Metamorphosis from Evolution: A few questions

> Just out of curiosity, how would one account for the evolution of the
> metamorphosis process of animals such as frogs and butterflies? It has
> always seemed improbable to me that such an elaborate process could have
> evolved due to natural selection for random genetic mistakes. However, I
> readily admit that this is a topic in which I am utterly ignorant! Perhaps
> you would be willing to share your insights?

Evolutionary advantages of the process include the lack of competition between larvae and adults and the different dispersal and survival potential of different lifestyles in various situations. Thus, once such a system develops, it may be fairly successful (e.g., beetles).

Insects appear to be derived from crustaceans. Like all arthropods (and closely related phyla such as Nematoda), they have a hard cuticle and grow by a series of molts. The most primitive forms, such as silverfish among the insects, have larvae that are essentially miniature adults. The next stage fo complicaiton comes in the winged insects, in which the last (in mayflies, perhaps the most primitive extant flying insects, the last two) stages have wings. However, the rearrangement is relatively gradual and small.

The more dramatic changes of having a pupa stage and a larva that differs greatly from the adult are found in a group of advanced insects, including butterflies, beetles, flies, wasps, etc. Advantages include having a relatively durable stage that may survive adverse conditions (e.g., limited food in cold/dry/etc. season) and the developmental opportunity to go through a more radical rearrangement than is typically feasible between stages of other insects.

Although there are now a few insect genomes and at least one nematode genome known as a distant non-metamorphisin comparison, evolutionary studies on details of their metamorphosis have a long ways to go to establish genetic causes. Evolution is not a major focus or strong point of much biochemical research, as evidenced by the sweeping conclusions often drawn from comparisons of the handful of genomes available.

(Note that primitive and advanced are technically only applicable in a strictly relative sense. E.g., silverfish have specializations of their own; their simple metamorphosis seems to reatin the ancestral state, however.)

In amphibians, the aquatic (or at least high humidity-dependent) egg is a holdover from fish ancestors. Thus, the young are going to start out in water in all the species with distinct metamorphosis. As Vernon has pointed out, good swimming abilities may be useful in aquatic species, so increased specialization for aquatic life makes sense for the young. Meanwhile, the adult can use increased specialization for terrestrial life. The transition is fairly gradual, as can be seen by keeping a tadpole or young salamander in an aquarium. We just had a tadpole grow legs in the lab here.

This is also the evolutionary problem with Haeckel's embryos. He thought that development was a museum of past adult stages. However, this ignores the selective pressures acting on the juveniles, in addition to making incorrect assumptions about the way in which new features are added (they are not invariably additions in the adult stage).

Metamorphosis is sufficiently complex to have some appeal for ID advocates. However, probably the most complex metamorphoses are associated with parasites. Special design of schistosomiasis, etc. runs afoul of claiming, e.g. that God would not have created with death as part of the process.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL 35487-0345 USA
    bivalve@mail.davidson.alumlink.com

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa
Received on Wed Jun 23 21:16:46 2004