From: Scott Chase (osteopilus@yahoo.com)
Date: Sat 08 Oct 2005 - 00:37:23 GMT
--- Dace <edace@earthlink.net> wrote:
> Derek,
>
> > Ted
> >
> > To be specific, you commit two kinds of errors in
> your essay:
> >
> > 1) misrepresentation of neo-Darwinism
> > 2) premature declaration that some kind of
> > "memory" theory would 'explain' ontogenesis
> >
> > The grossest misrepresentation of neo-Darwinism
> > (there are several more minor ones) is in your
> > use of Hoyle's fallacy. For instance,
> >
> > "The Hyacinth macaw can crack a nut with its beak
> > that you or I would need a sledgehammer to open.
> > Is all that colossal strength nothing more than a
> > side-effect of a chance mutation in the macaw's
> > genetic toolkit? How many millions of such coding
> > mistakes had to come and go before the right one
> > announced itself, and at last the bird got its
> meal?"
> >
> > and again later:
> >
> > "Like a toy in a cereal box, every defining trait
> > of every species on Earth comes with a special
> > mutation hidden inside. Genes, you might say,
> > work in mysterious ways. We don't know why the
> > right mutation comes along at the right momentit
> just does!"
> >
> > and again:
> >
> > "What about the creation, from scratch,
> > of trillion-celled furry animals with big
> > ears and buck teeth? Apparently, DNA is the one
> > thing that really can pull a rabbit out of its
> hat."
> >
> > No scientist proposes any of the above 3
> > scenarios. None ever has - even the
> > "mutationists" of the 1920s had a much more
> > refined and sophisticated view than the one you
> > claim we have today. Mutationism went out the
> > window when Fisher showed that Mendelian genetics
> > did after all fit natural selection.
>
> You've taken my statements out of context. I make
> it plain in my article
> that such mutations merely provide the raw material
> for natural selection.
> The question is the source of the variations
> environmentally selected.
>
Provision and source are one and the same here.
Mutation provides variation and it is a source.
>
> Is
> it random mutations in our nuclear "determinants,"
> as Weismann claimed, or
> is it the adaptations made by creatures in the
> course of their lives, as
> Darwin claimed? Clearly, the second accords better
> with common sense. A
> theory that can account for transmission of acquired
> adaptations is
> inherently more plausible than Wiesmann's
> alternative.
>
I think you're getting confused about two separate
phenomena. There's variation and there's a selection
process resulting in adaptation(s). Mutation is a
significant source of the raw genetic variation in a
population. We can think of changes being represented
as alleles at gene loci. A specific locus can have
more than two alleles for a givwn locus, but each
individual organism, if diploid, will have only two
alleles represented. If homozygous they will have two
of the same allele at the locus. Anyway beyond
mutation there's also recombination, due to crossing
over during meiosis when sex cells are formed. This
shuffles alleles in a population with regard to those
at other gene loci. When loci are really close on a
chromosome they might be considered linked, thus have
a higher probability of being carried together. It's
this phenomenon of linkage that facilitated mapping of
chromosomes in fruitflies during the days of Morgan
and his students.
Anyway, between mutation and recombination, there's
some variation in a population. For a simple scenario,
we have two alleles at a locus that determines if a
plant is tall or short. Let's say the environment is
variable enough at time 0 so that there's no
significant advantage to a plant being tall or short.
At time 1 a change occurs that favors short plants.
The tall plants have a harder time reaching sexual
maturity (survival) and passing their tall alleles
(reproduction) to the next generation. They tend to
not pass their alleles to the next generation. In a
sense, this natural process is much like a farmer
culling the tall plants in favor of the short plants.
The farmer doing it would be artificial selection. The
natural process whereby the tall plants are disfavored
by the changed environment is natural selection. If
the tall plants are totally winnowed over a series of
generations, there is a reduction in variation. That's
the way directional selection works. It favors one
allele (short) at the expense of another (tall) and
takes a formerly variable population down a path to
reduced variability. Mutation was the original source
of the alleles that determined tallness and shortness
in the plants. Mutation was the creative spark, so to
speak. All selection did was mold the average height
of the population until it reached a generally reduced
stature. Thus, mutation and selection are separate
phenomenon and in this context of directional
selection, selection doesn't produce variability, it
removes it.
If OTOH this population of plants had very small
effective size, the allele that is related to tallness
could have been lost due to a sampling error over the
time of several generations and the short plant allele
thus drifted to fixation. This too (genetic drift)
reduces variability in the population.
In either case, the only instance of tall plants in
the population would be if a mutant allele related to
tallness were to appear by....MUTATION.
At time 2 the environment might change again to favor
tall plants. If the allelic frequencies of the
population hadn't shifted (evolution by definition)
completely towards small plants via selection and/or
drift there might be enough variability for the
population of plants to be molded over time where the
average height increases as a result of the new
seleective regime. And the rare mutation that results
in a tall allele will provide a small number of these
now important alleles and mutation will facilitate the
evolution of the population via selection. But
selection didn't provide the variability. It would
shift it towards 50%/50% then eventually towards
extinction of the short allele.
Individual organisms don't adapt, but they can adjust
to an environmental change if they hae a wide enough
reaction norm or possess enough plasticity in their
phenotype. This makes things a little fuzzier than my
simplistic tall versus short allele above. If say
multiple gene loci are involved in the development of
a given trait and if there's enough feedback from the
environment where individual plants can adjust their
height to an environmental change (like if the
environment has had a history of being variable, so
plants tend to have the ability to adjust height and
have evolved the right response mechanisms to
facilitate this change) the environmental change will
be buffered a bit and instead of a harsh either or
selective regime where tall plants are culled at the
expense of short plants, the individual plants go
through a process where their height is reduced during
their lifetime. But there will possibly be individual
variability in the ability to make this adjustment. So
some plants will still tend to be favored, those
making the best response to the environmental trigger
by decreasing their height. If the environment stays
the same over multiple generations, the subsequent
descendants may slowly lose their ability to make a
response towards tallness, thus the reaction norm
reduces towards the shorter end of the spectrum and
individual plants begin having less plasticity.
If above you were hinting at the Baldwin effect, maybe
we can shift to an hypothetic situation where behavior
can lead the way for evolution via selection. We have
a pond of fish (not completely unlike walking catfish)
that are able to emerge from the pond when the need
arises. Mutation and recombination may have resulted
in some variability in this population as far as
abilities to live in drier conditions. Let's say these
fish eat insects found at the shoreline. Some fish
stick to feeding while in the water, but others
venture more into the shallows and the drier or muddy
areas. Those with a tendency to prefer drier areas may
start eating insect species that are more often than
not found on land, but sometimes in the water.
Individuals (if this genetic variabiility exists) that
tend to venture out of the water more will eat more
and since eating helps them survive to reproductive
maturity they might leave more offspring than those
staying more in the deeper water, especially if
something adverse happens to the insects that tend to
stay at the water surface, versus those on land. The
water preferring fish start dying off, but those
staying neearer land flourish. In this population
there might be some variability for food preferences
and for ability to withstand dryness. Over time, the
behavioral shifts in preference towards the land
insects may result in birth of individual fish that
can stay on land and eat more insects to pass their
traits to offspring. Over thousands of years the
population tends to stay away from the water more.
Some individuals may vary in the ability of eggs to
withstand dessication. Those that have eggs that can
withstand drier conditions will pass these traits to
future generations. Another trait with variability is
appendage morphology and strength. Those with fleshy
fin lobes that have more musculature will pass these
traits to subsequent generations. Over time, if the
population continues to flourish mutational variation
may be randomly injected into the population that
results in variation for ability to survive on land
(tetrapod limbs, shelled eggs, thicker skin less
permeable to water). The water source (ie pond) may
shirink or disappear, thus creating a do or die
situation for subsequent generations and individual
born with variation confering greater ability to live
on land will escape the selective grim reaper long
enougn to mate and pass their alleles.
This had started back when individuals in an ancestral
population exhibited a preference for the shore and
the prey items located there. This initial behavioral
variation resulted in changes in descendant
populations beyond behavior itself. Yet muations were
at the base. Mutations provided the initial behavioral
variability. Mutations also provided the varability in
other important traits advantageous to drier
environmental conditions. Recombination surely helped
too.
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