Re: The evolution of "evolution"

From: Scott Chase (
Date: Sat 08 Oct 2005 - 00:37:23 GMT

  • Next message: Kenneth Van Oost: "Re: The evolution of "evolution""

    --- Dace <> 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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