An evolutionary perspective on human genetic variation also
allows us to make predictions about the future, both of
biological research and of our species. We are able to pose
many more questions within biology than we are presently
able to answer. An evolutionary perspective tells us how we
might go about answering these questions, and about what
kinds of answers we might expect.
Phenotypic traits of humans, be they skin color, height or
disorders such as diabetes, are controlled by a combination
of inherited and environmental factors, and stochasticdevelopmental and molecular processes. The easiest traits to
dissect genetically are those determined in large part by single
genes – so-called Mendelian traits. However, many of the
phenotypic traits of most interest to both anthropologists and
physicians are not so simple. These complex traits are
governed by interactions between chance events, multiple
genes and the environment, and disentangling these
interactions will help to relieve the considerable burden of
complex diseases on individuals and economies.
Genes involved in complex traits can be identified in one
of two ways. In a physiological or biochemical approach, we
identify the gene product as a causal factor, based on an
understanding of the physiological and molecular basis of the
trait. In the alternative positional approach, we locate the
position of the gene within the genome by identifying a
chromosomal segment that is consistently co-inherited with
the trait. The physiological approach has had limited success,
and as we shall see in this book, an evolutionary perspective
greatly enhances our ability to adopt the positional approach
to gene identification.
A knowledge of our past allows us to predict something
about the numbers and frequencies of gene variants expected
to be involved in a given trait and to choose the best strategy
of finding them: what populations to choose, and which
segments of the genome to concentrate on. Not only that,
but an evolutionary perspective helps to understand and
predict which individuals will respond best to each therapy
and how best to focus limited screening resources. Finally,
genes of medical relevance are often sites of past selection,
and evolutionary analyses of human diversity can offer a
shortcut to identifying these important regions of the
genome.