As we learnt from the text book, the four-winged Drosophila is the phenotype (a homeotic transformation of the halteres of the third thoracic segment changed into the wings normally associated with the second thoracic segement) that results from a Ubx gene mutation. But, of course, there are many insects, such as dragonflies, butterflies, and bees, which normally have four wings. Is two-winged flies evolved from four-winged ancestors, or opposite?
To investigate this question, I look up the text book of evolution. In fact, all the phylogenetic relationship of various insect groups and evidence from the fossil record indicate that the ancestor of all these winged insects was one with four wings. During the lineage leading to files, the wings of the third thoracic segment were modified into halteres. This is not surprising, at least for me. In my point of view, flies are very successful animals and should be more evolutionary than most insects. And also I think evolution of wings from halteres is harder than degeneration of halteres from wings. But, how this evolutionary event happened in history? Scientists have at least three hypotheses to explain the transition from four-winged to two-winged insects.
About 60 years ago, the first hypothesis is that the four-winged common ancestor of flies and dragonflies lacked the Ubx gene. Somewhere in the evolutionary lineage leading to flies, Ubx appeared and resulted in the transformation of the third thoracic wings into halteres. But after the development of molecular biology, it is discovered that Ubx gene is present throughout the insects, this hypothesis could be rejected.
Then, it is common to think that the Ubx gene is in all insects but expressed different between two-winged and four-winged insects. So the second hypothesis was that the evolution of two-winged from four-winged insects is caused by the mutations that affected the regulation of Ubx. But when the Ubx expression pattern was examined in a number of different insects, it was found to be highly conserved, even in wingless insects. So, the second hypothesis could also be rejected.
The third hypothesis, currently favored, focus on the downstream targets of Ubx gene. Evidence shown that in flies, various genes involved in processes such as vein formation, sensory hair growth, and cell proliferation came under the control of Ubx. Considering that the wings have veins and sensory hairs, whereas the halteres do not; the wing is far larger and contains a greater number of cells than does a haltere, it is very likely the expression of Ubx in flies’ third thoracic segment cause the formation of halteres instead of wings. But these same genes in butterflies are not regulated by Ubx, at least not in the same way.
Combining these three hypotheses and combining the basic rule of Darwinian Theory, these three kinds of mutations must be happened in history (and even nowadays), but most of them caused lethal larva, like the null mutation of Ubx gene. Some of these mutations lower the fitness of insects that might have less offspring than others, like wings are not developed in flies’ third thoracic segment nor are halteres, which make flies have trouble on flying. After one or more generation, these mutates are completely die out and we cannot see such population in wild nowadays. What we can see today is the only few of the collection of such mutations, which is rare events, but the result is the dominance of such mutates. This is the most attractive parts of evolution for me.
Nicholas H. Barton, Derek E.G. Briggs, Jonathan A. Eisen, David B. Goldstein, Nipam H. Patel. 2007. Evolution. Cold Spring Harbor Laboratory Press