International Balzan Foundation Luigi Luca Cavalli

Luigi Luca Cavalli-Sforza
and finds that species differ for many single genetic differences. The percentage
of DNA units which happen to be different in two species is related, in a very
simple way, to their evolutionary time separation. But human populations living
today have been separated for a relatively short time. Archaeology shows that
modern humans appeared only a little over one hundred thousand years ago, in
Africa, and spread first to Asia, between 100,000 and 50,000 years ago, and
from South East Asia to Oceania between 60,000 and 40,000 years ago. Europe
was reached about 40,000 years ago from both West Asia and North Africa, and
America from Siberia beginning 15,000 years ago.
Mutation first appears in one individual, and then it spreads to other individuals
only in successive generations, when the individual carrying the mutated type
has several progeny carrying the mutation, either because mutants are favored
by selection and survive easily or have more children, or they are favored by
chance. It usually takes a great number of generations before the mutation is
found in many individuals, and even more for it to replace completely the original
type. Thus between the first appearance of a mutation and the replacement
of the original type there will usually elapse a very long time. During that period
the mutant type is “polymorphic”, i.e. there coexist in the population both the
original (ancestral) type and the mutant one. There are extremely few mutations
that have reached the whole of the human species after its origin in Africa.
In fact, we established that, although the human population has enormous genetic
variation between individuals, 85% of the total human variation is within
single populations, and only 15% between them. We therefore cannot use for
the comparison of different human populations the same measure of genetic
distance useful for comparing different living species, for which an individual
from each species would suffice. For instance, we find that the RH negative type
has a frequency of 50% in some populations, e.g. in the Basque country, a lower
one in England (40%), and 0% in East Asia. Genetic replacement means
transition from 0% to 100%. The difference between Basques and English people
is only 50%-40% = 10% of a complete replacement, and that between
Basques and East Asians 50% . We actually employ slightly more sophisticated
formulas to calculate the genetic distance between two populations, which is, on
average, proportional to the time since the two were separated. It is also essential
to calculate the averages of many genes for these measurements to be valid.
When chance is one of the main factors responsible for change, the only way to
eliminate the uncertainty it causes is to apply the law of large numbers, that is,
use a large number of genetic differences. However, even if we could confine
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