Chemistry Reference
In-Depth Information
In these populations and in many other peoples, it seems that the lactose
concentration is moderated by cultural adaptation. Milk is processed to sour
milk, yoghurts and cheeses, which have a reduced lactose content, and
individuals also adapt their consumption habits by taking smaller quantities
of milk at a time. These cultural adaptations enable non-persistent indivi-
duals to benefit from the calorific, mineral and vitamin constituents of milk
without inducing the associated symptoms of lactose malabsorption, and are
complemented by adaptations of the large intestinal bacterial flora (see
Diagnosis of lactase non-persistence/persistence).
6.9.
Selective Forces
What were the selective forces that resulted in the elevated lactase persistence
frequencies observed in certain populations? A number of theories have
attempted to explain the apparent selection for lactase persistence. Because
of the worldwide distribution of lactase persistence and the generally coin-
ciding pattern of historically milk-drinking populations, Simoons (1970) and
McCracken (1971) independently suggested that the selective force for lactase
persistence was milk dependence. This has become known as the 'culture-
historical hypothesis', and suggests that the increase in lactase persistence co-
evolved alongside the cultural adaptation of milk drinking.
The model works only on the premise that the advantage was conferred
specifically by fresh milk, as non-persistent individuals are also able to benefit
from the calorific, vitamin and mineral contents of milk by processing it,
which reduces the lactose content (e.g. by fermentation to yoghurt or cheese),
and therefore selection is most likely to have occurred in populations for
whom fresh milk formed an integral part of the diet.
A problem with this hypothesis is the 'non-fit' populations, who have
either a high lactase persistence frequency without being milk dependent, or
who rely heavily on milk products but who have a low reported frequency of
lactase persistence. However, statistical modelling has suggested that an
incomplete correlation does not necessarily provide evidence against the
culture-historical hypothesis, and non-fits may be expected if some lactase-
persistent populations have recently stopped milking or other populations
have only recently adopted the habit, therefore allowing insufficient time for
lactase persistence to be driven to high frequency (Aoki, 1986). Furthermore,
this model does not account for migration, which may provide further expla-
nation for the imperfect correlation.
Other statistical analysis also provides evidence in favour of the culture-
historical hypothesis; Holden and Mace (1997) performed an analysis that
