Encyclopedia of Evolution.pdf - Online Reading Center

DNA (raw material of evolution)
DNA is the basis of inheritance and of evolution. DNA, or
deoxyribonucleic acid, is important to evolutionary science in
four ways:
• Because of DNA, traits are heritable.
• Because of DNA, traits are mutable.
• The study of DNA allows comparisons of evolutionary
divergence to be made among individuals (see DNA [evidence
for evolution]).
• The study of DNA allows the genetic variability of populations
to be assessed (see population genetics).
In order for natural selection to work on traits within
a population, those traits must be heritable (see natural
selection). Characteristics that are induced by environmental
conditions, though sometimes called adaptations, are
not heritable (see adaptation). Scientists, and most other
people, have long known that traits are passed on from one
generation to another, that organisms reproduce not only
“after their own kind” but that the offspring resemble their
parents more than they resemble the other members of the
population. Some scientists in the 17th century believed that
tiny versions of organisms were contained within the reproductive
cells, such as homonculi within sperm; that is, entire
structures were passed on from one generation to another,
a theory called preformation. Other scientists believed that
structures formed spontaneously from formless material,
a theory called epigenesis. They were both partly right and
partly wrong. In the 20th century scientists discovered that
the instructions for making the structure, rather than the
structure itself, were passed from one generation to another
through the reproductive cells.
In the 19th century, many scientists believed that characteristics
that an organism acquired during its lifetime could be
passed on to later generations. The scientist most remembered
for this theory is Jean Baptiste de Lamarck (see Lamarckism).
The inheritance of acquired characteristics was not his
special theory, but rather the common assumption of scientists
in his day. Until Gregor Mendel (see Mendel, Gregor;
Mendelian genetics), nobody had performed experiments
that would adequately test these assumptions. Charles Darwin
(see Darwin, Charles) was so perplexed and frustrated
by the general lack of scientific understanding of inheritance
that he invented his own theory: pangenesis, in which “gemmules”
from the body’s cells worked their way to the reproductive
organs and lodged there, carrying acquired genetic
information with them. His theory was essentially the same
as Lamarck’s, and equally wrong. Darwin either had never
heard of Mendel’s work or overlooked its significance.
How DNA Stores Genetic Information
By the early 20th century, data had accumulated that a chemical
transmitted genetic information from one generation to the
next, and that the chemical was DNA. In 1928 microbiologist
Frederick Griffith performed an experiment in which a harmless
strain of bacteria was transformed into a deadly strain of
bacteria by exposing the harmless bacteria to dead bacteria of
the deadly strain. Some chemical from the dead bacteria had
transformed the live harmless bacteria permanently into generation
after generation of deadly bacteria. Research in 1944 by
geneticist Oswald Avery and associates established that it was
the DNA, not proteins, that caused the transformation that
Griffith had observed. Research in 1953 by geneticists Alfred
Hershey and Martha Chase established that it was the DNA,
not the proteins, of viruses that allowed them to reproduce:
The DNA from inside the old viruses produced new viruses,
while the protein coats were merely shed and lost.
Many scientists doubted that DNA could be the basis
of inheritance and suspected that proteins might carry the
genetic information. DNA was a minor component of cells,
compared to the abundance of protein. Furthermore, DNA
was a structurally simple molecule, compared to proteins,
and was thus considered unlikely to carry enough genetic
information. It was not until the structure of DNA was
explained by chemists James Watson and Sir Francis Crick in
1953, based upon their data and data from colleagues such as
chemist Rosalind Franklin, that DNA became a truly believable
molecule for the transmission of genetic information.
DNA is an enormously long molecule made up of
smaller nucleotides (see figure on page 133). Each nucleotide
consists of a sugar, a phosphate, and a nitrogenous base. In
DNA, the sugar is always deoxyribose. The nitrogenous base
in a DNA nucleotide is always one of the following: adenine,
guanine, cytosine, or thymine. The general public has been
well exposed to the abbreviations of these bases (A, G, C,
and T). The nucleotides are arranged in two parallel strands,
like a rope ladder. The parallel sides of the ladder consist
of alternating molecules of phosphate and sugar. The rungs
consist of the nitrogenous bases meeting together in the center.
A large base is always opposite a small base, and the correct
bonds must form; therefore, A is always opposite T, and
C is always opposite G. Because of this, both strands of DNA
contain mirror-images of the same information: if one strand
is ACCTGAGGT, the other strand must be TGGACTCCA.
DNA not only stores information but stores it in a stable
fashion: All of the information in one strand is mirrored in
the other strand. If mutations occur in one strand, the base
sequence in the other strand can be used to correct them.
Mutations in DNA are usually but not always corrected. All
cells use DNA to store genetic information. Therefore the
mutations that occur in one strand are frequently corrected
by an enzyme that consults the other strand.
Mutations occur relatively infrequently, and evolution
proceeds slowly, in all species of organisms. Some viruses
(which are not true organisms) use a related molecule, RNA
(ribonucleic acid), to store genetic information. RNA is single-stranded,
and its mutations cannot be corrected. Because
of this, RNA viruses evolve much more rapidly than DNA
viruses. RNA viruses such as colds and influenza evolve so
rapidly that the human immune system cannot keep up with
them. This is why a new flu vaccine is needed every year.
Last year’s flu vaccine is effective only against last year’s
viruses. In contrast, DNA viruses such as the ones that cause
poliomyelitis (polio) evolve slowly enough that old forms of
the vaccine are still effective. The human immunodeficiency
virus is an RNA virus and evolves rapidly (see AIDS, evolution
of).
DNA is capable of replication. If the two strands separate,
new nucleotides can line up and form new strands that
exactly mirror the exposed strands. In this way, one DNA