The Enigma of Metamorphosis Is Hardly Limited to Butterflies - Evolution News & Views

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The Enigma of Metamorphosis Is Hardly Limited to Butterflies

Before systematically demolishing the only Darwinian explanations on offer for butterfly metamorphosis, Paul Nelson (Biola University) and Ann Gauger (Biologic Institute) provide the fascinating background to this devilish problem for orthodox evolutionary theory. It's all in their essay "Stranger than Fiction: The Riddle of Metamorphosis" in Metamorphosis: The Case for Intelligent Design in a Chrysalis, the just published and FREE companion e-book to the new Illustra documentary Metamorphosis. Download it here.

Excerpt from Nelson and Gauger:

The butterfly's life history involves the development of not one, but two sequential body plans. This transformation is known as metamorphosis (literally meaning "to change one's shape").

Strange as this story is, metamorphosis is not limited to insects. In fact, animals whose life histories involve two or more distinct body plans are the rule rather than the exception. Most marine invertebrates (animals without backbones) have a metamorphic life history, with one or more free-swimming larval stages, followed by a bottom-dwelling, reproductive adult stage. Parasitic organisms can also have two or more developmental body plans, each specially adapted for a particular host. Even some vertebrates, including frogs, toads and salamanders, go from an aquatic larval stage (tadpole) to a terrestrial adult. Given the widespread presence of metamorphosis as a developmental strategy, one would expect there to be good explanations for its evolutionary origin. Yet it remains an enigma, for a number of reasons.

What the Fossils (Don't) Show

According to standard geological models, multicellular marine invertebrates such as crustaceans, worms, mollusks, and echinoderms appeared for the first time during the Cambrian explosion 540 to 520 million years ago. The sudden appearance in the fossil record of animals with such complex, distinct body plans has spawned many theories but no satisfactory answers. Darwin himself acknowledged that this mystery was the single most difficult challenge to his theory. Even more mysteriously, it appears that the most ancient phyla were metamorphic from the beginning, based on the few larval forms that have been preserved. This suggests that these Cambrian animals had not one but two or more developmental stages at the outset, a small and free-swimming larva, and a bottom-dwelling adult with little or no resemblance to its earlier form. But how such transitions could have evolved, and from what, is completely unknown.

In contrast, insects arrived on the scene much later, in the late Silurian or early Devonian, and apparently developed metamorphosis secondarily. The most ancient insects were wingless, terrestrial animals that developed directly into mini-adults, and lacked any metamorphosis (this is called ametabolous -- literally, "without changing" -- development).

Winged insects such as dragonflies and mayflies appeared in the late Devonian or early Carboniferous. Scientists generally agree that all winged insects came from a single lineage, but debate still rages in the scientific community about how it happened. It appears, based on some fossilized nymphs and adults and from what we know of their modern relatives, that from the beginning these insects had a partial form of metamorphosis (hemimetabolous -- literally, "part changing" -- development). The nymphs resemble adults in many respects, but lack wings and reproductive structures. Through several successive molts their wings grow gradually, with fully developed wings and reproductive organs appearing only in the adult. Other familiar hemimetabolous groups include grasshoppers and crickets.

Insects that undergo complete metamorphosis, such as beetles, flies and ants, did not appear until the late Carboniferous or early Devonian. These insects have been fabulously successful. In fact, nearly 85 percent of all modern insect species have holometabolous -- literally, "all changing" -- development.

Butterflies and moths were among the last to appear on the scene. Their order, the Lepidoptera -- literally, "scaly wings" -- first appeared in the fossil record in the Jurassic, and more significantly in the Cretaceous. These insects have a dramatic -- and well-known -- holometabolous life history.

What distinguishes holometabolous species is their strikingly different life stages. The major stages of holometabolous (abbreviated Holo) metamorphosis are (a) egg, (b) larva (often given a different name, such as "caterpillar"), (c) pupa (or chrysalis), and (d) adult, in that sequence:Holo egg → larva → pupa → adultContrast this with hemimetabolous (abbreviated Hemi) development:Hemi egg → nymph → adultBecause hemimetabolous development appears simpler, and because fossil insects with this pattern of development appear earlier in the fossil record, it is thought by most scientists to be evolutionarily primitive, evolving prior to the more complicated pathway seen in most modern insects.

Thus, from an evolutionary standpoint, the problem of the origin of butterfly metamorphosis -- in particular, of the pupal stage -- is really the problem of the origin of holometabolous metamorphosis generally, not just in Lepidoptera.

Although, of course, "the pathway is especially dramatic in butterflies."


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