Encyclopedia of Evolution.pdf - Online Reading Center

DNA (evidence for evolution)
genes, which lost their genetic function further back in the
past. The age of a pseudogene can be determined by molecular
clock techniques.
DNA is found in the nucleus of each cell of complex
organisms (see eukaryotes, evolution of). It is also found
in the mitochondria of eukaryotic cells, and the chloroplasts
of photosynthetic eukaryotes, because mitochondria and
chloroplasts used to be free-living bacteria (see symbiogenesis).
Mitochondrial DNA (mtDNA) is inherited only maternally,
that is, through egg cells. Phylogenetic analyses based
upon mtDNA reconstruct only the matrilineal history of a
species. This can be a convenient simplification in the study
of the evolution of sexual species, in which nuclear DNA is
reshuffled each generation. In mammals, the Y chromosome
is inherited only paternally (see Mendelian genetics). Studies
of mutations in Y chromosomes have been used to reconstruct
the patrilineal evolutionary history of humans.
Mitochondrial DNA mutates 20 times faster than nuclear
DNA. More mitochondrial mutations accumulate per million
years than nuclear mutations. Noncoding DNA (which makes
up a large proportion of the Y chromosome) experiences
mutations at the same rate as genetic DNA, but the mutations
accumulate. Therefore mtDNA and the Y chromosome
are often used in evolutionary studies of the modern human
species, which has existed only for about 100,000 years (see
Homo sapiens).
Because DNA usually decomposes quickly, scientists
perform most DNA comparisons on specimens from living
or recently dead organisms. In some cases, they can obtain
enough DNA from ancient specimens to allow comparisons.
It is usually DNA from mitochondria or chloroplasts that is
sufficiently abundant in ancient specimens to allow such comparisons.
Mitochondrial DNA from Neandertal bones more
than 30,000 years old (see below) and chloroplast DNA from
leaves almost 20 million years old have been recovered and
studied.
How do evolutionary scientists compare different samples
of DNA?
• The actual base sequence (of A, C, T, and G bases) of each
sample can be determined. Then the base sequence of one
species is lined up with the base sequence of the other species,
for a specified part of a chromosome. The number of
similarities as a proportion of the total number of comparisons
serves as a measure of evolutionary relatedness. Once
a laborious process, DNA sequencing is now automated.
However, it remains expensive. For this process to be used,
the DNA must be amplified, that is, copied over and over
in a test tube. This can only be done if the correct primers
are used. Primers are needed because the enzyme that copies
the DNA cannot create a whole new DNA strand but
can only lengthen a strand that already exists; the primer
is that short strand that the enzyme lengthens. It usually
takes a lot of work to find, or design, the correct primers;
once this has been done, it is generally easy (at major
research sites with the right equipment) to determine base
sequences. Once the base sequences are determined, they
are submitted to a worldwide data bank. Researchers can
make computer-based comparisons among any of the millions
of sequences in the database (see bioinformatics).
• The DNA can be broken up into fragments of various
lengths. The fragments can be separated out from one
another on the basis of their size, on a gel subjected to an
electric current. The process of gel electrophoresis that is
used to separate DNA fragments from a mixture is very
similar to the process that separates proteins from a mixture.
This produces a DNA fingerprint that looks something
like a supermarket bar code. DNA fingerprints are
usually used to identify criminals; if the suspect’s DNA
fingerprint matches that of the DNA sample at the crime
scene, after more than one independent test, it is virtually
certain that the DNA came from this individual. DNA
fingerprints of different species can be compared by this
method. Two organisms with similar fingerprints are closely
related, while two organisms with very different fingerprints
are more distantly related, by evolutionary descent.
• Two samples of DNA can also be compared by reannealing.
Each DNA molecule in an organism consists of two
strands that match perfectly or almost perfectly. These
strands can separate if an investigator heats them in the
laboratory. The investigator can mix the DNA from two
different organisms, heat it, and allow the strands to come
back together, or reanneal, into pairs. Sometimes the DNA
molecules come back together in their original pairs. But
sometimes the DNA strands from the two organisms form
pairs. When the two DNA strands from the different species
match closely, they bind tightly, and a higher temperature
is required to separate them. When the two DNA
strands do not match closely, they bind loosely, and a lower
temperature is required to separate them. There is roughly
a 1 degree Celsius difference in reannealing temperature
for each 1 percent difference in the base sequences of the
two DNA strands. The original evolutionary comparisons
of DNA were conducted before the invention of biotechnology
techniques and relied on reannealing data, but the
method is much less common in evolutionary studies today.
DNA comparisons among species of organisms have
revealed some fascinating insights into evolutionary history.
The following are examples of research that has used the
techniques described above.
Evolutionary Patterns of Life
Origin of mitochondria and chloroplasts. The laboratory of
Carl Woese (see Woese, Carl R.) compared DNA sequences
of many species of bacteria, as well as the DNA from mitochondria
and chloroplasts. His results show that mitochondria
are related to aerobic bacteria and chloroplasts are related to
cyanobacteria (see bacteria, evolution of). Woese’s results,
and similar results of other researchers, confirm the symbiogenetic
theory (see Margulis, Lynn) of the origin of chloroplasts
and mitochondria.
Tree of life. Woese’s laboratory also found, by comparing
DNA sequences from many species of bacteria and
eukaryotes, that the diversity of life falls into three major
groups or domains: the archaea (see archaebacteria), the