Re: Bacterial Evolution Question


Ken Shackleton wrote:
Seanpit wrote:
Dave wrote:
Is the evolution of antibiotic bacteria micro- or macro-evolution?

I would think that it is macro- because the species has changed.

The terms "micro-" and "macro-evolution" are confusing because of this
very thing. The demarcation of genus and species is too subjective. It
is bit easier, though, to talk about different types of functional
systems. Can more and more functionally complex systems evolve? If so,
how much time, on average, does it take for functional systems at a
particular level of complexity to evolve as compared to systems at a
higher or lower level of minimum size and specificity requirements?

As it turns out, antibiotic resistance, when it does evolve de novo, is
almost always the result of a block of a pre-established system of
function, such as the interaction of the antibiotic with a target
sequence. Mutations that can give rise to interference between the
antibiotic-target interaction are relatively common and can be realized
in relatively short order. That is why bacterial resistance can evolve
to all kinds of antibiotics very quickly - because such evolution does
not require the formation of a novel function, only the breakdown of a
pre-established function/interaction. And, as even children
understand, it is much easier to break Humpty Dumpty than to put him
back together again - or even in the first place.

But why is this? Why is it easier to break something than to create it?
Because, there are so many different ways to break something or
interfere with a functional system relative to the very few ways there
are to put it together in such a way that it "works" properly.

Such forms of "higher-level" evolution are much harder to demonstrate.
Evolution that is not based on the disruption or loss of some other
pre-established function just doesn't happen as easily. In fact, as one
moves up the ladder of functional complexity, evolutionary powers of
creativity simply stall out in a rather dramatic manner - exponentially
in fact. Review the literature and you will find that there simply are
no observed laboratory examples of novel functions evolving that
require more than a few hundred fairly specified amino acid residues at
minimum.

Those novel independent functions that have been shown to evolve in
real time, like lactase or nylonase enzymatic-type functions, require a
minimum of no more than 3 or 4 hundred fairly specified residues. Even
these types of relatively "simple" functions have their significant
limitations when it comes to evolutionary potential. That's right, not
all types of bacteria can evolve the lactase function no matter how
much time is provided and no mater how much selective advantage would
be gained. Compare this limited evolutionary potential of many types of
bacteria for lactase evolution with the potential to evolve resistance
to a particular antibiotic by every single type of bacteria in short
order. What's the difference?

The only difference is that one type of function requires the loss of a
pre-established function while the other type requires a gain of a
truly novel function that is not based on the loss of any other
function.

Beyond such low levels of functional complexity, evolution just doesn't
happen. Statistically, functions that require just a few thousand
fairly specified residues working together at minimum cannot evolve
this side of trillions upon trillions upon trillions of years of time -
on average. Why? - Because of what I like to call the
neutral/detrimental gap problem. If you are interested I discuss this
concept in much greater detail on my website.

When I read this.....the message I get is this:

*Evolution doesn't happen in huge, single leaps.*

This does not seem like an unreasonable position to take.....but how
does this prevent small, incremental changes each generation from
adding up to very large changes over time?

Ken



Sean Pitman
www.DetectingDesign.com

Some small percentage of bacteria are naturally more resistant to the
drugs. If the drug is administered for too short a time, the most
resistant ones will recover and restart the infection, but the second
infection will be of descendants of the resistant forms. Subsequent
adminsitration of the drug will be less effective in eradicating the
infection. This has nothing to do with 'evolution' per se, but rather
with selecting the more resistant forms. The 'evolution' occurs in the
mutations that make the bacteria resistant to start with. In nature,
those forms would typically be a small percentage. Anti-biotic drugs
are selected and tested against the bacteria for efficacy. Those that
has no efficacy are rejected. The bacteria do the same thing. The forms
that are susceptible die off, leaving those that are less so. It's
basically a war.

.

Relevant Pages

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  • Re: Bacterial Evolution Question
    ... infection will be of descendants of the resistant forms. ... This has nothing to do with 'evolution' per se, ... Anti-biotic drugs ... are selected and tested against the bacteria for efficacy. ...
    (talk.origins)