>You (Pim van Meurs) wrote:
>> [...] it has since been shown that IC systems can "evolve" gradually.
>
>Please refer me to a source that shows that irreducibly complex systems can
>evolve gradually through non-artificial selection or other mechanisms that
>do not employ intelligent causal agents.
There have been a few groups trying to understand how antibiotic
resistance can persist in bacterial populations. In many cases,
when the resistance traits are moved into previously sensitive
strains, it is found that the traits can reduce the relative
fitness of such strains when grown in environments free from the
antibiotic. If that was the whole story, then one would expect
that cells carrying the resistance genes to be reduced in number
or completely eliminated from a population which is no longer
exposed to the antibiotic.
However, people have found that antibiotic resistant strains can
persist in the population long after the antibiotic is gone.
Further, they've found that these strains grow just as well as
the original, "wild-type" strains that lack the resistance
genes. Researchers found that secondary mutations arose in these
strains which compensated for the problems generated with the
original resistance trait.
at least one case, if the antibiotic resistance mutation is
"fixed" (returned to the original state), the remaining compensatory
mutation can actually reduce the growth rate of the bacterium. In
these cases, both mutations are required for optimal growth.
Either way, these systems may be taken as examples of irreducibly
complex systems (minimallly of two components) that apparently
evolved gradually (ie. in steps) and naturalistically. The function
in this case is the the ability to grow well both in the presence and
absence of the antibiotic. The "IC nature" of the mutations is most
obvious in the cases where "fixing" only one of the sites in the
doubly-mutant strains leads to a bacterium with reduced growth rates.
This example was discussed briefly in this group last September.
See Glenn Morton's post of:
http://www.calvin.edu/archive/evolution/199809/0446.html
In talk.origins, Ian Musgrave probably gave the best and
most recent description. This article/thread provides also
provides better references to the literature. See:
http://www.dejanews.com/[ST_rn=ps]/getdoc.xp?AN=444806765
Aside: Suppressor mutations of this sort have been used as tools
for a long time in studies of genetics and biochemistry.
Regards,
Tim Ikeda
tikeda@sprintmail.hormel.com (despam address before use)