Encyclopedia of Evolution.pdf - Online Reading Center

DNA (evidence for evolution) DNA is not only the molecule
that stores information about the physical characteristics
of individuals (see DNA [raw material of evolution]),
but also the molecule that stores a record of evolution in
past generations. From generation to generation, mutations
accumulate in the DNA. If these mutations occur within
genes, natural selection may act upon the organism that
has the mutation. Natural selection may reduce the frequency
of a harmful mutation in a population, although it may never
eliminate it (see population genetics), or it may increase
the frequency of a beneficial mutation. Natural selection does
not affect the frequency of a mutation that has no effect on
the function of the organism (a neutral mutation). Mutations
that occur in the noncoding DNA will also be unaffected by
natural selection. Neutral genetic mutations, and mutations
in the noncoding DNA, accumulate in the DNA as it is passed
from one generation to another (see markers). A comparison
between the coding and noncoding regions of DNA indicates
that there are more differences between humans and mice in
their noncoding DNA than in their genetic DNA (see figure).
Organisms that are part of a single population interbreed
and share genes. Although each organism in a population
is genetically unique, all are genetically similar because they
draw their genes from a single gene pool. But when a population
is separated into two populations, new and different
mutations occur in each of the populations (see speciation).
The new mutations present in one population are not found
in the other. Over time, the DNA of the two gene pools
becomes different. The longer the populations are separated,
the greater the differences in their DNA. If the populations
diverge enough to become separate species, the DNA is even
more different; DNA of two different genera are even more
different; the DNA of two families even more different; and
so on (see Linnaean system). By quantifying how different
the DNA is between two organisms, scientists can determine
DNA (evidence for evolution)
how much evolutionary divergence has occurred since their
separation. If scientists assume that mutations occur at a constant
rate, these mutations constitute a molecular clock.
The DNA molecules have been keeping a cumulative record
of the passage of time that helps scientists to reconstruct evolutionary
history.
Because neutral mutations and mutations in the noncoding
DNA are not related to the structure and function
of organisms, comparisons of DNA and comparisons of
anatomy provide independent data for the reconstruction of
evolutionary relationships. DNA nucleotides, just like the
physical characteristics of organisms, can be used as data in
phylogenetic analysis (see cladistics). Usually, phylogenetic
reconstructions of evolutionary history based upon DNA correspond
closely to phylogenetic reconstructions based upon
physical traits, thus providing independent confirmation of
the evolutionary patterns. This could scarcely have happened
unless evolution really occurred. The DNA confirmation of
evolution is one of the best sources of evidence against creationism.
A creationist could explain the evolutionary patterns
of neutral and noncoding DNA only by claiming that
a Supreme Being created mutations that had an evolutionary
pattern but were otherwise useless.
Evolutionary hypotheses can be tested not only by comparing
DNA sequences but also the presence and absence of
segments of noncoding DNA. The DNA of an organism contains
the DNA from genes used by its ancestors, even though
the organism no longer uses these genes. The functionless
ancestral gene is now called a pseudogene. The presence of
a pseudogene can be evidence of evolutionary ancestry. It
can also be used to quantify evolutionary relatedness. Two
species that share a greater number of the same pseudogenes
share a more recent common ancestor (are more closely
related to one another) than two species that share fewer
pseudogenes. This is particularly true for the older pseudo-
This is a comparison of human and mouse nucleotide sequences for the same genetic region. Capital letters indicate nucleotides that have not changed
since humans and mice evolved from a common ancestor; lowercase letters indicate changes that have occurred since the common ancestor of humans
and mice. Dashes indicate loss of nucleotides in one of the species. There are more differences in the noncoding (intron) region than in the coding region
of DNA. (Modified from Watson)