As most people are aware, the genus Homo comprises a temporal sequence of hominine populations displaying divergent morphology and cranial capacities. Our genus is believed to have arisen about three million years ago from a late australopithecine. Historically, palaeoanthropologists utilized this phenotypic divergence so to decide whether the various phenotypic populations were isolated (breeding-wise) populations of the same species or different species. Morphology was utilized as genetic techniques were not available until recent times so to determine which hominine populations could interbreed and generate fertile offspring. Fertile breeding, being the sin qua non, differentiating one species from another. However, hybridization between closely related species is recognized in both animals and plants, but, it not generally successful, and is not the cause with the genus Homo.
A rather complete definition of the biological meaning of the term ‘species’ was reiterated by the great evolutionist, Ernst Mayr, in the mid 20th century in his article, What is a Species, and What is Not?, Phil Sci, (June 1996); 63:262-277. In this article, Prof. Mayr noted, “Among the attributes shared by members of a species, the only ones that are of crucial significance for the species definition are those which serve the biological purpose of the species, that is, the protection of a harmonious gene pool. These attributes were named by Dobzhansky (1935) isolating mechanisms. It is immaterial whether or not the term isolating mechanism was well chosen, nor is it important whether one places the stress on the prevention of interbreeding with non-conspecific individuals or the facilitation (“recognition”) of breeding with conspecific individuals. The concept I have just developed is articulated in the so-called biological species definition: “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” The isolating mechanism by which reproductive isolation is effected are properties of individuals. Geographic isolation therefore does not qualify as an isolating mechanism.
However, modern wisdom teaches that comparisons of morphological variants (from one or a few specimens) so to decide speciation is seriously flawed. For instance, modern genetic testing shows that the modern European H. sapiens genome contains about 2 – 4% derived from H. neanderthalensis (possibly up to 10% in selected populations). Other genetic studies suggest that other archaic hominins interbreed successfully such as H. denisovans – H. neanderthalensis and H. denisovans – H. erectus admixtures. Maternal mtDNA suggests that H. heidelbergensis was the common ancestor for the three named populations above.
Such evidence seriously challenges the wisdom of the earlier morphological classification scheme. Current genetic evidence (including, mitochondrial, Y-chromosomal, and somatic DNA) suggests that modern humans could interbreed successfully with its archaic ancestors dating back hundreds of thousands of years (including, perhaps, Asian H. Erectus).
Moreover, the wide range of variant “population-specific” phenotypes observed in our species, distributed over large geographical areas and long time periods, suggests that the human genome is highly adaptable to rapid changes in its geographical / climatic niche. Such adaptations, and perhaps exaptations, arising not from individual genetic mutation, but, from a generally pluripotent genome. Based upon such history, the various ethnic groups of modern man should be considered to comprise at best, a minor degree of phenotypic segregation arising from population isolation over long time periods.
A similar situation appears to exist for the species Canis lupus, currently having some 40 interbreeding subspecies (the various wolves, dingo, and dogs). Moreover, all of us realize the ease of generating large phenotypic variation in domestic dogs without jeopardizing breeding. Such characteristics in a particular species are called ‘polytypic.’
Similarly, under forced environmental disruption of normally occupied niches, it is likely that many of the variant “species” of Lake Victoria Cichlids will be forced to abandon their highly unique environments and interbreed with other “species” so to inhabit a new niche. As they can interbreed, they are actually subspecies which are polytypic.
However, such extreme phenotypic variation between individuals in a single species is not common. More consistent characteristics being responsible for the general observation that evolutionary changes occur solely via advantageous modifications in individual genomes and not in the species as a unit. Subsequently, new species arise, survive mainly unchanged, and eventually face extinction as a unit. The unit experiences no transformations to “richer” phenotypes over its existence, as seems to be the case with existing species of hominin.
Interestingly, the genetic studies of archaic and modern man suggest that we are an atypical species, being capable of great phenotypic improvement (increase in functioning cranial volume and organization) via minor restructuring of the human genome. Such improvement, most assuredly arising from to our unique capacity for enculturation, language, and technological innovation. I guess one could say that humans are rich transformationally.
The potentially good news about our species is that our genetic structure may be innately capable of further cognitive-affective transformation, given the proper geopsychosocial environment and rational technology. Perhaps, we can move beyond our self-centered and aggressive behaviors one day and become a better species before we eliminate ourselves.
Moreover, history shows us that conscious evolution is a realizable activity for an awakened man and woman. We have all we need right now to accomplish the deep purpose hidden within the evolutionary process.
Tomorrow, I shall begin speaking so to unfold the source and the methodology of this hidden purpose.