Metamorphosis

Another Severe Challenge to Darwinian Evolution

Classical evolutionary theory describes how the diversity of plants and animals that we see today developed gradually over time in form and function from a single universal common ancestor. In his book, On the Origin of Species, Charles Darwin reasoned that, just as mankind can develop desirable traits in domestic plants and animals by selectively breeding for these traits, nature can develop desirable traits by a similar process (albeit, undirected), which he called "natural selection" acting on random variation. The key point is that each step needs to provide some increased survival benefit for the individual, or there is no reason to retain it for future generations.

We need to realize that the word evolution by itself has a range of definitions. Colloquially, we speak about a situation "evolving." Frequently, shifts within an existing species' population that are due to environmental changes are referred to as examples of evolution (e.g., peppered moths). A more expanded understanding of the word is the idea that one species can actually morph into a new, closely related species over many generations (e.g., brown bears and polar bears). The most extreme definition of evolution is Darwin's theory, where he states that all the varieties of plants and animals are descended from a single primordial cell by way of an undirected, incremental process.

The first few of these definitions are generally accepted, but the last one is still subject to some controversy, simply because there are large gaps in the data, and some findings don't seem to fit the existing models and descriptions. That is why research continues.

Gradual Development

Researchers in the field of evolutionary development (Evo-Devo) study the developmental processes of various organisms and seek to determine from their findings how various organisms are related and how they evolved. It seems reasonable to imagine that changes during development in an individual life cycle could reflect how a species might evolve into another species over time.

For example, tadpoles slowly grow legs while their tails are absorbed, and their gills disappear as their lungs develop. The development of a tadpole into a frog or toad can be used as an analogy for how a species could evolve from living in a water environment to living in a land environment through long-term evolution over many generations.

Another example is crickets that hatch from eggs. Although newly hatched crickets look similar to adults, they go through several stages of growth, known as "instars," before reaching full maturity. As they continue to eat, they fill out their exoskeleton until there is no more room. They then ­completely shed this exoskeleton (even over complex shapes, such as eyes, mouth parts, and legs), and let the new exoskeleton harden (a matter of days), so they are protected while they continue to grow.

Similarly, grasshoppers progress through a series of instars, usually about six. Unlike crickets, they change incrementally from the initial nymph stage to the adult form, with their wing-buds increasing in size at each stage. After the final molt, the wings are inflated and become fully functional. Here we see the same amazing ability to shed the exoskeleton that crickets have, but with a new level of complexity—gaining the ability to fly.

These developmental processes seem reasonable to many people as analogies to Darwin's evolutionary process. One can imagine the existence of a prehistoric grasshopper without wings, since it would be able to survive by moving about with just its legs, like a cricket. One can then speculate as to how wings might gradually develop, giving the insect a survival advantage by making it even more mobile.

Abrupt Metamorphosis

However, some organisms show abrupt changes during their lifecycle, which present significant hurdles to evolutionary theory. The butterfly is one such creature; its change during development is so abrupt that we call it a metamorphosis (see Figure).

The first part of the lifecycle of butterflies is similar to that of crickets and grasshoppers in that they begin as eggs. Then, unlike crickets and grasshoppers, they develop into caterpillars, which go through a series of molts (usually about five), shedding their skin as they grow in size by two or three orders of magnitude (effectively doubling in mass every couple of days over a period of a few weeks). During this stage, their external shape stays basically the same through each instar.

The next stage for butterflies, however, is an abrupt transition, far greater than the transitions experienced by crickets or grasshoppers. Right before beginning its last molt, the caterpillar attaches itself to a twig. Under the skin that it sheds, there is a different type of skin that hardens to form the chrysalis. Inside this chrysalis (or pupa), the caterpillar completes its transformation into an adult butterfly. This stage involves the simultaneous development of a whole variety of new body parts and shapes—all of which are necessary for a successful butterfly. This process appears much more difficult to explain by an evolutionary framework.

Let's look at the various changes that occur inside the chrysalis, and then decide which ones are essential to the functioning of the butterfly, and how they could form incrementally. We will use the Monarch butterfly as our example, as it is well studied and most people are familiar with it.

The first thing that happens inside the chrysalis is that the caterpillar's body digests itself from the inside out, using some of the same digestive juices it used to digest the leaves it previously ate. It then proceeds to assemble a new body with new structures—amazingly, while it is still alive. The new structures include:

  • A thinner, lighter, multi-segmented body: Three segments of thorax develop, each of which has a pair of legs attached to it, while the second and third segments each have a pair of wings attached as well.
  • Six long, thin, segmented legs, instead of sixteen stumpy ones: The hind four legs grip vegetation and flowers when the butterfly lands on a plant. The two front legs are held close to the body most of the time, but are used to taste-test milkweed before the butterfly lays its eggs.
  • Four wings: Two forewings and two hindwings are attached to the second and third body segments respectively, each with its necessary muscles and a system to inflate them. The butterfly can move these wings either by using muscles in the thorax (for flying) or by flexing the thorax itself (to move the wings out of the way when it is feeding on flowers). Recently it has been shown that each of the larger veins in a butterfly wing contains a central tracheal tube and one or two hemolymph channels; these control temperature and give the two-dimensional wings structure, strength, and support.1
  • Two-part hollow tongue: The straw-like proboscis is the butterfly's tongue, through which it sucks in nectar and water (instead of chewing leaves) for nourishment. The tongue is formed in two halves and must be assembled after the butterfly leaves the chrysalis. Since the proboscis is very long (sometimes up to 1.5 times the length of the body), it would be in the way and might be damaged when not in use, so it conveniently is able to be curled up.
  • A much smaller digestive system, modified for digesting nectar instead of leaves.
  • An improved sense of smell and taste: While the caterpillar has tiny tentacles near its mouth-parts, which provide some sense of smell, the butterfly's antennae, palpi (a pair of protrusions on the front of the head), legs, and feet have abundant sensory receptors with which it can locate flowers that have nectar, potential mates (producing pheromones), and milkweed plants where it can lay its eggs.
  • An improved sense of sight: A butterfly's compound eyes are made up of thousands of ommatidia, each of which senses light and images, instead of six pairs of simple eyes that barely detect light versus dark. This allows the butterfly to maneuver effectively at speeds thousands of times faster than a caterpillar.

Which of these structures is not essential for the butterfly to function (and so would not need to appear until a later generation)? Which of these changes might happen first and allow the caterpillar to make the initial step toward developing the capabilities we see in a modern butterfly? We submit that all of them are essential and that none of them appears to be beneficial by itself.

Too Much Change, Too Little Time

You may recognize this problem as one of irreducible complexity, a term coined by Lehigh University biochemist Dr. Michael Behe in his book Darwin's Black Box (Simon and Schuster, 1996). As Behe explains, if a system needs all its component parts to be in place simultaneously in order to function, there is no pathway for this to occur through a gradual Darwinian process. This is what Behe means by an irreducibly complex system. His specific examples include:

  • Blood clotting: a cascade of clotting factors, including calcium ions, twenty-odd proteins, and other factors, all of which must be present for the system to function.
  • The bacterial flagellum: a multi-component molecular motor that rotates at speeds up to 100,000 rpm to permit motion.
  • Transport mechanisms within the cell: multi-component processing and signaling systems that are used to transport a newly made protein to where it's supposed to go.
  • The immune system: another complex system by which the body defends itself against invasive pathogens.
  • The production of nucleic acids: these are the building blocks that make up DNA and RNA and are involved in energy production.

Behe also makes passing references to other cellular mechanisms (DNA replication, photosynthesis, transcription regulation, etc.).

The characteristic that marks all these systems as irreducibly complex is that every single component in the system must be present at the same time in order for it to function, just as multiple, simultaneous changes must occur within the chrysalis for the emerging butterfly to survive and function. In the butterfly, all these changes occur within the two-week period it spends inside the chrysalis. The combined probability of these necessary components coming into existence virtually at once is astoundingly low. Even billions of years is not enough time to give all this a reasonable chance of happening by a random process.2

The complexity becomes even greater when we move down to the genetic level and examine the molecular changes that trigger the functional changes we just listed. In the journal Genetic and Molecular Research, Shun Yao and others describe the developmental changes in genetic expression in another species of butterfly, the swallowtail butterfly.3 They found 1,723 differences in genetic expression between the caterpillar and chrysalis stages, and 1,162 such differences between the chrysalis and adult butterfly stages. They identified these differences as being related to the changes that occur in the digestion, cuticularization (skin formation in the caterpillar stage), chemoreception (for the sense of smell), wing formation, and other systems. Since mutations are relatively rare (and most would negatively affect the ability of the insect to live), the chance of all of these needed mutations occurring simultaneously, and correctly, is far beyond an astronomical number.

In a previous Salvo article,4 we reported on the time it would take for ten small proteins to form that would allow a rudimentary eyespot to detect light—a length of time, it turned out, that is far beyond the estimated age of the universe. In other words, it is basically impossible. A calculation of the time it would take for all the necessary genetic changes to occur that would allow a caterpillar to transform into a butterfly would yield a figure far, far greater. In fact, the time span is so incomprehensible that it strains reasonable analogies, making the random development of the necessary changes effectively impossible.

Sheer Cliffs on All Sides

Yet, Richard Dawkins, in his book Climbing Mount Improbable (Norton, 1996) suggests a way in which it could be possible. On one side, Mount Improbable, as he calls it, rears straight up from the plain in a sheer cliff face, making its towering summit seem unreachable. But around the other side of the mountain, there are gently inclined meadows, over which one can climb steadily and easily to the top. The height of the peak does not matter, says Dawkins, so long as you do not try to scale it in a single bound. Locate the mildly sloping path, and the ascent is only as formidable as the next step.

Following this guidance, let's try to imagine a scenario in which the caterpillar could slowly morph into a butterfly through successive "gentle gradients"—first developing wings, perhaps, then longer legs, then a long tongue, and so on until all the butterfly's features are in place. The problem is that no single one of the new features would be retained for succeeding generations of butterflies, since, by itself, the feature would not present a survival advantage. In fact, any of these features, developed singly or even in pairs, would be detrimental to the caterpillar's survival. Can you imagine a caterpillar with a long, straw-like tongue and sixteen stumpy legs?

Thus, we seem to have arrived at a total impasse in trying to find a Darwinian evolutionary path for a butterfly to evolve. It seems as though this Mount Improbable has insurmountable cliffs on all sides.

So how can these changes occur? The remaining explanation is that the information needed to trigger them was present in the caterpillar from the start. In other words, the butterfly's life cycle was intelligently designed from the beginning.  

Notes
1. Cheng-Chia Tsai et al. “Physical and behavioral adaptations to prevent overheating of the living wings of butterflies,” Nature Communications (Jan. 28, 2020): https://doi.org/10.1038/s41467-020-14408-8.
2. The short video An Everyday Miracle, showing the stages in the Monarch’s life cycle can be found at: https://thejohn1010project.com/an-everyday-miracle.html.
3. Shun Yao et al., “Preliminary Analysis on the Developmental Transcriptomes of Swallowtail butterfly Papilio Polytes,” Genetics and Molecular Research (Apr. 12, 2018).
4. Tom and Elizabeth Siewert, “Age Doesn’t Matter” Salvo 48 (Spring 2019), 28–32.

is a (mostly) retired metallurgist. He received a BS in math and physics, before moving into materials science, then finishing with a Ph.D. in metallurgy. He is now spending some free time in exploring some of life’s generally accepted, but poorly supported ideas. For topics in biology, it helps a lot to have a wife who knows biology.

earned a BS in math and chemistry. She worked in the field of molecular biology at the University of Colorado for 15 years, carrying out research in genetics, where she learned a lot about biological systems. After her children were grown, she went on to earn a Ph.D. in biostatistics, thus combining her love for both math and biology. Her interest is in quantitatively modeling biological systems.

This article originally appeared in Salvo, Issue #58, Fall 2021 Copyright © 2021 Salvo | www.salvomag.com https://salvomag.com/article/salvo58/metamorphosis

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