Not at all. For instance, it is thought that H. sapiens × H. neanderthalis hybrids, while viable at least some of the time, were almost never fertile. To explai

Not at all. For instance, it is thought that H. sapiens × H. neanderthalis hybrids, while viable at least some of the time, were almost never fertile. To explain the Neanderthal contribution to the genome of modern humans, only a very low frequency of successful hybridization events is required.