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.