Encyclopedia of Evolution.pdf - Online Reading Center

molecular clock
these scientists did not reject gradualism as an evolutionary
mechanism, they did reject it as a macroevolutionary pattern.
They claimed that major evolutionary changes occurred
when rapid microevolution (directional selection) occurred
(punctuations), followed by long periods of stasis (stabilizing
selection). Evolutionary scientists are increasingly accepting
punctuated equilibria.
Another assumption that accompanied the modern synthesis
in the minds of most scientists was that the variation
upon which natural selection acted was supplied entirely by
mutations in existing genomes. At the same time that the
modern synthesis was forming among Western scientists,
some Russian geneticists such as Boris Kozo-Polyansky were
proposing that a few major evolutionary innovations had
occurred by what is now called symbiogenesis. Lynn Margulis
(see Margulis, Lynn), building upon the work of Russian
scientists such as Kozo-Polyansky and upon the work
of the American biologist Ivan Wallin, demonstrated that
mitochondria and chloroplasts were the evolutionary descendants
of endosymbiotic bacteria. While symbiogenesis in no
way contradicts the mechanism of natural selection, it does
present new possibilities for the origin of new genetic variation
upon which natural selection acts. Margulis continues
to point out that symbiogenesis may be much more common
than evolutionary scientists have generally appreciated.
The modern synthesis not only brought Mendelian genetics
together with Darwinian evolution but has also revolutionized
conservation biology. Rare species often suffer from
a lack of adequate genetic diversity in their populations; this
affects both their genetic characteristics (harmful mutations
show up in the organisms) and their ability to keep evolving
in response to environmental changes and other species (particularly
parasites) (see extinction; red queen hypothesis).
Modern ecologists, as well as modern evolutionary scientists,
can thank the pioneers of the modern synthesis, because they
now know that to save a species, one cannot merely take two
of every kind onto an Ark but must save whole populations.
Further Reading
Dobzhansky, Theodosius. Genetics and the Origin of Species. New
York: Columbia University Press, 1937.
Fisher, R. A. The Genetical Theory of Natural Selection. Oxford:
Oxford University Press, 1930.
Haldane, J. B. S. The Causes of Evolution. London: Longmans,
Green, 1932.
Huxley, Julian S. Evolution: The Modern Synthesis. London: Allen
and Unwin, 1942.
Mayr, Ernst. Systematics and the Origin of Species. New York:
Columbia University Press, 1942.
———, and William B. Provine, eds. The Evolutionary Synthesis:
Perspectives on the Unification of Biology. Cambridge, Mass.:
Harvard University Press, 1998.
Simpson, George Gaylord. Tempo and Mode in Evolution. New
York: Columbia University Press, 1944.
Stebbins, G. Ledyard. Variation and Evolution in Plants. New York:
Columbia University Press, 1950.
Wright, Sewall. “Evolution in Mendelian populations.” Genetics 16
(1931): 97–159.
molecular clock A molecular clock technique uses
changes in biological molecules as a measure of the passage
of time. Two species that differ only slightly in their
molecular makeup diverged from a common ancestor more
recently than two species that differ greatly in their molecular
makeup. Molecules such as DNA (see DNA [evidence
for evolution]) can thus be used to reconstruct evolutionary
history (see cladistics). If the assumption is made and
confirmed that the molecules change at a constant rate over
time, the degree of divergence between two species can also
be used as a molecular clock to indicate how many years ago
the divergence occurred. The technique was first proposed by
chemists Emile Zuckerkandl and Linus Pauling in 1962.
The rate at which molecules change over evolutionary
time can be influenced by the population size and the generation
time. In larger populations, in which there is less genetic
drift (see founder effect; population genetics), the molecules
may change more slowly over time. If the molecules
change each generation, the changes would occur more rapidly
in species with short generation times. As Japanese geneticist
Tomoko Ōta pointed out, species with large populations
tended to have short generation times, and species with small
populations tended to have long generation times. It is therefore
possible that the effects of population size and generation
time on the rate of molecular evolution effectively cancel
one another out.
Among the difficulties encountered by the molecular
clock hypothesis are:
• The rate of change may not be constant over time (the
clock speeds up or slows down).
• The rate of change is different for different kinds of molecules
(some clocks are faster or slower than others). For
example, among proteins, fibrinopeptides evolve faster
than globins, which evolve faster than cytochrome c, which
evolves faster than histones. Histones are components of
chromosomes and are constrained from evolving rapidly
because their exact structure is important in the processes
of cell division such as mitosis and meiosis.
• The rate of change can be influenced by natural selection.
Molecular clock studies use neutral variations that
experience genetic drift rather than natural selection. For
example, the DNA sequences used in molecular clock studies
often come from noncoding DNA.
The clock must be calibrated. To do this, investigators
must correlate the time at which the divergence of the variant
forms of the molecule began with a date in the fossil record
provided by radiometric dating. One problem with this
approach is that the divergence of the molecules begins earlier
than the visible differences among organisms in the fossil
record. A molecular clock can be calibrated only by a minimum
age (that is, the time of divergence must be older than
the date determined from the fossil record). Once a molecular
clock is calibrated, it can be used for comparisons among species
for which the fossil record is inadequate.
Linkage disequilibrium can also be used as a molecular
clock (see Mendelian genetics). Two DNA sequences
(such as markers) that are linked on the same chromosome