First problem: It’s inapplicable to bacteria and Archaea, which don’t “interbreed” in anything like the way implied by Mayr. Second problem: How can “reproductively isolated” be an absolute standard if genes are continually transferred horizontally (by viral infection and other mechanisms) and if, furthermore, members of one species sometimes breed with members of another, producing new lineages of hybrid offspring? Answer: Reproductive isolation is a useful and intuitive standard, yes, but not an absolute one.
Take Homo sapiens, the species most dear to our hearts. In the era of DNA sequencing, scientists have recognized that the human genome contains evidence of hybridizing events. Homo neanderthalensis was discovered in 1856, named in 1864, and for many decades considered a discrete species, closely related to us within the hominid family, but distinct. Some experts now consider the Neanderthals to have been a subspecies of Homo sapiens, more properly called Homo sapiens neanderthalensis, but others argue that Homo neanderthalensis is still the right label, representing the group as a full species.
In any case, our lineage diverged from theirs sometime between about 300,000 and 600,000 years ago, maybe more, when pioneers left Africa and colonized Eurasia. From those pioneers would descend several non-African species, including Homo neanderthalensis. Our own lineage, known as “modern humans,” sent another wave of dispersers out of Africa and colonized Europe again, but much later, around 50,000 years ago. Then, for one reason or another, the Neanderthals disappeared.
Paleoanthropologists have long speculated that either our ancestors killed off the Neanderthals, by direct aggression, or forced them to go extinct, by competition, or else absorbed them to some degree, by interbreeding. But there was no conclusive proof. Nowadays, since the recovery and sequencing of Neanderthal DNA by a team including the Swedish biologist Svante Pääbo, analyses indicate that hybrid matings did occur between Neanderthals and modern humans. The human genome, especially as found among non- African peoples descended from those hybrid matings, now contains about 1 to 3 percent Neanderthal DNA.
A twist in our sexual encounters with other ancient humans
And it’s not just Neanderthals in our genome. The human lineage diverged from what became the chimpanzee lineage 7 million, or 10 million, or maybe 13 million years ago—nobody knows the timing with any precision. But recent genomic analysis suggests that, sometime well after the big split, hominid ancestors and chimp ancestors came back together for hybrid matings, and that those hybrid matings have left genuine chimpanzee genes (not just close human equivalents) in parts of our genome.
Of course, the presence of chimpanzee genes, or Neanderthal genes, isn’t the half of it. There’s also that viral DNA—including syncytin-2, a gene co-opted from a retrovirus, repurposed to enable human pregnancy. The fact that DNA from endogenous retroviruses constitutes 8 percent of the human genome certainly complicates our sense of Homo sapiens as a species of primate.