>>2287935 #>>2287940 #>>2287943 #>>2287945 #>>2287951 #>>2287968 #>>2288268 #>>2288512 #>>2288538 #>>2288546 #>>2288551 #>>2288554 #>>2288600 #>>2288690 #>>2288893 #>To understand the continuous nature of human variation, it isuseful for the initiate to consider the concept of a cline, a term
proposed by Huxley in 1938 [73]. A cline is a measurable gradi-
ent in a single characteristic of a species across its geographical
range. Cline is not a term used frequently in biology, but greatly
facilitates a basic understanding of the gradient of genetic vari-
ation in humans as we migrated out of Africa. In brief, a portion
of the African population migrated out of Africa -
30 000–
50 000 years ago, settled at some distance from the parent
population, and became a reproductive group. A portion of this
second group migrated, settled at some distance and again
reproduced. Migration, settling, reproduction and further mi-
gration resulted in a gradient of alleles across a geographical
range defining clines, hence, the term clinal genetic variation. A
cline in what is present day Turkey had a different allele fre-
quency from a cline in present day China or India, even if all of
the occupants are considered Asian. Similarly, a cline in what is
present day Morocco had a different allele frequency different
from what is represented today in Atlanta, Georgia. Thus, it is
impossible to specify a specific allele or an allele frequency that
would typify ‘African ancestry’ for 1.3 billion Africans on the
African continent or 45 million African Americans in the USA.
Because allele frequencies vary across geographical space, it
follows that people of ‘African ancestry’ do not represent a
homogeneous group. Only those who share direct ancestors—
parents, grandparents and all generations of great grandpar-
ents—are genetically related.
