Biology Reference
In-Depth Information
To conduct a population history study, the most straightforward way is to deliberately
search out genomic regions that have not been affected by natural selection, that is, regions
that are “selectively neutral.” A classic study by Cann and colleagues (1987) provides a good
example: These authors were the first to use mitochondrial DNA (See What Constitutes
Genetic Data?, below) to argue that modern humans must have arisen in Africa about
200,000 years ago. They did this by looking at variation within the mitochondrial genomes
of 147 people distributed worldwide, and found that individuals from the sub-Saharan
part of Africa exhibited the most amount of genetic variation. More variation requires
more time to accumulate. From an estimate of the average mitochondrial DNA mutation
rate, the authors were able to say something about how long that variation must have taken
to accumulate
d
about 200,000 years.
In contrast, for a study of adaptive history, researchers target genomic regions thought to be
in some way largely genetically responsible for specific trait(s). A recent example is the study
by Perry and colleagues (2007), which tapped into the idea that individuals can (and do) have
multiple copies of a gene, and that population differences can exist in the average number of
copies that individuals will have. In their study, the researchers looked at the number of copies
of the particular gene responsible for the production of salivary amylase proteins among indi-
viduals in different populations. It turns out that higher numbers of copies of the salivary
amylase gene tend to be found in populations that have historically relied on high starch diets,
and lower numbers of copies in those that have not. The inference here is that natural selection
has favored higher numbers of copies in those groups relying on starchy foods.
Genotype and Phenotype
Genotype refers to the heritable information carried by all living organisms. Phenotype is
the product of the interaction between genotype and “environmental” factors. Some pheno-
types are almost completely determined by heredity, such as blood groups (discussed
earlier), but most, such as human height, are also greatly impacted by nonhereditary factors,
such as nutrition.
At a fundamental level we are concerned with changes in the genotypes that make up
a population from generation to generation when talking about evolution. Additionally, since
an organism's genotype generally affects its phenotype, the phenotypes that make up the
population are also likely to change.
To Learn More
.
There is much, much more to learn about the topics of evolution and evolutionary theory,
including population genetics. Unfortunately this cannot all be covered within a subsection
of a single book chapter. Please consult the myriad of existing textbooks on evolution (such as
Futuyma, 2009
or
Ridley, 2004
) and population genetics (for example,
Hartl and Clark, 2006
).
2. Method
What Constitutes Genetic Data?
What follows is a detailed explanation of those aspects of the human genome on which
researchers are currently focused. This explanation applies to researchers who deal with
DNA from living, recently deceased, and ancient organisms.
