Buried in the most recent scientific literature there is a story of love, sex, and intrigue that has all the makings of a hearty Mills & Boon novel. The central characters of this plot are not lovers wrapped in each others arms but fruit flies that choose their sexual partners according to the microbiota that line their guts (1,2). Lactobacillus plantarum is the ‘cupid gut bug’ that seems to have greatest influence on sexual preferences (1,2) And it appears to do so by influencing the release of a class of Drosophila pherormones known as cuticular hydrocarbons (1,2). For evolutionists this finding is cited as one possible avenue through which speciation might take place in Drosophila (1,2). For those of us who are critical of such work however there exists one small but important catch. That is that the sexual preferences observed are easily eradicated by simply treating fruit flies that had been raised on different diets, with antibiotics (1,2). In other words no genetic changes that would ensure irreversible reproductive isolation, and hence speciation, have taken place.
The late paleontologist Stephen Jay Gould was convinced that for speciation to occur so-called peripheral populations would simply have to become ‘locked up’- isolated from the gene flow of their larger ancestral stocks- in such a way that new adaptations would become stable and not watered down by interbreeding (3). Gould’s close colleague Niles Eldredge posited that new species could form, “by the mere accumulation of genetic differences in the two segments of a single ancestral species” on the basis of some sort of hypothetical, (however slight) change in the reproductive system (4, p.116). He added that “some modification of the reproductive system is required for speciation to occur” (4, p. 121).
Unfortunately for Gould and Eldredge such an exit glossed over key mechanistic questions. After all what mechanism could we come up with that ensured irreversible reproductive isolation from ancestral stocks? Attempts to correlate speciation events with some sort of unguided change in the genetic makeup of an isolated population have proved largely unfruitful. In their studies on fruit flies Laura Reed and Therese Markow suggest that reduced sperm motility as well as sperm storage, recovery from storage, or ability to penetrate the micropyle might play an important role in the hybrid incompatibility that results from interspecial crossings (5). But they readily admit that the lack of a known genetic causality for speciation today represents “a major challenge to evolutionary biology” (5). They further add that “no study has yet characterized levels of naturally occurring variation for factors causing postzygotic isolation in any animal taxon” (5). Such a challenge is of fundamental importance if, as Darwin did, evolutionary biologists are to confidently claim that speciation is merely an extension of population variation.
Today some scientists have posited that a small number of genes in individual species might somehow maintain populations as reproductively isolated units. Biologist Daniel Barbash and his colleagues from Cornell University put forward a gene called ‘Hybrid Male Rescue’ (hmr) as a possible candidate speciation gene in fruit flies (6,7). They demonstrated that hmr was responsible for hybrid incompatibility between different species, seemingly acting in concert with an unknown autosomal factor (7). Yet they also recognized that the 13% interspecial amino acid divergence observed in the HMR DNA Binding protein was a tall order for ‘relaxed selective constraints’ to have achieved on their own (7). According to their statistical analysis of mutational frequencies, and in line with the Dobzhansky-Muller (D-M) speciation model, they were adamant that some sort of positive selection must have been at work although they were unable to suggest what selective pressures might have acted to favor reproductive isolation (6,7).
Darwin himself recognized that whatever factors might be involved in ensuring reproductive isolation, they could not have arisen through natural selection. He wrote in The Origin of Species that “the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired…by the preservation of successive profitable degrees of sterility” (8, p.361). As he subsequently noted “it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection” (8, p.379). In other words whatever genetic factors had maintained reproductive isolation would have had to have been found purely through chance alone- a blind walk through genetic space in search of those mutations that would prevent reproduction between some individuals and allow reproduction between others (9).
Rather than supporting evolutionist dogma the picture of coordinated changes being effected in individuals numerous enough so as to ensure the creation of novel species resonates more closely with the tenets of intelligent design. It is the height of irony that evidence-lacking meanderings over evolution and speciation should become most apparent in the “gut feelings” of the humble fruit fly.
Further Reading
- Jeff Akst (2010) Gut Bugs Affect Mating, The Scientist, 15th December, See http://www.the-scientist.com/news/display/57793/
- Gil Sharon, Daniel Segal, John M. Ringo, Abraham Hefetz, Ilana Zilber-Rosenberg, Eugene Rosenberg (2010) Commensal bacteria play a role in mating preference of Drosophila melanogaster Proc Natl Acad. Sci USA Vol 107, pp. 20051-6
- Stephen Jay Gould (2002) The Structure of Evolutionary Theory The Belknap Press of Harvard University, Press Cambridge, Massachusetts, pp. 800-802
- Niles Eldredge (1985) Time Frames: The Rethinking of Darwinian, Evolution and the Theory of Puctuated Equilibria, Published by Simon and Schuster, New York
- Laura Reed, Therese Markow (2004) Early events in speciation: Polymorphism for hybrid male sterility in Drosophila, Proc Natl Acad. Sci USA Vol 101 pp. 9009-9012
- A Gene Responsible for Hybrid Incompatibility in Drosophila, PLoS Biology Vol. 2, Issue 6, p. 709
- Daniel A. Barbash, Philip Awadalla, Aaron M. Tarone (2004), Functional Divergence Caused by Ancient Positive Selection of a Drosophila Hybrid Incompatibility Locus, PLoS Biol. 2004 Jun;2(6):e142.
- Charles Darwin (1859) The Origin of Species By Means of Natural Selection Or The Preservation of Favored Races In the Struggle For Survival Modern Library Paperbacks Edition (1998), New York
- Darwin stipulated that in order for speciation to have segregated populations into reproductive isolates, so-called ‘disturbances’ in the genetic makeup of offspring must have occurred whenever there were crossings between species. But natural selection could not have been the mechanism that led to hybrid incompatibility. Since natural selection favors those traits that are advantageous to an individual within a population, what advantage could be gained from one individual becoming reproductively isolated from the rest of its neighbors? Without the ability to reproduce no genes would be carried over to successive generations. Several individuals would have had to evolve their speciation genes in precisely the same way as to give a genetic constitution that was sexually compatible, this through chance alone.