- A hybrid is defined as the offspring produced between two different variations of organisms. Usually the term hybridization is employed to describe mating between two different species, as the variation that exists within a species is not an encumbrance to mating. It can, however, refer to the mating between subspecies, which are usually distinct enough genetically to earn separate classifications.
- Sexual reproduction can best be described as the blending of two different genomes to create a unique mix, which has the benefit of promoting diversity and adaptability. However, sexual reproduction has its boundaries; beyond those edges organisms are no longer compatible genetically. In fact, species are often classified based on the ability to mate with other members to create viable offspring. Isolation and sexual selection are usually powerful enough to keep species apart, but not always.
- If two disparate genomes mix, a number of problems may arise. Proteins may no longer express properly, hampering their ability and creating potential disorders. Some genomes might be so different that egg and sperm don't even fertilize. Or if they do fertilize the offspring might be sterile and unable to reproduce. Often sterility is the product of differences in chromosome number, which prevents the production of viable gametes (egg and sperm).
- One known sterile hybrid is the mule, which is a cross between a male donkey and female horse. Only a few female mules are actually fertile. The reason for sterility is that a donkey has 62 chromosomes, while a horse has 64 chromosomes. Some feline hybrids such as tigons and ligers, two potential crosses between lions and tigers, are sterile to varying degrees, although they have been known to produce offspring.
- In the 1860s, Charles Darwin wrote broadly about hybrids and sterilization, as he considered these to be important concepts to the theory of evolution. He even devoted a chapter to hybridization in "The Origin of Species." Up until that time, sterility was thought to exist to prevent the mix of species. However, this was a time long before genetics was even known about, and Mendel's laws of inheritance had yet to be synthesized with evolutionary theory. Darwin himself equivocated often on whether to consider sterility a naturally selected characteristic. Only later did scientists discover that it was incidental.