Biology Reference
In-Depth Information
BOX 16.1
EVOLUTIONARY FORCES IN HUMAN HISTORY
As explained in the main text, the four
classic evolutionary forces
d
mutation, gene
flow, genetic drift, and natural selection
work in concert with each other, though
the evolutionary history of a trait or traits
can be shaped primarily by one or two of
these “forces” relative to the rest. Here are
some examples of how this can happen.
before these LP gene variants arose, they had
to remove the milk's lactose through fermen-
tation and at the same time lose 20
e
50% of its
calories. In situations of sporadic famines,
those additional calories obtained through LP
could aid tremendously in survival and
reproduction. This appears to not be coinci-
dental, as lactase persistence can be evolu-
tionarily advantageous.
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Mutation and Natural Selection:
The Case of Lactase Persistence
Lactase is the enzyme responsible for the
digestion of the milk sugar called lactose.
Lactase production decreases after the
weaning phase in most humans, at which
point the typical individual becomes lactose
intolerant and experiences digestive upset
(gas, bloating, and/or diarrhea) upon the
consumption of fresh milk. Some people,
however, continue to produce lactase into
adulthood, a trait known as lactase persis-
tence, or LP. LP is a genetically controlled trait
that is found at moderate to high frequencies
in European (particularly northern), some
African, Middle Eastern, and Southern Asian
populations.
4
It turns out that the condition has evolved
independently in at least four places around
the globe and a number of different mutations
have been found in association with the LP
trait. What explains the high frequency of LP
alleles in only certain populations? Genetic
drift and gene flow have probably played
a role, but here is a case in which natural
selection is a more compelling candidate. Age
estimates in terms of the antiquity of lactase
persistence-associated alleles coincide with
those for the origins of animal domestication
and the cultural practice of dairying.
5
Though
humans were likely consumingmilk products
Genetic Drift: The Case of the
Initial Peopling of the Americas
The topic of the peopling of the Americas
has captivated anthropological audiences for
decades now, and not without numerous
points of contention. Anthropologists do tend
to agree on one thing, however: that the
primary original entry point of humans onto
the American continents was likely through
the Bering Strait in northern Alaska. This
suggests that humans experienced a
founder
effect
, or a loss of genetic variation that occurs
whenaverysmall number of individuals from
a larger population establish a new pop-
ulation, in this case, in a new continent. This
could have profound implications on the
pattern of morphological and genetic varia-
tion in the Americas.
In the past decade and a half, genetic
studies have provided some evidence for
a strong founder effect. Broad, worldwide
surveys of genetic variation have demon-
strated that both ancient and modern Native
Americans harbor a subset of the variation
found in Asia. MtDNA provides the clearest
example: currently, populations in Asia
demonstrate ca. eight to ten broad mtDNA
lineages, whereas Native Americans show
five of those. For a recent review, see Kemp
and Schurr (2010).
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(Continued)
