I was previously aware that there is genetic evidence
indicating that humans descend from a population of several thousand
individuals (probably about 10,000) rather than just two. In Adam and the Genome: Reading Scripture after Genetic Science, Dennis Venema (a biology
professor and a Christian) explains this evidence in greater detail. The basic
idea is that humans are so genetically diverse in the present day that a large
ancestral population is needed to transmit that diversity to us. Venema
describes a few of the methods scientists have used that support this
conclusion:
- Allele diversity: With this method, scientists measure how
many alleles (gene variants) are found in present-day humans. They then
estimate the ancestral population size that would be needed to produce all
those alleles given the human mutation rate and the mathematical probability of
new mutations spreading in a population or being lost.
- Linkage disequilibrium: This method takes advantage of the
fact that there is a well-characterized relationship between (a) the distance
between two genes on a chromosome, and (b) the likelihood that “crossing over”
(a process of chromosome breakage and rejoining) will occur between those
genes. If two genes are located close to each other on the same chromosome, the
alleles present at both locations tend to be inherited together (i.e., a
crossing over event is unlikely to occur between them). So with this method,
scientists look at the allele combinations that are found in present-day
humans, and then they estimate the ancestral population size that would be
needed to produce those combinations given the crossing-over frequency.
- Incomplete lineage sorting: This method exploits the fact
that we expect the relatedness pattern of certain genes to sit at odds with
what we expect on the basis of species relatedness. For example, although
humans and chimpanzees are the closest living relatives of each other as
species, we expect that some human genes will be closer matches to those of
other great apes, such as gorillas. This allows scientists to infer what
genetic variants were present in the common ancestral populations, which allows
them to estimate population size.
