Macromutations and Hopeful Monsters
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Macromutations and Hopeful Monsters

Sometimes small mutations in genes that regualte early development can have profound effects. Most of the time these effects are detrimental to the survival of the organism. But not always...

Hopeful monsters, a term first used by Richard Goldschmidt in his book The Material Basis of Evolution (1940), have long been considered as freaks of nature that amounted nothing to general evolution. Recently, however, it seems that they might have been, or still are, more important than initially thought.

A hopeful monster is an organism that looks very different from its conspecifics, because of a macromutation that has taken place. One might think that this causes difficulty for the monster to reproduce. This is not the case, since a small genetic change can have major consequences. For example, let’s take a look at the so-called Hox-genes. These are the genes that, in early stages of development, regulate the orientation of the body axis. Crudely stated, these genes define what is the front and the back of the organism and also what is the top and the bottom. Furthermore, they define the number of segments and appendages. Knowing this, it is easy to imagine that a small change in genetic structure can have major effects on the phenotype of the animal or plant.

So much for the theory. But are there real-life, experimental examples? Yes, there is a study that examined the effects of a heat-shock on the phenotypical variation perceived in a laboratory population of the fruit fly (Drosophila melanogaster), one of the favorite test subjects of geneticists. Through a change in temperature during early development, the function of the protein Hsp90 was compromised. This protein is responsible for buffering naturally occurring genetic variation in several morphogenic pathways. When this protein is disabled, this variation is expressed. This lead to a variety of monsters, flies with extra or less legs, wings, eyes, a different eye color or the replacement of one appendage by another one. In addition, it was shown that these mutants and their offspring continued to express these new or altered traits, even after the function of the Hsp90 protein was restored.

Now could such monsters play an active role in the evolution of life on earth? This question, of course, is extremely difficult to answer. The lack of transitional fossils does not necessarily mean that a characteristic evolved through a hopeful monster. One possible event that might have been facilitated by a hopeful monster, is the sudden apparition of the turtle shield. According to the gradual theory, the shield is the result of an increase in the number of bony plates (called osteoderms) on the back of the turtle, which were finally joined together. There are two problems with this theory. Firstly, the oldest known turtle fossil (Proganochelys, 215 Mya) already possesses a full shield. Secondly, the shoulder blades lie within the rib cage. The gradual theory explains this through a backward of the pectoral girdle. But this does not seem to be the case. More likely, the ribcage moved outward, which would be caused by a change in embryonic development.

Perhaps there are many more events which required a sudden, profound change in phenotype. Think of the asymmetry in flatfishes, the bald head of vultures and the cheek pouches of several rodents. There is still much discussion about the importance of hopeful monsters, but it is becoming increasingly clear that small genetic changes can have unpredictably large effects. And do these effects always have to be detrimental to the survival and reproduction of the organism? Not necessarily. So could hopeful monsters could have played an important role in evolution? Possibly.

Sources

Gould, S.J. (1980) The Return of Hopeful Monsters. Natural History. 86, 22-30.

Rieppel, O. (2001). Turtles as hopeful monsters. BioEssays. 23, 987 – 991.

Rutherford, S.L. & Lindquist, S. (1998). Hsp90 as a capacitor for morphological evolution. Nature. 396, 336 – 342.

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