It's hard to believe that these 3 Heliconius butterflies are completely different species. They have very similar wing shape, color, and patterns, and are all perched on passion flower vines, their food source and a cozy place to lay their eggs. Heliconius have a good reason for resembling each other: predators know that bright butterflies make a really foul-tasting meal (the good-tasting ones just mimic the bad-tasting ones so they won't get eaten.)
Evolutionary biologists have studied Heliconius butterflies for more than a century--the first paper on Heliconius mimicry came out in 1879 and is a classic example of convergent evolution, environmental pressure causing different species to look or behave increasingly alike. Today, genetics inform new studies, but might change the way we think about their evolution.
A recent study reveals an amazing thing about Heliconius wing color: the evolution of only one gene is responsible for Heliconius mimicry across dozens of species. Little tweaks in one gene, over generations, are responsible for making dozens of different species similar. "This is our first peek into how mimicry and convergent evolution happen at a genetic level," researcher Robert Reed said in the press release. "We discovered that the same gene controls the evolution of red color patterns across remotely related butterflies."
Though these species look alike, we have to remember that they are classified as different species, and thus their genes are arranged and/or regulated differently. Which makes it astonishing that the same gene is responsible for changes in each species. "This is in line with emerging evidence from various animal species that evolution generally is governed by a relatively small number of genes," says Dr. Reed. "Out of the tens of thousands in a typical genome, it seems that only a handful tend to drive major evolutionary change over and over again."
But...if only one gene is involved...is this a case of convergent evolution or homology? Reed asks the same question in the abstract of his study's paper, commenting that it"blur[s] the distinction between convergence and homology."
Do these butterfly species come from a different-enough lineage so we can say they evolved to converge on wing color? Years ago, a study showed that non-sterile hybrids of Heliconius species were possible. These hybirds were first noticed in their habitat outside the lab, and so are an example of what scientists once though was mimicry but was really hybridization. The Reed study uses "distantly related species," though, and still finds the same gene involved.
The other possibility is that the wing color gene comes from a common ancestor of all Heliconius butterflies, and since evolution is acting that single gene to change wing color, it's an example of homology. Who knows--maybe Heliconius wing color is like whale flippers and human arms: they all come from the same ancestral part, tweaked, regulated, deregulated, and changed completely since they last had a common ancestor.
Maybe it's homology then convergence...the butterflies first evolved into different species and then converged on wing color. Truth is, I'm not really qualified to speculate. Any takers?
SOURCE -- Heliconius.org, UCI
Reed RD, Papa R, Martin A, Hines HM, Counterman BA, Pardo-Diaz C, Jiggins CD, Chamberlain NL, Kronforst MR, Chen R, Halder G, Nijhout HF, & McMillan WO (2011). Optix Drives the Repeated Convergent Evolution of Butterfly Wing Pattern Mimicry. Science (New York, N.Y.) PMID: 21778360
You are probably right,
ReplyDeleteUnder the most parsimonious hypothesis, optix was co-opted once into wing color patterning (single origin -> homology) but then, there was divergence within lineages and evolutionary convergence between sympatric butterflies in expression of optix, explaining diversity and convergence in wing patterns.
So the character "optix making colors" is homologous but the character states (where it is expressed -> shape of the color patterns) is polymorphic with cases of convergence.
-a co-author on the study-