RE: Evolution Statement

From: bivalve (
Date: Fri Dec 14 2001 - 20:06:34 EST

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    Here is a reply to several specific points raised in this discussion. I have put >< around quotes and blank lines between quotes and replies.

    Regarding the original statement:
    >Like all scientific theories, the theory of evolution is a current best explanation.<

    No, many theories are now discounted as worse than the current best explanation.

    >Nonetheless, no scientific hypothesis other than common descent with modification can account for and make predictions about the unity, diversity, and properties of living organisms.<

    Other hypotheses can account for and make predictions about these, but not as well as common descent with modification.

    Regarding various replies:
    >(True, genetic drift and neutral mutations possibly direct genetic exchange in very early organisms are sometimes invoked as change agents, but they are relatively minor causal factors.)<

    Direct genetic exchange turns out to be extremely extensive and important in bacterial evolution. It also occurs with some frequency between symbionts and hosts and between organelle and nuclear genomes. Genetic drift plays a role in all evolution, as even mutations that are selected upon by natural selection are also affected by the probabilistic outcome of reproduction.

    >If natural selection is not the major change agent in the overall course of DWM, please correct me and tell me what is.<

    It depends on the definition of change agent and of natural selection. Ultimately, inherited change must result from mutations (unless an organism is able to transmit information culturally), and so mutation might be seen as the change agent. Natural selection provides the principle pressure for change, but the development of two forms from one also depends on the establishment of some sort of reproductive barrier (geographic, behavioral, genetic, etc.). This barrier may result more from random than selective pressures, e.g. the occurrence of a mutation that affects reproductive compatibility. Also, if the pressure of natural selection is relatively low, random variation may have a greater effect than selection in producing change.
    Natural selection can be defined narrowly as environmental pressures or more broadly to include sexual selection, catastrophes, and other sources of selective pressure. These latter selective pressures may can play a greater role than the ordinary environmental pressures. For example, male great-tailed grackles have tails so big that they can cause difficulties flying in a strong wind because the females prefer them that way.

    >the only direct evidence we have that natural selection is the causal factor in DMW is bacterial resistance to antibiotics, and what Gould called "short-term evolution" (STE) studies<

    There are many other experiments using selective pressures to affect evolution of bacteria and viruses besides antibiotic resistance, in addition to the natural experiments of disease outbreaks. A disease parasite must balance competing demands. The more it exploits the host, the more it can grow in the host, but if the host dies before the parasite can transmit itself to the next host, all is lost. A well-studied example came from a virus introduced to control rabbits in Australia.
    I am not sure what would qualify as direct evidence of natural selection in the geologic past. Certainly there are numerous examples of a new opportunity leading to a rapid evolutionary radiation, or changes in organisms corresponding to environmental changes. For example, when the Isthmus of Panama formed, land plants and animals were able to travel between North and South America, resulting in extensive changes in the faunas as the two came into contact with each other. However, we do not have direct evidence that North American carnivores and ungulates directly outcompeted the South American carnivorous marsupials and notungulates.
    Organisms are functional for their habitats, which is the basic expectation of natural selection.

    >Or to say it differently, evolutionary biologists hold that so-called macroevolution, or large innovative changes in DWM is microevolution WRIT LARGE.<

    Is the allusion to Milton or 1066 and All That? Many evolutionary biologists hold that macroevolution (which they define as evolution at the species level and higher) involves additional factors besides the everyday microevolution, but many others accept microevolution as accounting for all common descent.

    >To continue, such an extrapolation is unwarranted because of the simple fact that changes observed in STE studies and bacterial resistance are _reversible_.<

    No, since on the morphological level macroevolution (in the sense of large innovative change) can be reversible. The development of limbs in vertebrates was reversed in snakes, and the change from flipper to foot in the early amphibians was reversed in whales, pleisiosaurs, some crocodiles, ichthyosaurs, etc. Molluscs evolved a shell and then lost it in several lineages. The ancestors of primates went from terrestrial life to arboreal, and baboons, gorillas, and humans have climbed back down again.

    >what is more difficult to explain is _disparity_, or the highly organized nature of the organic world, with its deep discontinuities between the major families of organism, roughly designated as phyla, using natural selection as the operating change agent.<

    Natural selection favors some degree of specialization. This can be achieved through separation of a population into separate subpopulations and eventual speciation. Once two lineages are separate, they will continue to encounter different selective pressures and different constraints. They will also independently acquire mutations. If they separated a long time ago, e.g. the early Cambrian, by now they will be very different. In addition, organisms specializing for very different niches will be very different. By the end of the Cambrian, most broad categories of niches were filled, leaving no space for radically new types of organisms except on land. These are two ways in which natural selection explains the existence of distinct phyla.

    >One must distinguish the past evolution of the characteristics of the moon from that of its original creation. Evolutionary theory deals with the appearance of new complex forms of life from simpler ones. The analogous statement for the moon would be to find a simpler moon to which the present more complex moon evolved from. It is the problem of origins that I strongly believe is not a scientific question.<

    Although I am not sure that it is simpler, the impact theory of the origin of the moon describes a pre-moon system, the transition, and then the post-formation situation.

    >For instance, the following puzzles me, why did Jesus say that from the beginning God created them male and female? Did He think we were not ready for a class in biology or evolutionary theory? <
    Certainly there had to be male and female at the beginning of humanity, or else there would have been no continuation of humanity. However, scientific knowledge of the first century Jewish rabbis was not adequate to make them ready for a class in biology or evolutionary theory, and it would have been an extraneous diversion from the question at hand about the legality of divorce.

    >2. The lack of mathematical thinking in the "story" of the evolutionary tree. Perhaps I have not read the right stuff, of course. But I do recall some analyses of this made at the Darwin Centennial some years ago, which were not kind to the "grand scheme." <

    Mathematical analysis of evolution has taken off in the past three decades owing to three developments. Vast improvements in our ability to obtain biomolecular data, especially the development of DNA sequencing, have given us much more data suitable for qualitative analyses. New mathematical models have been developed to analyze the data, and there has been an emphasis on (if not obsession with) taking more mathematical approaches. This general approach is referred to as cladistics. Finally, the development of computer power has allowed the implementation of these models and analysis of much more data than previously feasible.

    >4. The fact that "microbe to man" is the ONLY naturalistic (i.e. scientific) possibility possible to conceive of. So, as a scientist, I accept it; there is no competition (in science). As a philosopher, I remain skeptical. <

    Multiple abiogenesis of advanced organisms would be another naturalistic explanation, though it is not supported by available evidence.

    >While I can understand the mechanism of selection, I find it difficult to understand what drives neutral drift. I'd appreciate it if you would explain it. It seems to me to be a radical departure from natural selection. What is the significance of their "faster accumulation in the genome" that you mention? <

    The probability of any mutation becoming established in a population is proportional to the advantage that it conveys to an organism that has it and to its frequency at the starting time. An organism has a certain chance of reproducing. If it is a normally sexually reproducing species, then the mutation has a 50 percent chance of being passed to any one descendant. Over time, the mutation will either be lost from the population or will take over the population, if no other mutations affect the results. A non-genetic example from humans comes from the pattern of last names. If children always receive the last name of their father, then each last name will continue to be inherited unless there is a generation with all daughters. This has a certain probability, depending on the family sizes. Over time, some last names will disappear from the population. For example, although J. S. Bach had 20 kids, no one since the mid-1800ís has inherited the last name Bach from him. !
    All his sons had either all daughters or sons that only had daughters or sonís sons who only had daughters...
    This variation is especially evident in small populations, which produces the founder effect. No one individual is fully average for a population. If a small, isolated population is established, it will have a subset of the mutations present in the main population. A rare mutation present in one of the founders of the small group has a much better chance of becoming common in the small population than in the large one. For example, some city parks have a high population of albino or melanistic squirrels. Out in the woods, the population is large enough to swamp such mutations, but if a park was initially populated by just a few squirrels, inbreeding would help increase the frequency of the rare form. The blue people in eastern Kentucky are one of the more unusual examples of a rare human mutation concentrated in an isolated population.
    Neutral variation occurs more rapidly than selective variation for several reasons. The majority of mutations are neutral or close enough not to matter, so they occur more frequently than mutations that have selective impact. Organisms have had millions to billions of years to refine their genes, so dramatic improvements are relatively rare. Only in the course of evolving a new feature (seen in experiments on bacteria, for example), do we often see significantly advantageous mutations appearing. The level of error checking in DNA varies, and DNA sequences with no sequence function often have little error checking. Some types of sequences are extremely prone to mutation; these occur in parts of the genome where they do not negatively impact the organism (or else the organism has problems). All of these tend to promote the high occurrence of neutral mutations.

    >This is not a ratchet-like evolutionary progression, as you claim. It is step one. Has anything been observed that builds on step one to produce step two?<

    A good example of multistep evolutionary progression documented from the evolutionary record comes from the discussion of the universal genetic code. Hemichordates and echinoderms are closely related phyla. In hemichordates, one standard codon has lost its meaning. In echinoderms, this same codon has gained a new meaning, as a result of a mutation in the tRNA. Thus, the hemichordates preserve an intermediate step in the multiste process of changing the genetic code.

    >Does your "bacterium [that] acquired streptomycin resistance" that you mentioned earlier have "mind-bogglingly huge morphology- space available to it"?<

    Bacterial morphology is so small that we do not have a good quantification of the cell-level variation due to difficulties in observation. The number of forms taken by bacterial colonies is certainly large.

    >Evolution abhors a discontinuity. Many evolutionary biologists are busy trying to iron them out.
    <<Discontinuity is a product of speciation (a mechanism of evolution) and extinction.>>
    Show me. <

    The definition of discontinuity here is not the same. Reproductive barriers between species are a discontinuity critical for the evolution of diversity. If all organisms are the same species, variety is quite limited. A significant discontinuity between kinds, however, provides a challenge for evolutionary explanations. The continual discovery of additional transitional forms, discovery of molecular intermediates or related genes, and new analyses all help fill these gaps.

    >descent with modification needs ... an unambiguous refutation of intelligent design.<

    Total refutation is rarely possible. Instead, the explanation that best matches the evidence is preferred. For example, the precession of Mercury could be explained by Newtonian laws plus some sort of fudge factor, e.g. the gravitational influence of an as-yet unobserved body, an additional force, a slight adjustment to the formula, etc. If you work at it enough, you can produce just as accurate a prediction as relativity. However, you can also predict any other value. Relativity, on the other hand, predicts what is observed without a fudge factor. It also predicts other things better than Newtonian formulas.

    >I am sure that if we knew all the science there is to be known, evolutionary theory would still have no ability to make forward in time predictions. First and foremost the complexity of the problem is cosmological in magnitude. Secondly, even knowing precisely all the basic laws of science, the derivation of complex system out of the basic science would still be intractable. The fundamental problem of a purely scientific approach to the origin of living things is beyond human imagination. Even the simplest facts in physics---for instance, the numerical value of the fine-structure constant---are extremely difficult to solve and one has not hint of how to even approach it. How much more difficult are the problems posed by the existence of life and its historical development. Of course, it may be that the latter is not even a scientific question but a purely historical one.<

    Evolutionary theory can predict forward in time for a simple enough system, just as in physics. The intractability of the math is a real problem, but it is a problem with the application rather than the scientific legitimacy. Meteorology might be a good analogy, with a greater role of physics than evolutionary biology.
    One simple system that has been studied involves populations of two closely related species of beetle in a lab. When placed in direct competition, one species wins out, with a probability than can be calculated based on details of how well each species does under different conditions. A more general prediction is that global warming will cause organisms to move, adapt, or die out as local changes push the limits of their tolerance. This is being observed in various organisms. Exactly what each species will do depends on its abilities and opportunities. For example, the inhabitants of the Maldives will move, die out, or live on boats when sea level goes up a little more.

    >long time scales involved in geological studies makes its finding more ambiguous than those in forensic science.<

    It depends on the finding. I am more certain that Cambrian bivalves had two valves than I am about most court cases. The configuration of their soft anatomy is less certain, though there are some traces left on the fossils (muscle scars, etc.) and some reasonable inferences from living mollusks. However, the soft anatomy of extinct orders of bivalves that occur in much younger deposits is just as uncertain.

    >I do not really know what you mean by a mechanism that causes evolution<

    For there to be an inheritable physical change, there must be a mutation. This is then the basic causative mechanism for evolution. Many things can cause mutations, and natural selection, neutral drift, and other factors affect whether a mutation becomes successfully established in a population. The exact cause for a given mutationís success is rarely certain, however. For example, the loss of legs by the ancestors of snakes was very successful-there are lots of kinds of snake today. However, it is not certain whether the impetus for this was for swimming or for burrowing. Both of these favor streamlining and are suitable for limbless locomotion. The fossil record of protosnakes is from shallow marine environments, but that is also the best place to get fossilized and then found by paleontologists.

    I hope this is helpful. It has been challenging trying to catch up on the discussion!

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