Re: mathematical models for speed of evolution
- From: "Alan Meyer" <ameyer2@xxxxxxxxx>
- Date: Fri, 29 May 2009 13:50:34 -0400 (EDT)
"kjetil1001" <kjetil1001@xxxxxxxxx> wrote in message
news:gvikif$fkp$1@xxxxxxxxxxxxxxxxxxxxxx
Does anyone have references to mathematical models explaining
speed of evolution, or basic info like how to define speed of
evolution, how to measure it?
I am especially interested in papers confronting theories about
speed of evolution with empirical data.
kjetil halvorsen
I don't have a simple answer to your interesting question. In
fact, I don't have any answer at all. But I may be able to make
some suggestions about the issues involved.
The basic issue, as I see it, with the question you raise is that
the speed of evolution varies with a number of different
variables. These variables have different values over time and
over species. Here are some of them.
1. The rate of mutation in DNA.
This rate depends, to some degree, on the presence of
mutagenic factors. For example, radiation, ultra-violet
light, heat, and different mutagenic chemicals in the
environment or food supply of various organisms.
2. The efficiency of repair of mutated DNA.
This varies from one group of organisms to another and has
evolved over time.
Some complex, "higher" organisms have the ability to repair
mutations so efficiently that only one in perhaps 10^7
mutations go unrepaired (I'm making up that number. I don't
remember the real one, but I think it's somewhere between
10^5 and 10^7, by which I mean ten to the fifth power or to
the seventh power.)
Other much simpler organisms such as bacteria repair far
fewer mutations, e.g., 10^4 (again, I don't remember the real
number) and, for that reason, have a much higher real
mutation rate. Bacteria can survive in spite of their less
capable mutation repair because first, they have fewer genes
so that there is less to go wrong, and second, because there
are so many of them that if a few gazillion die off, it has
no effect on the population.
3. The possibilities for gene flow.
In some organisms, for example bacteria, it is possible for
organisms to exchange genes even if they are not members of
the same species. This is why so many bacteria are now
antibiotic resistant. Plants have some of this capability too.
Mammals have very little of it.
Organisms that can import genes from other species can evolve
really fast.
4. The size of the population.
As I understand it, very large populations of a species have
a tendency to swamp mutations with extensive gene pooling. I
believe that there are mathematical formulas that can be
brought to bear to describe this, but I don't recall what
they are.
Conversely, small isolated populations, for example on
oceanic islands or on mountain tops, may change more rapidly.
5. The stress on a species.
If a species is under stress, for example due to gradual
climate change (a too rapid change may simply wipe out the
species), new adaptations to the stress will be selected and
come to dominate the population. On the other hand if the
environment for a species is the same over many millions of
years, there may be less change.
Some stresses can act very fast to change certain
characteristics of particularly adaptable species. For
example, commercial fishing has, in just the last few
centuries, reduced the size of many fish and shellfish
species because humans catch and eat the big ones.
Commercial agriculture with pesticides has spawned the
development of pesticide resistant insect and other pests,
sometimes within less than a decade of the introduction of a
new pesticide.
As for how to measure the speed of evolution, the most objective
measure that I can think of is divergence of the genome.
Now that we have the ability to sequence genes and whole genomes,
we can compare organisms and compute the degree of difference in
their DNA sequences. Closely related species like chimpanzees
and humans will have very similar DNA. More distantly related
species such as humans and monkeys, humans and mice, humans and
lizards, humans and fish, humans and ants, humans and oak trees,
etc. will have more and more divergence in their DNA sequences.
Under certain assumptions regarding the variables I describe
above, we can make mathematical statements about the speed by
which genomes vary. The speed varies significantly, depending on
what parts of the DNA we look at. Genes that code for life
critical functions are "highly conserved". When these mutate the
organisms die and the mutation dies with them. Other parts of
the DNA, for example parts that are not transcribed into RNA, may
evolve much faster.
Given any model of how fast such change occurs, we can (and
specialists do) compute the approximate date in the past at which
any two species diverged.
I hope some of that helps, and that I've got it right.
Alan
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