ABSTRACT
Generally, individuals or populations are considered as members of the same species when they can sexually recombine and generate offspring which, in their turn, are fully fertile. For instance, horses and donkeys can mate and produce offspring (mules), but as mules themselves are sterile, we can not consider horses and donkeys as belonging to the same species. The fertility problems of mules, or interspecic hybrids in general, are caused by inadequate chromosome pairing during meiosis, as maternal and paternal chromosomes are too different for this so-called synapsis.
