As a non-biologist, I&#x27;ll just go ahead and suggest that species are defined by a single genetic exemplar that is an actual common ancestor to <i>all</i> extant members of the species.<p>This individual then recruits additional members by hybridizing with its own ancestor species or sibling species in the same genus.  Any given organism&#x27;s species is determined by the relative genetic distance between it and the species exemplars.<p>Under this regime, if we postulate a new species <i>Homo superior</i> with, for instance, the mutant ability to read and write text messages while also being fully aware of its surroundings, the earliest known bearer of the mutant gene would likely be chosen as the exemplar.  If this person died and was not preserved in any way, we might be able to reconstruct a model of their genome via gene sequencing analyses, which would clearly show the mutant gene.<p>It would hybridize with <i>Homo sapiens</i>, the parent species.  Children lacking the mutant gene would be indistinguishable from H. sapiens.  Children with it would clearly be able to trace lineage to the exemplar.<p>Without barriers to breeding, the new species might just invisibly replace the parent species as a non-branching successor.  In this case, we might just let it keep the same species name, and retcon the original mutation into the appearance of a subspecies.<p>The fossil record makes selection of exemplars easier, because in many cases, only one instance even exists.  Only one tooth found in Denisov cave.  Only one Taung skull found.<p>If there is no gene in <i>Canis rufus</i> that is unique to a single species exemplar, there is no unique species left.  Even if it ever was its own species, having a purely theoretical exemplar with only a few known genes, <i>right now</i> it is just a <i>lupus-latrans</i> hybrid.<p>So for me, a species would be defined by the presence of one or more specific genes.  Those genes could be determined by sequencing DNA from actual remains, or inferred by analysis of living members of the species.<p>Being able to interbreed fertile offspring seems to me to be more like a <i>genus</i>-level thing, because that seems pretty common in plants.