I invited Ted Sargent to comment on Laurence Cook's recent comments. Here are Sargent's comments, received on August 10, 2004.
Sargent's 1st Reply to Cook
There seems to be here, as is often the case, a misunderstanding of my ideas on induction as they relate to the industrial melanism phenomenon in moths.
I envision a process similar to that which produces phenes (polyphenism) in many butterflies and moths. In these cases, some condition in the environment varies (e.g., light, temperature, humidity, etc.) and different phenes (forms) are associated with different levels of these variables. Thus, one might find different seasonal phenes, such as "spring" and "fall" forms, in some species; or perhaps ecological phenes; such as "dry" and "wet" forms, in others. The point, as I see it, is that these variations, striking as they may be, are not the result of different genes (alleles) being present or absent in these two forms, but are rather the result of different genes being expressed or not expressed in the two forms. The genotypes in these cases are the same in both forms, but the phenotypes are very different. Thus, something about the environment in which these insects develop results in the expression or non-expression of certain genes - and two very different phenotypes are the result.
In this same way, I have suggested that melanic polymorphisms may actually be polyphenisms, with the gene (or genes) for melanism being expressed under some conditions, and not expressed under others. The fact that melanism acts like a Mendelian dominant is irrelevent to my suggestion. I would agree that the gene is a dominant one, and is inherited as such, under conditions where this gene is expressed. And, indeed, selection would act to increase or decrease the incidence of melanism under these conditions, since at least two phenotypes ("typical" and "melanic") are being produced. But what I am also suggesting is that the gene in question (i.e., the dominant allele for melanism) may not be expressed under some conditions. This could give the impression that the "typical" form is due to a homozygous recessive genotype. But perhaps the moths in question possess the genetic potential to be either typical or melanic - and the dominant allele (for melanism) is not being expressed (just as the alleles for a "spring" form are not being expressed in the "fall" brood of species with seasonal forms). Thus, I am suggesting that gene (or allele) frequencies may not be changing in nature, at least to the extent suggested by the changing frequencies of the two forms that are seen in species exhibiting industrial melanism. Putting it another way, phenoypes may change without changes in the underlying genotypes. Or, at least there need not be a simple relationship between the two.
This is a very different suggestion than the one(s) that are often attributed to me. I do not use the word "induction" as, for example, Heslop Harrison did - implying that agents in the environment somehow induce mutations for melanism that are then somehow passed on genetically. I am using the word "induction" as cell or molecular biologists do when discussing gene expression (e.g., in the classical lac operon example). A gene may be induced to act (or not act) in any of a number of ways at the molecular level. Indeed, this is the fundamental process that underlies cellular differentiation during development. Perhaps, then, moths with the potential to exhibit both typical and melanic phenotypes have succeeded in the course of evolution, as the advantages of being typical and melanic may recur in nature (I can't help but think of the subtitle of Kettlewell's book, "A Recurring Necessity"). And perhaps the moths respond (in a developmental way) to conditions that predict the advantage of each form. A theoretical possibility would be responding to some product of forest fires (smoke, soot, heat, etc.) that precedes a period of blackened tree trunk backgrounds. Another possibility might involve responding to different chemical compositions of food plants that are correlated with different ecological conditions that, in turn, affect the resting substrates that the moths will eventually encounter. Thus, many tree species have very different bark characteristics depending on such things as age, altitude, associated species, moisture, and other habitat variables. And moths might be sensitive to any chemical differences that underlie these different ecological conditions.
Finally, I believe that my suggestions do yield testable hypotheses. For one thing, they predict that some distinct and heritable phenotype differences in various species may not be associated with detectable differences in the DNA of the forms (phenes, or morphs) in question. They also predict that species may sometimes be "tricked" into phenotypic changes that are not presently adaptive because normal ecological associations and outcomes have been altered (perhaps by human activities). This should, in turn, suggest a variety of experiments involving various rearing regimens. Whatever the case, I always endeavor to advance testable scientific hypotheses, and I will always yield to convincing scientific evidence.