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Uncooperative Fruit Flies Refuse to Speciate in Laboratory Experiments

Casey Luskin January 28, 2012 7:00 AM

The TalkOrigins Speciation FAQ spends a lot of space citing papers trying to document speciation through laboratory experiments with fruit flies. Its section on "The Fruit Fly Literature" is the single longest section of alleged examples of speciation in the FAQ.

As we saw in a recent article, to establish speciation, these scientists simply wish to establish a completely reproductive isolated population. While some limited degrees of reproductive isolation were sometimes produced, complete reproductive isolation -- i.e. speciation -- was never observed in any of the examples analyzed inmy response to the FAQ.

More importantly, none of the examples in the FAQ documented significant biological change. As the notable evolutionary biologist Theodosius Dobzhansky (1972) admits, "Reproductive isolation evidently can arise with little or no morphological differentiation."¹

A few examples below tell some of these stories with regards to fruit fly studies cited by the FAQ:

Case 1: "Drosophila paulistorum"

Fruit fly breeding experiments published by Dobzhansky and Pavlovsky in 1971 showed that if you start with "semispecies" within a fruit fly species which are "indistinguishable morphologically," and then subject the strains to artificial breeding experiments, then "in none" of the experiments "has anything like complete isolation been achieved."² These results led a later reviewer to list this study among various studies where "none has succeeded in establishing complete sexual isolation."³

Moreover, there was no suggestion that the populations were no longer "indistinguishable morphologically" after the experiments. In fact, after reviewing this example, Dobzhansky concludes that sometimes "reproductive isolation and speciation precede differential adaptedness,"⁴ suggesting they had not diversified. At best, only a "new race or incipient species"⁵ was created; some authorities have challenged even the partial isolation, claiming the results "may have been due to contamination of cultures by other subspecies."⁶

Case 2: "Selection on Courtship Behavior in Drosophila melanogaster"

This experiment took two pre-existing strains of fruit flies from within the same species --Drosophila melanogaster -- and sought to determine whether changes in mating preferences could be induced. This included artificially killing hybrids between the strains (a process that does not necessarily mimic nature).

Incomplete reproductive isolation was established. One paper cited by the FAQ (Knight et al. 1956) called only "[p]artial sexual isolation,"⁷ another paper in the FAQ (Halliburton and Gall, 1981) lists this study among various studies where "none has succeeded in establishing complete sexual isolation."⁸

The most biological change that this example documented was small-scale behavioral differences pertaining to courtship, specifically changes in the amount of "licking" that males do to females to initiate mating. One paper cited by the FAQ (Crossley, 1974) showed just how unimpressive the sort of change observed in this experiment was:

Quantitative analysis of male and female behavior revealed the underlying causes of changed mating preferences and faster mating. In the LS experiment male courtship became more stimulating because percentage licking of both males and percentage licking plus vibration of e males increased.⁹

All that was observed were changes in the courtship initiation behaviors (licking and vibrations) changed between the strains. The two strains were "similar" before the experiments, and apart from slight changes in mating behaviors, remained very similar after the experiments.

Once again, not only was significant biological change not observed, but complete reproductive isolation (e.g. speciation) was not established.

Case 3: "Isolation Produced as an Incidental Effect of Selection on several Drosophila species"

In another section, the FAQ cited three studies on fruit flies which reported "slight" or "incipient" and "not complete" sexual reproductive isolation. None showed complete reproductive isolation or speciation, and none showed significant morphological change.

In one example, the FAQ discusses a paper (del Solar, 1966) which reported experiments that artificially selected for certain behavioral traits but produced only "slight" sexual isolation, or "incipient reproductive isolation," due to "changes in sexual behavior."¹⁰ The paper thus reports that complete reproductive isolation was not found:

Whether selection for geotaxis and phototaxis always and necessarily produces a change in the sexual behavior, and whether continued selection may carry the sexual divergence anywhere near complete isolation, can only be decided by further experiments.¹¹

Not only was "anywhere near complete isolation" not achieved, but significant biological change was also not achieved. As the paper reports: "The geotactically and phototactically positive and negative strains appear to be indistinguishable in external morphology."¹²

Another example discussed by the FAQ in this section pertains to Dodd (1989) which reported experiments on populations of the fruit fly Drosophila pseudoobscura. Schluter and Nagel (1995) described Dodd (1989)'s findings by stating that only "some premating isolation evolved," and "Reproductive isolation between divergent lines was not complete."¹³ Speciation was also not said to have occurred. Aside from mating and food preferences, there were no claims of biological change between the populations. Again, we see that not only has reproductive isolation not been demonstrated, but significant biological change did not evolve.

Despite the aforementioned underwhelming results, the FAQ then discusses another paper which it admits reported "Less dramatic results." According to the paper cited by the FAQ (de Oliveira and Cordeiro, 1980), the experiment only produced "incipient isolation"¹⁴ -- not complete reproductive isolation. As for the degree of morphological change, aside from a preference for a certain pH level in food, no biological change was reported. In fact, the paper notes that among three long-standing natural races of D. willistoni, "These flies are morphologically indistinguishable." This study certainly did not change that observation: Once again we have not seen complete reproductive isolation, nor have we seen significant biological change.

Many other examples from the FAQ discuss fruit fly selection and breeding experiments, and all have essentially the same results: complete reproductive isolation is not established, and significant morphological change is not observed.

For additional details, please see the full response to the TalkOrigins Speciation FAQ.

References Cited:

[1.] Theodosius Dobzhansky, "Species of Drosophila," Science, Vol. 177 (4050):664-669 (August 25, 1972).

[2.] Theodosius Dobzhansky and Olga Pavlovsky, "Experimentally Created Incipient Species of Drosophila," Nature, Vol. 230:289-292 (April 2, 1971).

[3.] Richard Halliburton and G. A. E. Gall, "Disruptive Selection and Assortative Mating in Tribolium castaneum," Evolution, Vol. 35 (5):829-843 (September, 1981).

[4.] Theodosius Dobzhansky, "Species of Drosophila," Science, Vol. 177 (4050):664-669 (August 25, 1972).

[5.] Quoted in Jonathan Wells, The Politically Incorrect Guide to Darwinism and Intelligent Design, p. 56 (Regnery, 2006).

[6.] Jerry Coyne and H. Allen Orr quoted in Jonathan Wells, The Politically Incorrect Guide to Darwinism and Intelligent Design, p. 56 (Regnery, 2006).

[7.] G. R. Knight, Alan Robertson, and C. H. Waddington, "Selection for Sexual Isolation Within a Species," Evolution, Vol. 10 (1): 14-22 (March, 1956).

[8.] Richard Halliburton and G. A. E. Gall, "Disruptive Selection and Assortative Mating in Tribolium castaneum," Evolution, Vol. 35 (5):829-843 (September, 1981).

[9.] Stella A. Crossley, "Changes in Mating Behavior Produced by Selection for Ethological Isolation Between Ebony and Vestigial Mutants of Drosophila melanogaster," Evolution, Vol. 28 (4): 631-647 (December, 1974).

[10.] Eduardo del Solar, "Sexual Isolation Caused by Selection for Positive and Negative Phototaxis and Geotaxis in Drosophila Pseudoobscura," Genetics, Vol. 56:484-487 (1966).

[11.] Eduardo del Solar, "Sexual Isolation Caused by Selection for Positive and Negative Phototaxis and Geotaxis in Drosophila Pseudoobscura," Genetics, Vol. 56:484-487 (1966) (emphasis added).

[12.] Eduardo del Solar, "Sexual Isolation Caused by Selection for Positive and Negative Phototaxis and Geotaxis in Drosophila Pseudoobscura," Genetics, Vol. 56:484-487 (1966) (emphasis added).

[13.] Dolph Schluter and Laura M. Nagel, "Parallel Speciation by Natural Selection," The American Naturalist, Vol. 146 (2):292-301 (August, 1995).

[14.] Alice Kalisz de Oliveira and Antonio Cordeiro, "Adaptation of Drosophila willistoni experimental populations to extreme pH medium," Heredity, Vol. 44 (1): 123-130 (1980).