A growing number of studies has been presented as evidence that two animal species can combine to produce a third, sexually viable species in a process known as hybrid speciation. Newly identified examples include both insects and fish.
This evolutionary process, while known to be common in plants, has long been considered extremely rare among animals. Animals are generally thought to evolve the opposite way, when a single species gradually splits into two over many generations. But some scientists now believe that the behavior that has been called animals' sexual blunders could be an important force in their evolution.The article concludes:
Critics say that the likelihood of a hybrid establishing in reproductive isolation from its parents is very low, and that hybrids form less than 0.1 percent of animal populations. Given this low number, animal hybrid species are likely to always be rare no matter how sophisticated or exhaustive the genetic analysis is.And isn't it blindingly obvious that no matter how many different species an organism has sex with, the chance of producing viable offspring is pretty remote? Not only that, but the choice of partners who might actually produce a viable hybrid must be fairly limited - I don't suppose it matters how many times you have sex with a donkey, you're not going to produce viable offspring! Whilst hybridisation may not be the hidden secret of evolution, it may well be more important between related species and I wonder whether early humans successfully hybridised? Could a Homo sapien get a Neanderthal girl pregnant?