The Molecular Clock Hypothesis
A few questions....
I. RATES OF MUTATION PER UNIT TIME
As I understand it, the molecular clock hypothesis asserts that the
amount of interspecific variation in molecular sequences (eg., amino
acid sequences in haemoglobin, cytochromes etc.) is proportional to
the time which has elapsed since the separation of the evolutionary
lineages leading to the two species being compared from their common
ancestor. In other words, the mutations effecting these molecular
sequences take place at very nearly a constant rate per unit time, in
widely different species. If this were not the case, the observed
pattern of interspecific sequence variation would be quite different.
Now, mutations may be caused by external factors (e.g., cosmic rays),
or by copying errors, either in the lifetime of the cell (during
which time, I've been informed, a certain amount of replication goes
on) or during cell division. If we suppose that cosmic radiation is
fairly constant, then mutations due to cosmic rays will occur at a
roughly constant rate per million years.
However, different species reproduce at vastly different rates, e.g.,
elephants, field mice, bacteria, bristlecone pines... Can we assume
that the rate of cell division in the cellular lineage leading up
to reproduction is exactly the same in a bristlecone pine as it is
in a field mouse? I doubt it, but I have no information on the
rates of cell division in different species. I would imagine that
they vary considerably, perhaps by several orders of magnitude,
making the rate of cell-division copying errors far greater in the
more rapidly multiplying species.
I have no knowledge of the frequency of genetic replication during
the lifetime of a cell (i.e., between cell-divisions), so I can only
wonder as to its magnitude relative to cell-division copying errors,
and whether or not it is the same in different cells, and in in
different species. I don't even know whether or not it actually
effects all the genes in a cell, or just a proportion of them, which
are directly connected with its functioning.
Suppose that, in any particular species, in any one year:
1). cosmic radiation causes X mutations
2). there are Z cell divisions in the cellular lineage leading up to
reproducution, and at each cell division, (on average) Y copying
errors take place.
3). Between cell divisions, there are P replications of the genetic
material of the cell, and at each replication (on average) Q
copying errors take place.
Then the total number T of mutations per year is given by:
T = X + PQ + YZ
I want to know:
1). the relative magnitudes of P, Q, Y and Z
2). whether or not the terms P, Q, Y and Z are the same in different
species (especially, in species which reproduce at different
rates). Examples of different values would be useful.
Clearly, T can only be the same for two different species if the
terms on the right hand side of the equation are all constant, or
if the term which varies is very small, relative to the other terms.
If any term is large and varies considerably, then T must also vary
indication - a serious flaw in the molecular clock hypothesis.
Another question:
If X (the cosmic radiation mutation rate) is a major factor, one
would expect slower "rates" of the molecular clock in species which
are protected from cosmic rays - e.g., cave-living, and deep-sea
fish. Is there any evidence of this?
II. INTRA-SPECIFIC SEQUENCE VARIATION IN "LIVING FOSSILS"
If, as the molecular clock hypothesis asserts, sequence variations
between two organisms are dependend only on the time which has
elapsed since they separated from their common ancestor, one would
expect that in species which have survived for a remarkably long
period of time, such as the lungfish, coelocoanths and horse-shoe
crabs, there would be much more intra-specific sequence variation
than there is in more recently evolved species. Is there any data
on the amount of intra-specific variation in these species, as
compared to, say, homo sapiens?
III. STUDIES OF ANCIENT DNA SAMPLES
I read a news item in _New Scientist_ a few months ago, about a
study being carried out on DNA samples taken from bees which had
been preserved in amber, some 17 million years ago. Have the
results been published yet? I would be very interested to know
the extent of the sequence differences between these DNA samples
and the DNA of the most closely related modern bees. Is it
consistent with the molecular clock hypothesis (i.e, a degree of
difference proportional to the time elapsed, according to the
currently estimated rate of change) - or is it closer than was
originally expected?
I would like to thank, in advance, anyone who can help me on these
points. I do not have access to a university library (I could, but
it would cost me 100 pounds a year), or I would look up the data in
the Journal of Molecular Evolution, etc..
Andrew :-)*
