Race (human categorization)

===== Clines and clusters in genetic variation =====
Recent studies of human genetic clustering have included a debate over how genetic variation is organized, with clusters and clines as the main possible orderings. {{harvtxt|Serre|Pääbo|2004}} argued for smooth, clinal genetic variation in ancestral populations even in regions previously considered racially homogeneous, with the apparent gaps turning out to be artifacts of sampling techniques. {{harvtxt|Rosenberg|Mahajan|Ramachandran|Zhao|2005}} disputed this and offered an analysis of the Human Genetic Diversity Panel showing that there were small discontinuities in the smooth genetic variation for ancestral populations at the location of geographic barriers such as the [[Sahara]], the Oceans, and the [[Himalayas]]. Nonetheless, {{harvtxt|Rosenberg|Mahajan|Ramachandran|Zhao|2005}} stated that their findings "should not be taken as evidence of our support of any particular concept of biological race&nbsp;... Genetic differences among human populations derive mainly from gradations in allele frequencies rather than from distinctive 'diagnostic' genotypes." Using a sample of 40 populations distributed roughly evenly across the Earth's land surface, {{harvtxt|Xing|et al.|2010|p=208}} found that "genetic diversity is distributed in a more clinal pattern when more geographically intermediate populations are sampled".

[[Guido Barbujani]] has written that human genetic variation is generally distributed continuously in gradients across much of Earth, and that there is no evidence that genetic boundaries between human populations exist as would be necessary for human races to exist.{{sfn|Barbujani|2005}}

Over time, human genetic variation has formed a nested structure that is inconsistent with the concept of races that have evolved independently of one another.<ref>{{cite journal |last1=Hunley |first1=Keith L. |last2=Healy |first2=Meghan E. |last3=Long |first3=Jeffrey C. |author-link3=Jeffrey C. Long |date=18 February 2009 |title=The global pattern of gene identity variation reveals a history of long-range migrations, bottlenecks, and local mate exchange: Implications for biological race |journal=[[American Journal of Physical Anthropology]] |volume=139 |issue=1 |pages=35–46 |doi=10.1002/ajpa.20932 |pmid=19226641 |hdl=2027.42/62159 |hdl-access=free |url=https://deepblue.lib.umich.edu/bitstream/2027.42/62159/1/20932_ftp.pdf}}</ref>
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