To what extent are race and ethnicity social constructed categories, versus something fundamentally real in society?

General the term "race" is used to denote biological groupings and "ethnicity" cultural ones, but both concepts are not ones that can be established by clear, empirical demarcations. ``

On a biological level, there is very little variation in human DNA, with most identifiable physical differences being on an almost purely cosmetic level and affecting only quite malleable traits such as hair and skin color. If you compare human variations, for example, to that of dogs,...

General the term "race" is used to denote biological groupings and "ethnicity" cultural ones, but both concepts are not ones that can be established by clear, empirical demarcations. ``

On a biological level, there is very little variation in human DNA, with most identifiable physical differences being on an almost purely cosmetic level and affecting only quite malleable traits such as hair and skin color. If you compare human variations, for example, to that of dogs, one can see that relative uniformity of the human race; the differences between beagles and huskies are far greater that the differences among humans. Many of the theories of race that led to racial discrimination or even genocide in the past have no real scientific basis, but were political or ideological constructs.

Ethnicity has a cultural rather than a biological basis. For some civilizations, ethnicity was based on culture; in Hellenistic society, "paideia" or education was thought to distinguish the Hellene from the barbarian. Other national groups, such as Germany under Hitler or Imperial Japan conflated race and culture and strove for ethnic and racial purity.

The reality of human history, though, is that modern humans have existed for over 200,000 years and traveled, interbred, communicated, and influenced one another so much that no particular genetic pool or cultural tradition is independent of the rest of the human race, but all cultures are to a degree hybrids. Even in recorded history, the nation states of the early modern period, which argued for cohesion on the basis of shared language, ethnicity, and traditions, were in fact hybrids. The shifting geography of earth's land masses has meant that even areas that are now isolated islands were, in the past, connected to other land masses. 

Although in certain historical periods and geographical regions, groups do spring up that share languages, customs, and traditions, these are not permanent categories of being, but rather transitory phenomena, constantly subject to outside influences and internal mechanisms of change.