>
>
>On Mon, 09 Oct 2000 08:25:28 -0500, Susan Brassfield Cogan wrote:
>
>[...]
>
> >SJ>Susan said "we can watch ... natural selection happen every day ..."
> Wesley
> >>said "Natural selection, though, is notoriously difficult to empirically
> >>isolate as a mechanism of action."
> >>
> >>How can Susan "watch" something "happen every day" that is "notoriously
> >>difficult to empirically isolate"? IOW how does Susan know she is watching
> >>"Natural selection"?
>
>SB>Wesley was talking about isolating natural selection from drift and my
> >understanding was that he was talking about populations over a large amount
> >of time.
>
>Agreed. And I am talking about how Susan isolates "natural selection from
>drift" and how Susan can "watch ... natural selection happen every day ..."
>in only individuals in her home (see below).
>
>Why does Susan think that Wesley *needs* to see "natural selection" in
>"populations over a large amount of time"?
>
>SB>You see natural selection every time you spray for roaches or
> >don't take all your antibiotics.
>
>I would have no problem with this if it *was* natural `selection', i.e.
>differential survival and reproduction.
>
>But as a matter of interest how does Susan *see* "natural selection
>*every* time you": 1. "spray for roaches" or 2. "don't take all your
>antibiotics"?
>
>In the case of 1. if a "roach" survives after Susan sprayed it with
>insecticide, how does Susan "see" that it did not survive because she
>did not spray it properly? Also how does Susan "see" that the "roach"
>does not die later without offspring?
Chris
You've inverted selection. Organisms are selected *out* of the population,
not *into* it. If roaches are sprayed and some die, they are selected out.
It really doesn't matter if she sprayed "properly" or not. We know that
many roaches *are* killed by the poisons, and that some are not. In nature,
selection is not determined on the basis of whether some poison is
"properly" applied.
The effect over many sprayings is that the ones that are resistant, or that
move elsewhere, or that hide better (etc.) will be the ones that, on
average, are less often sprayed "properly."
But, in fact, though it doesn't effect the principle, it often *is*
possible to make such determinations, at least scientifically. That's why
they have to keep coming up with better roach poisons; the old ones are now
much less effective. Theoretically, the bug-sprayed bugs could just
*happen* to be the populations that genetically "drifted" into being more
resistant (while populations that were not sprayed just *happened* to be
the ones that did *not* genetically drift into the same resistance), etc.,
but that highly unlikely, especially if it occurs over many species of
insects.
Further, it does not take a degree in genetics to understand that dogs that
get run over and killed by cars are less likely to reproduce after that age
than other dogs that do *not* get injured at all, other things being equal.
Seeing a dog get killed by a car *is* seeing selection, unless it happens
mainly to dogs past puppy-spawning age, or unless there is some special
advantage (such as getting across the street to a mate) to risking death in
traffic.
>As for 2. how does Susan "see" bacteria, let alone that they have been
>naturally `selected' against the effects of the "antibiotics"?
>
>Finally, if Susan did manage to "see" the above, how does she know
>that what she sees is not natural `selection' but instead genetic "drift"?
Chris
Of course, she did not mean physically seeing bacteria. Or do you think she
*did* mean that?
As to your drift question, the same answer I gave above applies, although,
very likely, it could be extended by observing that, *if* it were mere
drift, the physical changes would have to be such that they are nearly
costless to the bacteria. So, we can test for drift by checking to see how
two populations of the "same" bacteria do under conditions in which the
antibiotic resistance is totally useless. If they consistently do *not* do
as well as the non-resistant ones, we know that there *is* a cost to the
resistance, a cost that is only worth "paying" if there is a serious risk
of an antibiotic.
Suppose you start wearing a full suit of armor in swim meets to protect
yourself against rabid jousters. But, then you enter a different swimming
meet still wearing the suit of armor while the other contestants are
wearing only their Speedos, because there are no rabid jousters around. You
will most likely lose the contest because of the suit of armor. But, if
rabid jousters *do* show up and kill off all of your opponents, you may
win. This is a silly example, but the point is that drift only works as
long as there's no special cost to it.
Further, bacteria become resistant to *specific* antibiotics (with some
resistance to closely-related antibiotics). If a population of bacteria
becomes resistant to antibiotic A and not to B and another population
becomes resistant to B and not to A, and if it happens that the first
population was treated with A and not B, and the second with B and not A,
will you be arguing that they just *happened* to drift in the *specific*
ways needed to counteract only the antibiotics they were treated with? If
so, you *might* be right, but it would be stupid to bet that way, because
the odds would be against you, especially as we increase the number of
quite distinct bacteria that have developed resistance to antibiotics (this
would be truly a "miracle" of genetic drift, for it to be uniform (but with
different genetic mechanisms?) across a wide range of bacteria).
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