Now in the artificial case of the fruit flys, the supposed demonstration would have proved nothing unless the offspring continued to not mate after they had been fed identical food for the same number of generations. No thinking person could possibly believe that any significant change in the genetic structure of these two strains could have taken place in only eight generations. The fact that the change was forced by an environmental change (the food supply) is proof that it was not caused by mutations.
(emphasis added) Be careful what you wish for, buddy.
You said in another thread that you are "a pure scientist by education and an applied scientist by profession". As such, you should have access to journals and journal websites of some kind. I found this article through JSTOR and you should be able to access it:
Dodd, D.M.B. (1989) "Reproductive isolation as a consequence of adaptive divergence in
Drosophila pseudoobscura."
Evolution 43:13081311.
In it, the authors quite clearly set out their experimental method: [Notes added.]
Starch-adapted populations were tested against maltose-adapted populations in every possible combination [1] to determine whether adaptation to the two new regimes could have induced the development of ethological isolation [2]. Multiple-choice tests were performed using mating chambers modeled on those described by Elens and Wattiaux (1964). All flies used in the mating-preference tests were reared for one generation on standard cornmeal-molasses-agar medium [3]. Virgin males and females were anesthetized with CO2, isolated from the opposite sex, and aged on standard medium [3] for 3-6 days. Twelve females from each of the populations to be tested were placed in the chamber. Twelve males from the two populations were then introduced as nearly simultaneously as possible. The flies were not anesthetized for this procedure [4]. The tests were perfored at room temperature (no higher than 25 degrees C), under bright (but not direct) lighting. The chambers were observed for 60-90 minutes.
[1] Namely, maltose males against either maltose females or starch females; starch males against either maltose females or starch females.
[2] "Ethological" meaning behavior; that is, if changes had occurred in behavior that precluded mating.
[3] Here's the punchline. The individuals for mating
had actually been reared on the standard medium before the testing! Now, if you'd taken flies right from feeding on starch and flies right from feeding on maltose, you might have a case that it was their diet and not their genes that had affected them. Indeed, the authors themselves note this in a later section:
Significant behavioral isolation between starch-adapted and maltose-adapted populations was observed. The isolation was not a result of conditioning of the flies to the two media, since all tests were performed using flies that had been reared on a common medium and had experienced neither starch nor maltose. Nor was physical isolation alone responsible for the changes in mating behavior, since there was no evidence of behavioral isolation between any pair of the four starch-adapted populations nor between any pair of the four maltose-adapted populations. The ethological isolation was a pleiotropic by-product of the adaptation of the populations to the two media, confirming one of the basic tenets of the Modern Synthesis.
(emphasis added)
[4] Obviously not! Scientists can be so anal at times.
And of course, maltose vs. starch isn't the only way to do this kind of experiment.
Many experimental studies have looked for isolation as a correlated response to divergent selection. For example, Burnet and Connolly (1974) divided a founder stock of D. melanogaster into three groups. The first and second were selected for increased and decreased locomotor activity, respectively, and the third was an unselected control. After 112 generations, the selected groups manifest markedly divergent locomotor activity, in the selected directions, whereas the controls remained unchanged. When the lines selected for increased or decreased activity were tested for nonrandom mating, a 50% excess of homotypic mating was observed (i.e., the percentage of homotypic matings was about 75 instead of the random-mating expectation of 50). In a similar type of study using a Musca domestica (common house fly) model system, Hurd and Eisenberg (1975) selected for positive and negative geotaxis. After 16 generations of divergent selection under allopatric conditions, a response to selection in the appropriate direction was found in both the positive and negative selection lines. When positively and negatively selected lines were tested for prezygotic isolation, a 60% excess of homotypic mating was observed.
...
When we surveyed 14 studies from the literature in which divergent selection was applied to allopatric populations and then a measure was taken for the development of prezygotic isolation, we were surprised to find such a large excess of positive results (10 positive to 4 negative; part A of table 1). While allowing for the fact that negative results are less likely to be published, it still remains clear that it is not unusual to find prezygotic isolation as a fortuitous byproduct of adaptation to divergent selection regimes.
(from W.R. Rice and E.E. Hostert (1993). "Laboratory experiments on speciation: What have we learned in forty years?".
Evolution 47: 1637-1653.)
In other words, "speciation happens all the time, even when we weren't actually looking for it!"
