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evolutionary biologist Sir Julian Huxley in 1938 and refers to a gradual change of a character
or feature in a species over a geographical area ( Lieberman, 2008 ). For example, Relethford
(2009) argues that human phenotypic features such as skin color and craniometrics are
patterned clinally (e.g., skin color being darkest near the equator and gradually lightening
as latitude increases), and that natural selection controls traits such as expression of skin
color from environmental pressure. Similarly, craniometrics are partially controlled by
natural selection in some instances and selectively neutral in others, although craniometric
differences still demonstrate more variation within local populations than between them.
Relethford (2009) argues that while geographic patterning is evident in these traits, placing
them into broad racial categories masks the true diversity of human variation.
The other school of thought explains variation as resulting from complex factors that
contribute to evolutionary forces, such as migration, bottlenecks, and population divisions
(e.g., Hunley et al., 2009 ). These complex factors interrupt gene flow as larger populations
are split up. This leads to the founder effect, where the genes of a smaller segment of the
larger population become overly representative of the parent population, resulting in genetic
drift. 17 While workers such as Hunley and colleagues (2009) and Long and colleagues (2009)
contend that the pattern of human variation is nested (the diversity in one population is
a subset of the diversity found in another) rather than clinal, they come to the same conclu-
sion as those in the cline camp: namely, that the traditional racial classification system is not
adequate for explaining human variation.
Regardless of what the actual geographic pattern of variation turns out to be, the impor-
tance of the environment in expression of phenotypic traits cannot be overstated. There is
a complex interplay between genotype, phenotype, and environment, which will never be
fully teased apart. Remember that the environment consists of sociocultural and physical
aspects to which an individual is exposed, beginning in utero and including but not limited
to postnatal factors such as nutrition, diet, exposure to pathogens, climate, education, and
physical and psychosocial stress. The environment essentially influences which traits will
be beneficial, harmful, and neutral, factors that change as the environment changes.
Cartmill (1999) contends that culture also plays a strong role in how genes and the envi-
ronment interact. The reader is encouraged to refer to that paper for an excellent discussion
on the interplay between environment and heredity. Lieberman and Kirk (2004:137)
further emphasize that one of the reasons the race concept has been rejected is due to the real-
ization that cultures are “a dynamic expression of their history and ecology” ` la Boas.
Research that examines human variation must account for environmental factors and
acknowledge that it is likely that not all of the different aspects of the environment's influence
on trait expression will be uncovered.
Therefore, to restate the overall research problems currently under investigation: (1) What
is the true nature of human population history that has led to the range of existing variation?
(2) How can geographic patterns explain human variation? (3) How does geography and
evolutionary forces contribute to the patterning of phenotypic and genotypic variation?
Many avenues are being used to address the numerous questions inherent to these prob-
lems, including from the field of DNA and from a biological distance perspective. Refer to
17 See Cabana et al. (Chapter 16), this volume for further definition and discussion of evolutionary forces.
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