Hybrid big cats are artificial creations. They are unlikely to occur in the wild except in unnatural situations e.g. in very isolated populations where there is no mate of the appropriate species available. Because of the fertility issues, valuable genes may be lost by breeding dissimilar species together. Most conservationists condemn deliberate hybridization as wasteful in terms of genes and in terms of money. So why are they bred?

Many are bred out of curiosity. Exotic animals, especially ligers (the largest big cats on the planet), are great crowd-pullers. Pony-sized striped big cats and leopard-patterned lions are undeniably magnificent creatures. Others occur by accident where two animals are housed together from an early age in the belief that they won't mate with each other. The mating instinct is strong enough that a puma allowed herself to be mated by an ocelot one third of her size! This occurs where there is limited accommodation e.g. private collections, travelling circuses etc. Even experienced zoos have accidentally bred hybrids this way e.g. the servical. Believing that hybrids are always sterile, some keepers have housed a hybrid big cat with pure-bred big cats only to discover that hybrid females are fertile.

Private menageries also breed hybrids, sometimes as exotic pets. Some are bred to bypass restrictions on ownership of purebred big cats. Loopholes in some legal systems means that hybrids are not subject to the same legal restrictions on ownership or transportation as pure-bred tigers or lions! Many privately-owned curiosities end up at rescue centres when they grow too large, become to expensive to keep or prove to be temperamental. There is also an element of salacious - as well as genuine scientific - interest in the act of inter-species copulation.

There is a limited amount of hybridization for scientific reasons. This may be for research into how physical or behavioural traits are inherited or to discover how closely two species are related. The ability of pumas to produce offspring with ocelots (South American cat) and also with leopards (African cat) helps scientists to work out the taxonomy of pumas i.e. how closely they are related to other cat species. More worryingly, big cats have been hybridized in an attempt to create domestic big cats.

Hybrids do not generally give rise to new species. Because hybrid males are mostly infertile, female hybrids are mated back to pure-bred animals. In only a few generations, the "alien genes" are absorbed into the gene pool of the species she is bred back to. Theoretically, a new sub-species may arise if the population is isolated, but they will only have subtle differences such as lions retaining spots into adulthood as a result of a few lurking leopard genes in the gene pool from a leopon many generations back.

Speciation (one species evolving into two) is usually an excruciatingly slow process. Different species usually cannot mate and reproduce (reproductive isolation). If the species are closely related, such as certain cat species, they can produce hybrids, but those hybrids have reduced fertility. The more easily two species form hybrids, the more closely they are related in evolutionary terms. One way reproductive isolation occurs is genetic mutation. One group of animals might be geographically isolated from others of the same species. Each group accumulates slightly different mutations over many generations - some genes affect appearance, others affect behaviour. Many generations later, the two groups have diverged and are different enough that even if they can mate, they can't produce fully fertile offspring.

Sometimes, one species splits into two through behavioural isolation. Some individuals develop behaviour patterns which limit their choice of mates e.g. they might be attracted to certain colours or might be active at different times of day. Though they are fully capable of interbreeding with the other group, their different behaviours keep them apart. If their habitat changed, behavioural barriers might break down and allow interbreeding; the hybrids might become new species.

Another way reproductive isolation occurs is when fragments of DNA accidentally jump from one chromosome to another in an individual (chromosomal translocation) The mutant individuals can only reproduce with other mutant individuals - not much good unless the individual has mutant siblings to mate with! There are also "master genes" which govern general body plan (Hox genes) and those which switch other genes on and off. A small mutation to a master gene can mean a sudden big change to the individuals that inherit that mutation. Sometimes, those radical mutations can "undo" generations of divergent evolution so that two unrelated species can mate with each other and produce fertile young (so far, this has only been seen in micro-organisms).

Hybridisation is frequently a dead end because the hybrids are not fully fertile. If the hybrids are fertile, they are usually absorbed back into the population of one or other parent species and most of the alien genes are bred out. More rarely, hybrids can become new species or new sub-species. In the hands of breeders, some domestic/wildcat hybrids can become breeds; these are not new species because the wildcat genes are largely bred out by crossing with domestic cats, until only the wildcat pattern remains.

Although big cat species rarely, if ever, form hybrids under natural conditions, in other species, hybridisation might possibly play a larger role in evolutionary biology than previously believed. Most hybrids face handicaps as a result of genetic incompatibility, but the fittest survive, regardless of species boundaries. Life may be a genetic continuum rather than a series of self-contained species.

In wild sunflowers, hybridisation causes an explosion of genetic variation; some hybrids become new species capable of exploiting new ecological niches. In this case, hybridisation may be more important than genetic mutations in causing rapid, widespread evolutionary transitions because hybridisation creates variations in many genes or gene combinations simultaneously. Laboratory hybrids of annual sunflowers were back-crossed over one or more generations to one of the parent species. Enough "alien" genes were retained in later generations to allow them to thrive in conditions where neither parent species could live. Computer simulations suggest that the successful hybrids could evolve into new species within 50 to 60 generations. Similarly, genes from GM crops will inevitably leak into the wild gene pool.

In Heliconius butterflies genes have leaked from one species into another through hybridisation. Heliconius hybrids are relatively common and are a long way from the biology textbook stereotype of a sterile and deformed hybrid. These hybrids can successfully breed with either parental species or with other hybrids. However, there is natural selection against hybrids. Pure-bred Heliconius butterflies have warning colouration recognised by predators. The hybrids, equally unpalatable, have an intermediate pattern which is not recognised - the predators have not yet adapted and so the hybrids are disadvantaged.

Natural hybrids are found among butterflies, birds and fish. Blue whales will hybridise with fin whales. Interspecies matings have been witnessed in dolphins. Wolves, coyotes and dogs all produce fertile hybrids, so much so that some wild canids are becoming increasingly mongrelised. Usually, where there are two closely related species living in the same area, less than 1 in 1000 individuals will be hybrids because animals rarely choose a mate from a different species (unless mates from their own species are in short supply, the reason some endangered species are further threatened by hybridisation).

So why don't genetic leaks cause species boundaries to break down altogether? One, seen in butterflies, is because predators may not recognise the hybrids as inedible. Another is because hybrids cannot compete against the parent species for resources. Healthy hybrids between Darwin's Galapagos finches are relatively common, but their beaks are intermediate in shape and less efficient feeding tools than the beaks of the parental species so they lose out in the competition for food. In a 1983 storm, changes to the local habitat meant new types of plant began to flourish and some hybrids had an advantage over the birds with specialised beaks.

The hybridisation of the native European white-headed duck and the introduced American ruddy duck means that pure white-headed ducks are being hybridised into extinction. While humans want to protect the white-headed duck; evolution wants to utilise the ruddy duck genes. Once a species is introduced into a new habitat and the process starts, any Endangered Species legislation is trying to work against the inexorable and far more ancient forces of nature.

In 2011, it was reported that hybridisation between different species of mice resulted in strains of mice resistant to rodenticide poisons. German and Spanish mice have rapidly evolved the poison-resistant trait by breeding with an Algerian species. The European and Algerian mice separated as species between 1.5 and 3 million years ago. Fast-track evolution through hybridisation is uncommon in mammals because many of the hybrid offspring are sterile. Mice resistant to Warfarin (which causes fatal bleeding) have evolved naturally - and slowly - in parts of the world, but the German and Spanish mice developed resistance rapidly by cross-breeding to the Warfarin-resistant Algerian mice.

While male mammalian hybrids are generally sterile, the females may be fertile and able to breed with either parental species. Some of those fertile female hybrid mice evidently bred with local European mice, passing the Warfarin-resistance gene into the gene pool. As result, most of the mice in Spain are now Warfarin-resistant and the number of resistant mice in germany is growing. It is due to human travel that the Algerian mice came into contact with the European species. Isolated from their own species, but with a biological urge to mate, they would have bred with the European mice instead. Thanks to their genetic advantage over Warfarin-susceptible mice, the strain resistant mice are likely to spread throughout Europe.

Also in 2011, it was shown that hybridisation between the House Sparrow and the Spanish sparrow had given rise to a new and separate species known as the Italian Sparrow. DNA studies have found that the species originated as a hybrid, but it no longer breeds with either of the parental species with which it shares habitat, therefore meeting one definition of "species". Italian Sparrows and Spanish Sparros live side by side, but appear to have a reproductive barrier that prevented the hybrids being absorbed into the Spanish Sparrow population. Scientists believe this shows that the crossing of two species to form a new species might be more common in nature than previously realised.

If a fertile female tigon or liger offspring were mated back to the lion, the percentage of lions genes in the offspring increases and the percentage of tiger genes decreases. Assuming that the offspring at each generation were fertile females, the tiger genes would eventually be swamped by continually back-crossing to a lion. The end result (after several generations) would show no visible signs of tiger ancestry. The same would happen if the tigon or liger was backcrossed to a tiger and the fertile female offspring of each generation were backcrossed to a tiger etc. After several generations of backcrossing to tigers, the percentage of lion in the offspring would decrease to such a point that the offspring would appear to be wholly tiger. After several generations of backcrossing to one parental species, the offspring become indistinguishable from that species as the effect of the alien genes is swamped out. This applies to the nuclear DNA - the DNA which transmits physical and physiological traits.

There is a second type of DNA which is found only in the mitochondria and which is inherited only from the mother. In simple terms, mitochondria are the energy factories inside cells. They are present in the egg and sperm cells, but when fertilization occurs the sperm mitochondria are left outside the fertilized egg. If a female liger and her female offspring were repeatedly backcrossed to a lion (as detailed above), the nuclear DNA becomes almost all of lion origin. However, the mitochondrial DNA come from the original tiger mother. In later generations, the male hybrids become genetically close enough to being all lion to be fertile and can breed with lionesses. Only at that point is the original tiger mitochondrial DNA lost. The same happens when tigons are backcrossed to tigers over many generations. The mitochondrial DNA is lion DNA until such time as fertile males are produced and breed with tigresses who pass on tiger mitochondrial DNA .

Where there is suspicion of hybridisation in the past, it is possible to test the mitochondrial DNA. Unless something prevented all of the later generation female hybrids breeding (so that they couldn't pass on their mitochondrial DNA), there will be traces of the other species mitochondria DNA in what appear to be pure-bred lions or pure-bred tigers.

A female liger is 50% lion and 50% tiger. This is backcrossed to a purebred male tiger. At each generation, the female offspring is backcrossed to a purebred male tiger. The percentage of tiger genes goes up in each generation until they reach 99% at which point it could be considered purebred. It will never quite reach a round total of 100%.

The lion and tiger genes won't be inherited in neat 50/50 splits as the genes are shuffled about (though over several individuals it averages at 50%), so breeders talk of "pure-bloodedness" instead. How close is each generation to being a pureblooded tiger? The arithmetic here is rounded up to one or two decimal places.

Because the female hybrids are often fertile, it is theoretically possible to create a very complex hybrid. It would depend on the females of each generation being fertile - conceivably, there could be a point where there are so many different or incompatible genes in the mix that the offspring are no longer fertile.