From Wikipedia,
the free encyclopedia.
The molecular clock
(based on the molecular clock
hypothesis (MCH)) is a
technique in
genetics, which researchers
use to date when two
species
diverged. It deduces elapsed
time from the number of minor
differences between their
DNA sequences.
The notion of a "molecular
clock" was first attributed to
Emile Zuckerkandl and
Linus Pauling who, in
1962, noticed that the
quantity of
amino acid differences in
hemoglobin between lineages
roughly matched the known
evolutionary rate of divergence
based upon fossil evidence. They
generalized this observation to
assert that the rate of
evolutionary change of any
specified
protein was approximately
constant over time and over
different lineages. It has been
applied to
DNA
sequence evolution also,
particularly
neutral evolution.
Later
Allan Wilson and
Vincent Sarich built upon this
work and the work of
Motoo Kimura (1968) observed
and formalized that rare
spontaneous errors in
DNA replication cause the
mutations that drive
molecular evolution, and that
the accumulation of evolutionarily
"neutral" differences between two
sequences could be used to measure
time, if the error rate of DNA
replication could be calibrated.
One method of calibrating the
error rate was to use as
references pairs of groups of
living species whose date of
speciation was already known from
the fossil record.
Originally, it was assumed that
the DNA replication error rate was
constant--not just over time, but
across all species and every part
of a
genome that you might want to
compare. Because the enzymes that
replicate DNA differ only very
slightly between species, the
assumption seemed reasonable
a priori. As molecular
evidence has accumulated, the
constant-rate assumption has
proven false--or at least overly
general. However while the MCH
canot be blindly assumed to be
true, it does hold in many cases,
and these can be tested for. For
example, molecular clock users are
developing workaround solutions
using a number of statistical
approaches including
maximum likelihood techniques
and later
Bayesian modeling.
The molecular clock technique
is an important tool in
molecular systematics, the use
of
molecular genetics information
to determine the correct
scientific classification of
organisms. Knowledge of
approximately-constant rate of
molecular evolution in particular
sets of lineages also facilitates
establishing the dates of
phylogenetic events.
See also
References
-
Kimura, M. (1968).
Evolutionary Rate at the
Molecular Level.
Nature 217: 624-626.
-
Sarich, V.M. and
Wilson, A.C. (1967).
Immunological time scale for
hominid evolution.
Science 158: 1200-1203.
- Zuckerkandl, E., and
Pauling, L. (1962).
Molecular disease, evolution,
and genetic heterogeneity,
pp. 189–225 in Horizons in
Biochemistry, edited by M.
Kasha and B. Pullman. Academic
Press, New York.
- Zuckerkandl, E., and
Pauling, L. (1965).
Evolutionary divergence and
convergence in proteins, pp.
97–166 in Evolving Genes and
Proteins, edited by V.
Bryson and H. J. Vogel. Academic
Press, New York.
