Butterfly Speciation: Mountains from Molehills - Evolution News & Views

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Butterfly Speciation: Mountains from Molehills

Heliconius.jpeg

Tropical butterflies tend to show remarkable variability in their wing patterns. Even males and females of the same species can look very different. Most strikingly, some species are able to mimic other species with uncanny precision. We last considered this in 2011 (here, here and here) to see whether the variations are best explained by Darwinism or design. Now, once again, a new genome comparison study tries to extrapolate small-scale variations in wing patterns of butterflies to grander claims about macroevolution.

News from the University of Chicago describes genome comparisons of Heliconius butterflies in Central America, referring to a paper in Current Biology by Dr. Marcus Kronforst, an assistant professor of ecology and evolution. Kronforst and his team found evidence for evolutionary divergence of three species over time. It sounds like classical Darwinian "origin of species" by natural selection:

To reveal genetic differences critical for speciation, Kronforst and his team analyzed the genomes of two closely related butterfly species, Heliconius cydno and H. pachinus, which only recently diverged. Occupying similar ecological habitats and able to interbreed, these butterfly species still undergo a small amount of genetic exchange. (Emphasis added.)

A map in the paper shows that H. cydno lives on the east side of the central mountain range, and H. pachinus on the west side.

The researchers found that this regular gene flow mutes genetic variants unimportant to speciation -- allowing them to identify key genetic areas affected by natural selection. The butterfly species, they discovered, differed in only 12 small regions of their genomes, while remaining mostly identical throughout the rest. Eight of these coded for wing color patterning, a trait important for mating and avoiding predation, and under intense selection pressure, while the other four remain undescribed....

The team also compared the genomes of these two groups to a third species, still closely related but further removed on an evolutionary time scale. Here, they found hundreds of genomic changes, indicating that the rate of genetic divergence accelerated rapidly after the initial changes took hold.

"Our work suggests that a few advantageous mutations are enough to cause a 'tug-of-war' between natural selection and gene flow, which can lead to rapidly diverging genomes," Kronforst said.

At first glance, this indeed looks like strong support for speciation. But a number of important questions went unanswered by the stated conclusions:

  1. Despite the striking differences in wing patterns between H. cydno and H. pachinus, if they are able to interbreed, does that not make them members of the same species according to the biological species concept? Humans show striking differences in skin and hair color, but all are members of one species -- Homo sapiens. Would it not be ridiculous to assume that blacks and whites are undergoing a speciation event?
  2. Are the genomic differences between the most distant of these species comparable to the genomic differences one might find between human haplotypes?
  3. Are the genomic differences tied to new functional information, as opposed to mere shuffling of existing information, or random genetic drift?
  4. How is it assumed that these two species are only "recently diverged" compared to the third species that is "further removed on an evolutionary time scale" without reasoning in a circle with Darwinian notions of time and branching speciation patterns?
  5. How do these findings support Darwinian theory without also supporting the views of Darwin skeptics who allow for significant variability within species or genera but do not believe variations can be extrapolated endlessly across higher taxa?
  6. Do the data refute the possibility that variability within species or genera is a design feature to permit adaptability to new environments?

It would seem highly presumptuous to use these findings as support for Darwin's grander claims that butterflies themselves -- along with giraffes, petunias and swordfish -- are all descended from a microbial ancestor by unguided natural processes. Yet Kronforst holds that his study supports that grander claim. The paper says the work demonstrates "the link between microevolutionary processes acting within species and the origin of species across macroevolutionary timescales."

Nevertheless, the news release reveals striking ignorance about speciation after 154 years of Darwin's theory:

"Speciation is one of the most fundamental evolutionary processes, but there are still aspects that we do not fully understand, such as how the genome changes as one species splits into two," said Marcus Kronforst, Ph.D., Neubauer Family assistant professor of ecology and evolution, and lead author of the study....

 "It is possible that this type of speciation, in which natural selection pushes populations apart, has been important in the evolution of other organisms. It remains to be seen whether it is a common process though," Kronforst said.

We don't wish to diminish the value of genomic comparisons. The team did yeoman's work gathering the butterflies, sequencing the genomes, and analyzing the differences. But the Darwinian conclusions are nothing new; they're like the grand claims made from other microevolutionary studies that imagine mountains growing from molehills. Remember the hubbub over those other lepidopterans, the peppered moths? Generations were taught it was a classic example of evolution in action, only to hear scientists more recently dismiss the study as irrelevant to Darwin's theory (and poorly done, at that). This new paper is silent about the peppered moth myth, but continues the tradition of assuming enough microevolution (given enough time) will add up to macroevolution.

What, in the end, has been demonstrated? Two or three butterfly species show different amounts of genetic variation in wing patterns. Marvelous. Now tell us how butterflies, with all their complex organs, metamorphosis, flight, navigation skills and multitudinous integrated complex systems arose without design. To understand those challenges to Darwinian thinking, see Illustra's documentary Metamorphosis: The Beauty and Design of Butterflies.

Heliconius butterfly/Illustra Media.