The Science of Hybrids
The Importance of Hybrids
Ligers are just one example of natural hybridization. Ligers historically existed in the wild, and in a small way, they still do (more on that in a minute). Natural hybridization is one of the tools used in the evolution of species. It helps alter life to keep up with a constantly changing world. With the discovery and invention of increasingly more sophisticated gene analysis, we are finding many more hybrids in the wild than we ever expected.
When two related species (those sharing a recent common ancestor) reproduce, the first-generation hybrid is called and F1. Sometimes the F1 is less fit for the environment than either parent species. If this is the case, the offspring will likely be less successful and less likely to breed and pass on its genes. Sometimes the effects of hybridization on the F1 is neutral, neither reducing or increasing fitness. Other times, hybridization results in increased fitness in the offspring. The inheritance of increased positive traits is called "heterosis" or "hybrid vigor" and makes the offspring more successful and more likely to pass on its genes.
There are many unsubstantiated claims about liger health, including that "ligers suffer from chromosomal disorders because their parents are different species". Results of a recent chromosomal analysis of liger DNA indicate that the tested liger has the correct number of diploid chromosomes (38, or 19 from each parent). This is what you would expect to find in both healthy lions and tigers. Although these results were expected, they highlight that hybridization of lions and tigers does not result in chromosome abnormalities in liger offspring. As reported by the laboratory, "The liger cell line has normal male cat karyotype and is chromosomally stable".
Natural hybridization is one of the ways by which new species can emerge, a term known as "speciation". Speciation through hybridization can happen quickly. In just a few generations, hybrids can become a genetically isolated group, separate from their parent species. This is a way for evolution to make a major leap in a short amount of time. This type of speciation is fairly common with plants. Although hybrid speciation occurs less frequently among mammals, it is not unheard of. The "red wolf", as species designated as endangered in 1967 by the U.S. Fish and Wildlife Service, is a hybrid between gray wolves and coyotes. The red wolf exists solely because of natural hybridization.
It is not uncommon in hybrids for one of the sexes to be less common, less fertile or even sterile. This is referred to as the "heterozygous" sex and may act as a reproductive barrier. In mammals, this is often the males. Nature always seems to find a way though. While a heterozygous sex may exist among animal hybrids, the opposite sex is fully reproductively sound. If an F1 female mates with a male of the parent species, and that offspring mates with the other parent species, the third generation retains roughly the same amount of DNA from each parent species, but often males gain the ability to successfully reproduce. In just a few generations, hybrid animals can become every bit as successful (if not more) than their parent species.
While hybrid speciation does occur in mammals, it is fairly uncommon. The real power of hybridization happens when hybrids mate with their parent species, allowing the two species to inherit genes (and unique adaptations) from one another. If these genes are beneficial, they may become a permanent part of that species' genome. The passing of genes from one species to another is called "gene flow" and hybridization which passes genes between two species through hybrid offspring is called introgression.
While ligers have existed in the wild, they have not gone on to create a new species. Their most significant contribution seems to be as the vehicle which allowed lions and tigers to inherit adaptations from each other. Today, all lions carry tiger DNA and all tigers carry lion DNA. In order for this to happen, not only did ligers need to exist in the wild, they had to survive into adulthood and reproduce with both lions and tigers. The genes that were passed back and forth between species appear to have some beneficial purpose, otherwise they would not have persisted and spread to the full populations. Ligers exist in the wild today in as much as both lions and tigers are genetically partially hybrids.
Lions and tigers are not the only big cats to have hybridized. Historically (or prehistorically), all of the Panthera genus have introgressively hybridized. A research article published in Genome Research elaborates and explains the genetic evidence of ancient hybridization among the larger cats. Although the big cats share a common ancestor, and therefore a certain percentage of their genome, the big cats have also inherited genes from each other that did not exist in their common ancestor.
The number of introgressive hybrids is vast. Scientists are discovering more every day. One of the most widely distributed introgressive hybrids are North American gray wolves (Canis lupus). Ten thousand years ago all North American wolves were gray. As early humans expanded from Asia, across the Bering land bridge, they brought with them early domesticated dogs. These dogs hybridized with wolves and introduced the gene responsible for black coat color. This previously non-existent adaptation in wolves provided a benefit for survival to the wolf population. In addition to the obvious benefit of dark camouflage, mate selection started being made based on coat color. The inheritance of this gene through introgressive hybridization has spread throughout the continent. Today, nearly half of all gray wolves in North America are now black.
By far the most widely spread hybridized species is HUMANS! That's right. Every human on the planet has at least one non-human ancestor. Early humans hybridized with early hominins while in Africa. Upon leaving Africa, humans again introgressively hybridized in Eurasia with ancient Neanderthals. Every person (except those with 100% sub-Saharan ancestry) carry a percentage of Neanderthal DNA in their genome. A result section included in the 23andMe personal ancestry genetics test is dedicated to illustrating the number of Neanderthal DNA variants each customer carries.
Modern Tibetans have a unique ability among humans to thrive at high elevations. This genetic difference is the result of humans in the area hybridizing with an ancient race called Denisovans. Similarly, the Tibetan Mastiff (a breed of large dog) inherited similar adaptations from introgressively hybridizing with Tibetan plateau wolves.
Scientists are just beginning to scratch the surface of the prevalence of natural hybrids. They surround us. In fact, they are us. The more we look, the more we are beginning to realize the evolutionary importance of hybridization.