have allowed the swarm to move even further north.

So, single, random factors can influence whether a swarm succeeds or fails. Had the mite not been introduced from Asia, European honey bee genes may have prevailed in the southern United States—and the African strain’s genes would not have been particularly advantageous. But, since it was, the African genes carried by the swarm gave the hybrids an edge and allowed them to advance further.

A new set of swarms

Among vertebrate animals, fish are some of the most prolific hybridizers due to their reproductive habits. Many are broadcast spawners, meaning that females lay eggs and males spray their sperm over them. So they don’t actually “have sex” in the way humans do. They disperse their gametes into the environment in close proximity in the hopes that they will pair.

“Compared to many mammals, for example, where internal sex occurs, it's harder for hybrids to occur,” Fant says. “But in plants and in fish, it's more common. You release eggs and sperm into the environment and then hope that it finds a match.”

If closely related species spawn in the same area—as is the case with the aforementioned herrings and whitefish—they may cross-fertilize. Some of the most recently identified swarms are fish due to the prevalence of these types of events.

The introduction of the red shiner (Cyprinella lutrensis) to American river systems where it is not native has led to the formation of a hybrid swarm due to crossing with the related blacktail shiner (Cyprinella venusta), for example. Intriguingly, many of the hybrids most closely resemble the red shiner—a phenomenon called cryptic introgression. Had genetic analysis not been done to determine the parentage of these fish, appearances would have suggested that the red shiner had just pushed its relatives out. Instead, genetically, those relatives were swallowed by the swarm.

While these swarms of tiny fish range over miles of river, some swarms are compact. One study found that miniature swarms comprising various subspecies of the common wall lizard (Podarcis muralis) had formed in German cities—these swarms seemed to extend for ranges of only hundreds of meters. Some of the subspecies were native and others had been accidentally introduced from France and Italy. The swarms themselves appeared to have interbred with each other, further exchanging genetic material.

Here again, we face the challenge of defining hybrid swarms. If these are simply variations of the same species, are they really hybrids in a meaningful sense?

These localized phenomena among very closely related organisms pale in comparison to some truly massive swarms that seem to have developed right under our noses—as a result of our own activities. As it turns out, much of the population of wild rice (Oryzias rufipogon) may actually be a hybrid swarm, carrying a mix of domestic genes that escaped agricultural strains. The domestic species is believed to have been developed in Asia some 9,000 years ago, possibly on multiple occasions in different regions.

Humans have since carried this species, O. sativa, across the globe. Nearly all of what we now consider to be wild rice appears to carry genes from domestic rice. While rice is largely self-pollinating, it can also be pollinated by wind and insects. These mechanisms may have facilitated the movement of domesticated genes into the wild, ultimately resulting in the formation of swarms.

Compellingly, the major varieties of domestic rice, indica and japonica, share genetic sequences with wild rice found in the regions where they are cultivated. These sequences code for non-shattering grains and upright growth habits. While they are not always expressed in wild populations—grains that do shatter protect the seeds from complete destruction by insects feeding on them and are thus advantageous to wild strains—the genes remain in the populations. What was once thought to be the wild-origin species of cultivated rice may thus be the result of rice that has gone wild, breeding its parent species out of existence—or at least fundamentally altering it.

Swarms threaten endangered species

Hybridization is a powerful evolutionary force. While it has conferred numerous benefits to organisms, including ourselves, it also constitutes a significant threat to threatened and endangered species.

“Is hybridization good or bad? There's no answer to that,” Taylor said. “It's context-dependent. Especially in a changing world, hybridization might introduce genetic variation that's really adaptive. And we can't predict what's going to be good or bad under what scenario, even [in cases of] human-caused hybridization, which, I would argue, we should try to reduce as much as possible.”

There’s a long list of cases where hybridization is making conservation challenging. The recovery of the threatened Colorado greenback cutthroat trout (Oncorhynchus clarki stomias) and the related Colorado River cutthroat trout (Oncorhynchus clarkii pleuriticus) have been impeded by the incursion of introduced rainbow trout (Oncorhynchus mykiss) in the Arkansas River basin, for example. The entry of rainbow trout into the ecosystem has resulted in hybrid swarms that threaten the genetic integrity of the related species.

In Scotland, red deer (Cervus elaphus scoticus), while not threatened, are an emblematic and charismatic species. Introduced sika deer (C. nippon) from Japan have heavily interbred with the native species, with some populations composed of 40 percent hybrid individuals. The hybrids seem to be intermediate in physical appearance, tending toward the spotted coats and smaller stature of the sika deer.

Similarly, the endangered koloa maoli, or Hawaiian duck (Anas wyvilliana) has been nearly erased on several Hawaiian islands by the introduction of mallard ducks (Anas platyrhynchos) in the 1800s. While the population on Kauaʻi remains pure, those on other islands carry substantial portions of mallard DNA and are often intermediate in appearance. Every bird sampled on Oahu, Maui, and Hawaii had hybrid ancestry, indicating that mallard introgression had resulted in a hybrid swarm. In a similar case, the Mariana mallard, an island subspecies of mallard, which had formed a hybrid swarm by interbreeding with mainland mallards, was later completely erased by increased introgression of mallard DNA.

“When you become rarer, you become less picky,” Fant said. “There is a phenomenon where as a species becomes rarer and rarer, it becomes more and more likely to hybridize with a more common species.”

The ecological consequences of these events are poorly understood. While hybridization may confer adaptive benefits, it may also introduce deleterious mutations. Taylor points out that hybrids formed between Carolina chickadees (Poecile carolinensis) and black-capped chickadees (Poecile atricapillus) in Boulder, Colorado seem to suffer consequences. Not only do they have higher metabolic rates, but they may also be cognitively impaired. Chickadees cache food in order to survive the winter. Their inability to remember where their food is stored might have serious implications for their survival.

The consequences are clearer in the case of Spartina species, known as cordgrasses—though in this case, the species benefits but the environment itself suffers. These grasses grow in coastal salt marshes and readily hybridize with related species. In San Francisco Bay, hybrids between native S. foliosa and the introduced S. alterniflora have formed swarms that have aggressively intruded on valuable mudflat habitat, which is crucial for shorebird foraging. New programs are attempting to remove these hybrids and restore these ecosystems to prevent impacts on other organisms.

As human disruption of the natural world continues, hybrid swarms may continue to take advantage of our sloppy stewardship. “They will decide the best combination of genes and sometimes if habitats change sufficiently, [organisms] are no longer well adapted, and so the hybrid becomes better adapted,” Fant warned.

The human swarm would thus be well-advised to brace itself for all manner of new swarms if we continue our disregard for the natural world. We have already wrought killer bees. What other monsters might we unintentionally create?

Richard Pallardy is a science writer based in Chicago. He has worked with publications such as National Geographic, Science Magazine, New Scientist, and The Biologist.