Fruit Flies Fail to Exhibit Neo-Darwinism
The Neo-Darwinian Synthesis is the current reigning paradigm of Darwinian evolution. It teaches that random genetic mutations provide the raw material of variation, and that natural selection acting on these variations produces all the complexity of life. A corollary is that mutation is independent of selection; i.e., that mutations do not “conspire” with natural selection to produce new structures and functions. Since mutations are random, it should be possible to speed up the mutation rate and thus speed up evolution.
The fruit fly Drosophila melanogaster has been used since the 1920s in experiments on neo-Darwinism. The poor bugs have been irradiated, treated with chemicals, and subjected to numerous tortures so that scientists could monitor the effects of mutations. Experiments have yielded some freaks, like four-winged flies and flies with legs where the antennae should be. One would think that, of any lab animal, fruit flies should provide some evidence that neo-Darwinism actually works. But a paper in Current Biology1 seems to indicate that most of the work in decades of experiments on Drosophila is irrelevant to the way neo-Darwinian evolution works in the wild.
It was not the intent of John F. Y. Brookfield, a geneticist at the University of Nottingham, to argue against neo-Darwinism. From all appearances, he accepts it; he begins,
The neo-Darwinian paradigm of evolutionary change assumes that mutations occur independently of any natural selection that will subsequently act on them. While such independence has been challenged in some descriptions of adaptive mutation in bacteria, it is still generally accepted to apply in multicellular organisms. It follows that, were one to examine simultaneously the process of mutation and the process of evolution, the kinds of mutational change that one would see should not be different in kind from the sorts of changes one sees occurring over evolutionary time, unless different types of mutation had systematically different phenotypic consequences: only selection can create a systematic difference between mutational and evolutionary changes.
By evolutionary change, he means adaptive change. Mutational change could be neutral, producing no benefit to the organism, or it could be “downhill,” destructive to the species.
Brookfield’s topic in this paper is another question: whether mobile DNA elements can also be sources of adaptive change. In passing, though, he mentions several serious problems that neo-Darwinian theory has yet to explain.
- Dominance. How do mutations become established in a population?
A lack of agreement between mutation and evolutionary change was first noted in the context of dominance. In the 1920s, when the neo-Darwinian synthesis was being created, it was seen that mutations in Drosophila melanogaster are usually recessive to the wild-type allele. The paradox was that if genes are evolving, then the current wild-type allele would have been a mutant when it first arose, spreading to become the wild-type because of its advantageous phenotypic effect. Why should advantageous mutations generally be dominant, when their advantageousness depends on the particular environments that they will encounter?
Brookfield describes attempts by R. A. Fisher and Sewall Wright to address this paradox. Fisher thought that dominance evolves, but Wright, whose views have been “abundantly confirmed” in subsequent experiments, was that “mutations are recessive because they inactivate genes, so that their recessivity has a physiological, rather than an evolutionary, cause.” But this seems to lead away from adaptive evolution. Turning off a gene would not create the gene in the first place. That leads to another difficulty:
- Information Loss. “Major mutations represent losses of gene function, a change not often used in adaptive evolution — we do not evolve by successively losing more and more of our gene functions, but rather by subtly altering the ways in which genes work.” Instead of answering where the beneficial mutations come from, he changes the subject, leading to a third difficulty for neo-Darwinism.
- Source of mutations. Brookfield mildly announces a shocker: most mutations in Drosophila are not random mistakes, but rather genetic information from other sources: “Similarly, what are the evolutionary consequences of mobile DNA insertions? It has been estimated that 80% of the spontaneous mutations seen in Drosophila genetics result from transposable elements. Do mobile DNA insertions similarly create 80% of evolutionary changes in this species? Without question, they do not.”
- Fixation. One would think that a beneficial mutation would become established in the genome. But Brookfield says, “The most revealing observation is the almost complete absence of fixed sites of mobile DNAs in D. melanogaster. A mobile DNA insertion that created an advantageous phenotype would be expected to spread to fixation in the species by natural selection. This would create a site fixed for the element throughout the species. Such sites are very rare, although they have recently been detected for the S element family in heat shock protein genes.”
Brookfield turns to focus on the possibility that mobile DNA insertions might enhance the expression pattern of a gene, and that this could lead to adaptive (evolutionary) change. He discusses recent examples of possible clues in certain fruit fly genes. In particular, a population of California fruit flies suggests the occurrence of a past selective sweep, which “occurs when a new advantageous mutation arises and rapidly spreads through the population.” Finding evidence of a selective sweep is difficult, he admits, and is based on circumstantial evidence. But a selective sweep is not necessarily a positive sign of evolutionary change: it has a downside:
- Reduction in diversity. “Because the mutation arises initially in a single chromosome, as it spreads, this chromosome also spreads through the population, eliminating the standing crop of genetic diversity in the region.” In other words, as the chromosome containing the beneficial mutation spreads through the population, the genetic diversity goes down.
- Few examples. Even if selective sweeps indicated the fixing of advantageous mutations, there aren’t many examples. All biologists have are some subjective hints, but most of the evidence has disappeared:
If these apparent selective sweeps are indeed the result of mobile DNA sequence insertions, why are insertion mutations that alter the expression patterns of adjacent genes in a selectively advantageous way not more common? Why do these so rarely seem to spread through the species as a whole? One can clearly create a model in which insertions are eventually followed by imprecise excisions, leaving behind a small fragment only of the inserted sequence, or causing the loss of all the insertion, along with some flanking host sequences. Such a change might still create the advantageous phenotype, and thus one can imagine that an advantageous insertion is replaced by its deleted derivative.
- Artificial effects. Lastly, Brookfield asks whether the mutations biologists have studied in fruit flies were not evolutionary at all, but results of man’s interference through the use of pesticides:
The other, more disturbing, aspect of this study is that the species is responding to a very strong, man-made selective pressure, as is the case with many of our best examples of recent adaptive change in wild populations. Are these sudden man-made changes in environments typical of the environmental changes that wild populations encounter, and to which they respond through evolutionary change? Or do environments more usually change in such a gradual way that the adaptive response is qualitatively different at the molecular level. In other words, just as the mutations seen in laboratories are not typical of the mutational changes used in adaptive evolution, is it possible that the mutational changes used in adaptive evolution triggered by sudden man-made environmental changes are not typical of the mutational changes used in adaptation to the more gradual environmental changes normally encountered by wild populations?
On that note, Brookfield quits. So where is the evidence for adaptive evolution resulting from mutations in fruit flies? He doesn’t say. The article leaves us hanging on question marks.
1John F.Y. Brookfield, “Evolutionary Genetics: Mobile DNAs as Sources of Adaptive Change?” Current Biology, Vol 14, R344-R345, 4 May 2004.
Time for a joke. Wife: “Why do you always answer my question with a question?” Husband: “Why not?” Encore: Spike Jones picks up the phone in one of his skits and we only hear his side of the conversation. We hear him responding, with various inflections, “You don’t say? … You don’t say? …. You don’t say?” After he hangs up, his curious buddy asks, “What did he say?” Jones replies, “He didn’t say.”
For this whole paper, we were waiting for Brookfield to say how fruit flies evolved from non-fruit flies by mutations and natural selection through a neo-Darwinian process, since that is the official creation myth of the culture that provides the explanations for whale aerodynamics (see 05/11/2004 headline), cell locks and keys (see 05/13/2004 headline) and transgender rights (see 05/17/2004 headline). But all we got was (1) Everything you were taught is wrong, and (2) How about if I answer your question with a question?
Remember that fruit flies are not simple, but some of the most elaborate miniaturized high-tech robots you could imagine (see 12/18/2003 headline). Darwinians have a lot more explaining to do than just weaving stories about how something might alter the expression of this or that gene. Brookfield spouted a lot of hot air about mobile DNA insertions, but these are a far cry from random genetic mutations. Mobile DNA elements represent genetic information that had to come from somewhere else, and were apparently not inserted at random, but in association with genes, altering or inactivating their expression. But a gene cannot be expressed unless it exists. Where did the gene come from? You don’t say.
This article does not represent a definitive treatise on neo-Darwinian theory, but it sure exposes some gaping holes the textbooks conceal. That means it must be banned from the classroom (see 05/13/2004 headline). When it comes to teaching evolution, “you don’t say” what the experts say.