To: All Msg #157, Sep0193 01:11PM Subject: genes and evolution (for Joe) Joe Morlan (Joe.M

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From: Chris Colby To: All Msg #157, Sep-01-93 01:11PM Subject: genes and evolution (for Joe) Organization: animal -- coelomate -- deuterostome From: (Chris Colby) Message-ID: <26335b$> Newsgroups: Joe Morlan (Joe.Morlan@f216.n914.z8.rbbs-net.ORG) wrote: : I think the real reason that modern biologists define evolution : in terms of changes in genetic variation, is because the field : has been dominated by geneticists since the advent of the "modern : synthesis." This is correct. : The modern definition can be paraphrased "gene pools : are not immutable." Is this progress? IMO modern biologists : have confused a mechanism of evolution, with evolution itself. Is it progress? Only in the same way discovering atoms and arranging them in the periodic table was progress for chemistry. The emergence of the "modern synthesis" was a great leap forward. This brought the focus of evolution to where it should be -- heritable characteristics. In other words, the study of elements that transcend the lifetime of individual organisms. Your assertion (in a previous post) that you don't need to understand genetics to understand evolution was so outrageously stupid that I couldn't even bring myself to post a rebuttal. Allow me now to briefly explain why genetics is central to evolutionary theory. First, lets start with the painfully obvious. Genes replicate themselves, usually exactly. Genes are capable of being in- herited for many, many generations without alterations. Organisms, and the phenotypic traits that compose them, are not replicated with that fidelity. This is especially true in sexual populations where offspring are the expression of two half genotypes (haplotypes) in the context of the environment. The environment plays a large role determining the phenotype of an individual organism. In contrast, the environment plays almost no role in determining the genotype of an individual organism. Unless the environment is highly mutagenic, the alleles in any individual remain the same and are simply passed on. The rate at which they are passed on, of course, is highly dependent on the environment. But, the information content of any allele is immune to almost all influences from the environment. Thus, the phenotypic characteristics of a population can change due to genetics or environment. A sun and fitness craze could hit a population of (oh lets say) Scandinavians and the frequency of tanned and muscled guys named Sven or Ole could sky- rocket. A few years later, the government could warn that the ozone is gone and the sun is bad, and that people should stay inside, read Ibsen and listen to Grieg. The frequency of pale, skinny Norskies would then rise. However, these changes would leave no record of themselves in the biology of the descendants. Changes in the frequency of heritable elements (genes) do leave their mark on future populations. If the frequency of an allele conferring resistance to a deadly disease increases to a high frequency following a severe plague, it stays at a high frequency once the plague has gone. In addition, the frequency of many other traits may have changed "permanently" as well because they were linked to the beneficial allele. For example, if the original subset of people carrying the resistance allele all had blue eyes, the frequency of blue eyes would increase in the population as a side effect. This change in frequency would also remain once the plague was over. Genetic changes have a permanence that phenotypic changes due solely to environmental conditions don't. In addition to this, the fact that alleles are inherited as discrete, unchanging (except in the rare instance of mutation) entities allows them to be counted. Simple (and subsequently more complex) models can be derived to account for changes or stability in gene frequencies. Building quantitative models does a lot toward moving a field from stamp collecting and just-so stories to a real, productive branch of science. Furthermore, genes contain traces of their ancestry. The genome of any organism is a palimpsest of information. It has been rewritten and rearranged continually to confer reproductive advantage to it ancestors. In systematics, any useful trait must be heritable. Traits that are completely environmentally determined give no information of ancestry. Systematists for years have used pheno- typic traits, because that is all thatwas available. And, it has for the most part worked. But genes have a few distinct advantages over phenotypic traits. For one thing, you always know a gene is heritable. What about macroevolutionary changes? Well, at the very least we can say that microevolutionary change contributed to it. Some biologists would say that the models of genetic change within populations have the potential to completely describe the changes involved in macroevolutionary transitions. Others say they are a component, but other mechanism must also be invoked. So, how important changes within a gene pool in macroevolution are is controversial -- but you cannot deny that it did play _a_ role. I would say that, if you compare the number of different generations that lineages remain as an intact gene pool versus how many times they speciate (and the opportunity for higher level mechanisms is present) -- the conclusion that the bulk of macroevolutionary change is mostly cumulative micro- evolution change is a safe bet. In chemistry class, the prof tells his students about chemical reactions using the model of atoms as basic units of matter. They can be combined into molecules and molecules can like- wise be broken up into atoms. Knowing the characteristics of individual atoms is a first step in understanding the chemistry of larger molecules. And, more importantly, it allows a vast amount of knowledge about molecules to be (at least partially) condensed into a simple framework. A chem prof _could_ teach a class where you memorized reactions, but never mentioned atoms. The students might learn what happens when you heat chemical A and chemical B, but they would be missing a very central point to chemistry. Specifically, given two chemical names and the conditions which they are to be combined in, the students would have no hope of predicting the outcome. You are missing an extremely central point to evolution (and hence biology) if you don't understand the role of genetics in evolution. We could memorize that this form followed this form and so on, but where is the explanatory or predictive power? I see that you have recommended (to Mickey I think) Mayr's "Animal Species and Evolution". Guess what? I read (O.K. skimmed) it a few years ago; not a bad book. But, the field didn't die after Mayr wrote the book. At the t.o. party, your name came up and I proffered the opinion that you were proof of the maxim that a little (emphasis on little) knowledge is a dangerous thing. How about reading Futuyma and considering, just for a second, that the entire combined field of evolutionary biology composed of scientists might have a few insights into evolution that a bird watcher who has read one book by Mayr might not have? Either that or you might consider adding HTE to your .sig file 8-) Chris Colby email:


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