“Biologic species” does not have a satisfactory definition. Most often “species” is defined as “the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction”. The fertility “barrier” is however arbitrary, inadequate for closely related species, and irrelevant to the vast group of asexually reproducing organisms. Since organisms do not come with tree of life place cards, not just species classification but also taxonomy in general is more or less arbitrary as shown by grouping issues and the frequent revisions to its structure. Darwin admitted in his “Origin of Species”: “I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other.” (p52) and “We shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combination made for convenience.” (p485)
Why keep “species” or why not redefine the concept if failed? Redefining would likely have been done long time ago if better criteria were available, while discarding the concept of “species” is opposed by those fond of Darwin’s “Origin of Species”. Those that believe the “reproductive isolation” story point to minor adaptations, which they call “speciation” (implying stability) and then ask us to extrapolate these small changes into the dramatic transmutations imagined yet never observed by Darwin or his followers. This is a classic trick – employed extensively by magicians, cinematographers and con artists among others – where one thing is shown and the brain then “sees” another that is not there.
Biologic changes like appearance, metabolic, and antibiotic resistance are all limited in scope and reversible when the triggering stimulus is removed. If the arctic environment turned animals white while maintaining their resemblance to the family, how could an almost identical environment turn apes into humans when the same environment did not impact much the other African fauna? Observed adaptation limits include sugar content of beets, drosophila bristles, no black tulip, no blue rose, no green rabbit. Humans are quite diverse, from black to white, pygmy, those adapted to high altitude, and even Neanderthals, yet we’re all one “species”. In contrast, minor variants in other organisms are too readily branded “speciation”. This is reflected in the inflation of hominid “species” as everyone that found a bone or two claimed they discovered a new one. And even after some cleanup, we’re still left with Neanderthals and Denisovans that successfully mated (fertile off-springs) with Sapiens despite some considering them “separate species”.
Various “natural selection” speciation mechanisms have been proposed based on geography: allopatric (separated), peripatric (isolated peripheral), parapatric (adjacent), sympatric (overlapping). To these, a number of “artificial” methods like polyploidy, hybridization, gene transposition that produce new “species” significantly different than the original, but their relatives are none other than the original organisms and the difference are comparable to those observed in sexual dimorphism. In the end, all speciation “mechanism” lacks clear threshold criteria while “natural selection” is not a logical, coherent mechanism, leaving us with just a vague, unsupported “speciation” story.
https://www.edge.org/responses/what-scientific-idea-is-ready-for-retirement – see Dawkins Essentialism