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There are no wild ligers. Hybrids were once considered rare in nature, and of little evolutionary importance. But now we know that they can play an important role in speciation, the creation of new, genetically distinct populations.
As it turns out, hybridization is quite common in nature. About 25 percent of plant species hybridize and about 10 percent of animals do the same.
“Hybridization as an event is rare,” said Jeremie Fant, a conservation scientist at the Chicago Botanic Garden who has worked on plant hybridization. “But throughout evolutionary history, it's been very common. Hybrids in the plant kingdom are everywhere. They're spread across most lineages. When hybridization does occur, it can have important evolutionary consequences.”
Often, hybrids between two species are evolutionary dead ends. They may be sterile, or they may simply be absorbed into populations of one of the parent species, leaving only a few spare genes from their foreign parent in the gene pool. But in a few rare but significant cases, hybridization events can significantly alter the trajectory of evolution.
When two related species overlap geographically, they can form what are known as “hybrid zones.” Some of the most obvious hybrid zones occur at the boundaries of divergent ecosystems. A plant species adapted to one soil type can exchange genes with a related plant adapted to another, and their offspring 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 little introgression, or back-breeding, 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 it, so they don’t spread farther.
Sometimes, however, hybridization events become more. They turn into swarms. The first time the term “hybrid swarm” appeared was in 1926 in a Nature article on the flora of New Zealand.
“In terms of biologically defining the difference between that zone and a swarm, I've struggled to find a nice, clear definition,” Fant said.
“A hybrid flock is the eventual erosion of two species into something else that is a combination of both,” suggested Scott A. Taylor, an associate professor at the University of Colorado who has researched hybridization in tits.
Defining a swarm is challenging, because the definition of a species itself is controversial within the scientific community. A species is broadly defined as a group of organisms that can reproduce, but many organisms that are considered separate species are capable of reproduction. Take the lion and the tiger, for example.
The definition of a hybrid swarm is thus fluid: it is applied to situations in which separate populations of two or more species merge, situations in which all members of two or more species merge, and even to situations in which subspecies or regional variations between species merge.
It is best understood as a working definition of the ways in which two or more genetically distinct populations encounter each other, interbreed, and become an entirely new group that includes genes from all of the parental species. These swarms are often variable in their genotypic and phenotypic compositions, meaning that both their genetics and physical characteristics are intermediate between the parental species.
Sometimes these crosses are one-way. That is, the first hybrids can produce viable offspring by back-breeding with one of the parent species, but not the other. The resulting mixtures of genes can introduce new combinations that are better adapted to the environment shared by the parent species and the hybrids.
Unlike hybrids that form in hybrid zones, swarms are highly unstable. They can die out, or they can dominate and eventually wipe out the species from which they originate. The formation of swarms, even unsuccessful ones, is rare.
“In many cases in nature, hybrid swarms don't form,” Taylor said. “Hybrids do form, but for whatever reason, they don't do as well as either parent species.”
But when they do, they can become a powerful ecological force.
