If bounds are used that would distinguish modern humans and their direct ancestors from other taxa, those bounds would exclude many living humans from the taxon. ~ Camilo Cela-Conde & Francisco Ayala
Modern humans are remarkably diverse in both morphology and less-striking physical features, not to mention internal differences which are not visible. Ancestry, culture, climate, and diet are causal factors.
A genome has vast potential for variety. Changes in gene regulation and expression often reap major adaptive advantage.
There was statistically scant change in the genes between the earliest hominin and that of modern humans. There are less than 100 protein-coding gene differences between Neanderthal and modern man.
What was more pervasive were a raft of modifications embodied epigenetically. The rise of moderns was mostly a mass of modest genic tweaks that adaptively aggregated into significance.
Divergence in gene regulation can play a major role in evolution. ~ American geneticist Dawn Thompson et al
Human populations evolved various traits as habitat adaptations: not only because of local pathogens, but also owing to cultural differences which drove genetic diversity. Contrary to anthropologists’ recent proclivity toward lumping, speciation is defined biologically not as inability to interbreed, but unwillingness to do so.
Fish and birds that encounter each other frequently – biologically sexually compatible – choose not to interbreed by preference and are considered separate species. Modern humans have been treated otherwise. Genetic analysis has demonstrated that European Cro-Magnon and Neanderthal interbred, yet both are considered separate species.
By an impartial biologists’ definition, the so-called human ‘race’ comprises a multitude of subspecies (or downright species), separated by both biome and socioeconomic strata, including a classic trait of speciation: tribal behaviors (aka cultural mores) that inhibit interbreeding. Clearly is it easier for anthropologists to speciate hominins of the past than it is to do so with current populations.
Humans tend to mate with those of similar personality, education, social standing, and even genome. Such discriminatory preference is termed assortative mating.
Nonetheless, where peoples converge, interbreeding invariably occurs, albeit in limited numbers. Unlike choosier species, many men are strongly inclined toward sex with anything that is plug-compatible. All around the world, rape remains common.
If we consider all the races of man as forming a single species, his range is enormous. ~ Charles Darwin