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Modular Paint-Box Explains Butterfly Color Patterns


Butterflies of the Amazon exhibit astonishing beauty and diversity. The Heliconius genus, in particular, is striking for its examples of mimicry, where members of different species have converged on the same color patterns on their wings. Collections of these butterflies can be arranged into series where yellow and red spots grow and shrink, grading into one another. How did this diversity come about?

The usual answer is Darwinian evolution -- a mutation of a gene leading to a novelty, then common ancestry and natural selection. A new model, published in PLOS Biology, does not dispute that kind of evolution, but offers different mechanisms that fit with intelligent design. As the authors discuss in news from the University of Cambridge, it's a story of shared technology.

Research finds independent genetic switches control different splotches of colour and pattern on Heliconius butterfly wings, and that these switches have been shared between species over millions of years, becoming "jumbled up" to create new and diverse wing displays. [Emphasis added.]

Everything else about the butterflies appears normal; the body anatomy; the antennae; even the basic wing shape. It's primarily the color splotches that vary. Each one has a so-called dennis patch at the base of the wing, and red "ray" streaks that fan out across the hindwing. Within any individual, though, the patterns remain symmetrical: the left wing is a mirror image of the right wing.

The Cambridge researchers looked at the genomes of dozens of specimens.

New research on butterfly genomes has revealed that the genetic components that produce different splotches of colour on wings can be mixed up between species by interbreeding to create new patterns, like a "genetic paint box."

This is not the usual mutation-selection mechanism of Darwinian evolution, in other words. The researchers believe that existing genetic switches can be "shared" between species by interbreeding, hybridization and introgression, a process of gene flow from one species into another population that results from repeated backcrossing of an interspecific hybrid with one of its parent species.

Novelty is not originating by mutation, in short, but rather by new combinations of genetic switches that were already present. Interbreeding and hybridization shuffles these genetic switches that turn particular colors on and off in different parts of the wings. Is this type of sharing common in the world?

It has been known for some time that exchange of genes between species can be important for evolution: humans have exchanged genes with our now extinct relatives which may help survival at high altitudes, and Darwin's Finches have exchanged a gene that influences beak shape. In butterflies, the swapping of wing pattern elements allows different species to share common warning signs that ward off predators -- a phenomenon known as mimicry.

However, the new study, published today in the journal PLOS Biology, is the first to show such mixing of genetic material can produce entirely new wing patterns, by generating new combinations of genes.

One wonders if these researchers realize they have undermined Neo-Darwinism in these famous examples. Swapping of genetic elements does not require mutation and selection. Moreover, early humans, finches and butterflies had to be able to interbreed or at least hybridize to benefit from the shared information.

The authors state that these switches evolved just once, then are used in different parts of the wings. In different individuals, they can see little regions of color "jumping about all over the place." And because of pleiotropy -- the linkage of the color function to other functions on the gene -- the gene itself cannot evolve:

The key to this evolutionary butterfly painting is the independence of each genetic switch. "The gene that these switches are controlling is identical in all these butterflies, it is coding for the same protein each time. That can't change as the gene is doing other important things," said lead author Dr Richard Wallbank, also from Cambridge's Department of Zoology.

"It is the switches that are independent, which is much more subtle and powerful, allowing evolutionary tinkering with the wing pattern without affecting parts of the genetic software that control the brain or eyes.

"This modularity means switching on a tiny piece of the gene's DNA produces one piece of pattern or another on the wings -- like a genetic paint box," Wallbank said.

We must not get confused about the meaning of "evolution" in these papers. The wing patterns may vary, but all the species are members of one genus. We're looking at a mechanism for robustness and flexibility that is consistent with intelligent design. In the tropics where these butterflies live, the ability to try out new color patterns can be advantageous to attract mates and avoid predators. These are tiny changes to combinations of existing switches, not matters of great transformations, such as gaining a new organ. It's mere shuffling of what already exists.

The paper is even more emphatic about nature preventing novelty by mutation:

One of the major impediments to evolutionary innovation is the constraint on genetic change imposed by existing function. Mutations that confer advantageous phenotypic effects in a novel trait will often result in negative pleiotropic effects in other traits influenced by the same gene. Several mechanisms have been proposed by which evolution can circumvent such constraints, resulting in phenotypic diversification. In particular, the modularity of cis-regulatory elements means that novel modules can encode new expression domains and functions without disrupting existing expression patterns. This modularity underlying gene regulation has led to the assertion that much of morphological diversity has arisen through regulatory evolution.

What does this do to old-fashioned Darwinian gradualism?

This might seem to imply that the evolution of novel regulatory alleles is relatively gradual, requiring the evolution of many small effect substitutions, but recent adaptive radiations can show extremely rapid rates of morphological change. The role of regulatory modularity therefore remains to be tested in adaptive radiations in which morphological variation evolves very rapidly.

Although this paper is full of the word "evolution," it's really a different concept they are proposing.

We show that two patterning switches -- one that produces red rays on the hindwing and the other a red patch on the base of the forewing -- are located adjacent to one another in the genome. These switches have each evolved just once among a group of 16 species but have then been repeatedly shared between species by hybridisation and introgression. Despite the fact that they are now part of a common pattern in the Amazon basin, these two pattern components actually arose in completely different species before being brought together through hybridisation. In addition, recombination among these switches has produced new combinations of patterns within species. Such sharing of genetic variation is one way in which mimicry can evolve, whereby patterns are shared between species to send a common signal to predators. Our work suggests a new mechanism for generating evolutionary novelty, by shuffling these genetic switches among lineages and within species.

In addition, they appeal to "convergent evolution" to explain some spectacular cases of mimicry. But if this were all due to chance, why are the patterns symmetrical? What about Darwinian mechanisms would require the left and right wings match?

None of what they describe requires neo-Darwinism or supports macroevolution. The switches were already there. They only control how much color goes into each spot. They can be shared within and across species by shuffling mechanisms, allowing rapid changes in wing patterns. It sounds like a great way for butterflies to quickly send different signals in different situations, so that they remain viable. Think of a flagman, using the same flags but in different combinations to send different messages. The flags were already there, and the man (or a programmable robot) knows how to use them.

Evolutionists have focused long and hard on the patterns on butterfly wings, which indeed are interesting, but they need to explain the weightier matters: how an egg becomes a crawling caterpillar, enters a chrysalis, and emerges a flying butterfly. As illustrated in Metamorphosis: The Beauty and Design of Butterflies, that's like turning a Model T into a helicopter.

The very traits they have focused on (wing patterns) turn out to be non-Darwinian and restrained by pleiotropy, reliant on "switches" that look designed. How much more must the critical organs for digestion, reproduction, and flight require specific information targeted for those functions? Only intelligence creates that kind of information.

Image: Tiger Longwing (Heliconius hecale), by Robert Lawton [CC BY-SA 2.5], via Wikimedia Commons.