How do we define a "new" species.

ARE MULES EVER FERTILE?Mules are generally infertile, but there does appear to be evidence that a very few female mules are fertile. Such fertility is very rare. The Romans had a saying:Cum mula peperit- meaning "when a mule foals", roughly equivalent to "once in a blue moon".HOW MANY MULES HAVE PRODUCED FOALS?In 1990, Lorraine wrote:"Since 1527 approximately 60 live births of foals to mules have been reported, in Europe, the USA, South America, North Africa and China."One of the best documented earlier cases was Old Beck, investigated by Texas A&M (USA) in the 1920s.In the 1980s, there were cases of a fertile mule and a fertile hinny in China and mules in the USA and Brazil who produced more than one foal!More recently we have had reports of a fertile mule in Morocco and a fertile hinny in China. And in 1994, there were reports in the press about a mule in Albania having a miscarriage.WHY ARE MULES USUALLY INFERTILE?Basically, because the chromosomes of horses and donkeys are different:"The donkey has 62 chromosomes (31 pairs), the horse 64 (32 pairs) and the mule and hinny each have 63 chromosomes - of which many pairs are unevenly matched. It is not just the number of chromosomes which is different in donkeys and horses, but their structure: they have developed slightly differently over evolutionary time....The donkey and horse chromosomes are almost completely unable to pair up."