MadSci Network: Evolution

Re: Can we mathematically estimate how long evolution should take?

Date: Tue May 16 09:29:54 2000
Posted By: Donald Terndrup, Faculty, Astronomy, Ohio State University
Area of science: Evolution
ID: 958151396.Ev

Hi --

There have been many studies of how long it would take for some particular
evolutionary change to be achieved, but these normally are focussed on the
development of particular organs or the time it takes to adapt to new
environments.  Dawkins in several of his books discusses calculations for
how long (specifically how many generations) may be needed to convert a
crude eye into a more sophisticated imaging device, or other examples.

To calculate how long it would take to go from a single cell to a human
being, you would have to know ALL the genetic changes which took place
over history.  This is not possible yet, since we have not mapped out the
human genome fully, nor do we understand the interactions between genes
(There is rarely one gene "for" individual traits.  Genes act in concert,
and we are just now understanding some of the complexities of these

Even in the restrictive case of trying to compute (say) the development
of the eye, there are many surprises.  These calculations use "toy" or
"model" genes, which may or may not correspond to real genes, but the hope
is that these calculations give about the right time scales.  The
calculations show that evolution can proceed surprisingly fast.  For 
example, if a gene arises which produces a 1% improvement in visual
acuity, so that (say) there is a 1% differential reproductive success,
then the animals with the better eye will completely dominate the 
population in only 1000 generations.  Since many species reproduce annually
or faster, these 1000 generations can occur in a very brief span of
geological time.  If you have 10,000 or 100,000 generations, then there
can be a tremendous improvement in the eye (or any other function) in a
very short period of time.

When you are faced with the question "how could purely random mutations 
create such complex beings?" you need to remember that the random
mutations are only the first step.  The environment imposes a superstrong
filter on these mutations, so that only those which improve the chances
of surviving to reproduction are those which actually make it.  The
filtering by the environment is decidedly NONRANDOM:  true there are
occasionally accidents, but most of the time survival is determined by
an animal's (or plant's) success in finding food, keeping enemies away,
and reproducing.

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