Q: MUTANT FRUIT FLIES. Why do textbooks use fruit flies with an extra pair of wings as evidence that DNA mutations can supply raw materials for evolution — even though the extra wings have no muscles and these disabled mutants cannot survive outside the laboratory?
I don’t know that a lot of textbooks actually do this, but let’s have a look at Wells’ question in more detail anyway.
When the four-winged flies do crop up, they’re used as an example of the way that some types of mutation do indeed provide ‘raw materials’ for evolution. These mutations affect genes that control the way the various bits of a body are organised.
Fruit flies (Drosophila) are dipterans. This means that they belong to an order of insects that has a pair of knob-like structures – the halteres – where other insects have a second pair of wings. Mutations in one gene complex (the Ubx genes) influence the size of the halteres. if all three mutations occur together, then instead of halteres, the fly ends up with a second pair of normal-looking wings. The Ubx genes are an example of a ‘Hox complex’ – a group of genes that control the expression of other genes involved in building body parts. Because of their high-level control role, small changes in the Hox genes can have significant effects on body form. For example, Ubx genes prevent development of legs on the abdomen in flies (and all other insects), but not in crayfish.
Like Wells, other advocates of Intelligent Design that [the] mutations that produce four-winged fruit flies teach us something about the genetics of development, but because they eliminate structures and harm the organism they provide no evidence for neo-Darwinian evolution (de Hart, 2004: 11). This statement reflects the general creationist view that mutations are by definition harmful and can never give rise to evolutionarily useful novelty. It also suggests a misunderstanding of the significant role of Hox genes in evolution, given that mutations in control genes can lead to quite significant differences in form and function. The importance of these genes is highlighted by Eugenie Scott, who points out that
[mutations] in these primary on/off switches are involved in such phenomena as the loss of legs in snakes, the change from lobe fins to hands, and the origin of jaws in vertebrates. HOX-initiated segment duplication allows for anatomical experimentation, and natural selection winnows the result. ‘Evo-Devo’ – the study of evolution and development – is a hot new biological research area.
What’s more, Wells is again resorting to a red herring. Outside the lab, these 4-winged flies would probably be selected against – though it’s impossible to predict what might happen in some future, changed environment. (For example, one hypothesis for the evolution of insect wings proposes that wings began as outgrowths from the thorax that were functioned in thermoregulation & were later co-opted for flying.) But nonetheless they show us how mutations can produce novel features that natural selection can then operate on – so much for the common creationist claim is that mutations can’t produce novel structures…
de Hart, R. (2004) Icons of Evolution: the growing scientific controversy over Darwin. Coldwater Media.