Ligers are a special hybrid between a mother tigress and a father lion. Ligers appeared accidentally because tigers and lions shared the same territory, but they cannot live in freedom. They survive only in captivity, where are well taken care of. Ligers are not just an out of the common species, but they also are the largest felines. Any case, there is a simple and logical explanation that scientists discovered, related to the size of ligers. It is all about the genes, like at any being from this planet.
The explanation of the size of ligers is related to imprinted genes. These genes are under a higher selective pressure than regular genes and only one copy of these genes is active at a time. Thanks to this awkwardness, if the copy has variations, these variations will be expressed. Because a back-up copy does not exist, nothing masks the effects. These kind of genes evolve much more rapidly than regular genes, so thanks to this evolution and permanent change, ligers result. Tigers and lions are distinct, but not distinct enough to not be able to produce offspring. Their offspring is viable and against all expectations most ligers are fertile. Even so, female ligers cannot carry a pregnancy and until now, baby ligers that were perfectly born did not survive for a very long time. In captivity, several tries to prolong cubs life did not have any success until now.
Although rare, ligers are not the only hybrid animals. The cubs resulting from a male tiger and a female lion are called tigons and their characteristics are very alike to the ones ligers have. Also, tigons are in the same situation as ligers, they are not able to survive in freedom and they do not appear in freedom. Tigons and ligers are born just in captivity because in nature these two species do not live together. Tigers are solitary, while lions are very proud, but in captivity, the two species get along very well. The result of this mutual friendship consists in the birth of the special cubs -- ligers and tigons.