Convergent Evolution

A Monarch Butterfly caterpillar feedsIt’s official: ICR “news” articles are now called Creation Science Updates. Or rather “Update” – they haven’t really mastered this pluralisation thing. The first article of this brave new era is called How Some Insects Can Eat Poisonous Plants, by Brian Thomas.

The chemicals used by milkweeds (including swan plants) and similar as a poison to deter herbivorous insects are known as cardenolides. They work by blocking the sodium pumps in cell membranes, but a handful of organisms are resistant to its effects. The most famous of these is the monarch butterfly, which was already known to have a amino-acid substitution mutation (referred to as “N122H”) in the gene coding for the pump that contributed to the butterfly’s immunity. A new paper in PNASCommunity-wide convergent evolution in insect adaptation to toxic cardenolides by substitutions in the Na,K-ATPase (pdf) – takes a look at the underlying genetics of all 18 insects known to be resistant, along with a number of their relatives.

What they found was that many of the same substitutions were involved in immunity to cardenolides, even though the organisms weren’t all closely related (with relatives that did not have resistance tending to lack the substitutions). According to the ScienceDaily press release:

By examining molecular changes in the sodium pump gene, the researchers found the mutation N122H in all four orders of insects studied. Furthermore, they discovered a second mutation in the same gene that also conferred resistance in 11 of the 18 species.

(While N122H was found in four orders, it was only found in five species all told.) This is, to state the obvious, an example of convergent evolution – when two different species evolve similar adaptations. Mr Thomas, however, isn’t happy with that term being used.

How is it that such different insects have exactly the same DNA base changes in this gene?

The PNAS study authors attributed it to “convergent evolution,” a term suggesting that the identical genetic substitutions occurred separately “at least four times” in insect groups over the course of 300 million years. However, they offered no details describing how this could occur—even in theory.

Did they really have to? We know how mutations work, and we know how convergent evolution works. This is basic stuff – both aspects will probably feature in my highschool biology exam in a fortnight (Mr Thomas was once a biology teacher).

The mutations that have been here found to confer resistance all work individually, and the two that were tested together were found to function even better in tandem. They were also not all found together in every resistant organism, as Brian implies, and his “at least four times” quote refers specifically to the N122H substitution. Mutations occur all the time – natural selection gets rid of many, but some are beneficial and proliferate. So a beneficial mutation like the N122H substitution can fix itself in the population (become present in all surviving organisms). If there’s another possible mutation that’s also helpful then it too can become fixed – there’s nothing strange going on here beyond the normal processes of evolution.

What makes this case interesting is that those same mutations were beneficial in multiple organisms, and so the same substitutions have arisen and taken hold multiple times. This is convergent evolution, at least in the realm of genetics.*

Still, Brian is not happy:

Convergent evolution is conceivable, but it is scientifically meaningless unless researchers can actually detect it. Otherwise, to claim convergent evolution as these authors did is merely to beg the question of convergent evolution. In other words, the study authors ignored all non-evolutionary explanations for how these remarkably specific DNA differences arose.

Perhaps the DNA differences were directly created, or perhaps well-designed cellular systems put them in place at some point after creation. The first possibility is blind to scientific experiment, which cannot directly investigate the past. No scientific experiment has verified the second possibility, but no experiment showed that these systems arose by convergent evolution either.

Imagine, please, what would happen if we let this kind of reasoning pass in a forensics investigation? I mean to say, creationists love bringing up court cases as analogies to “historical science” – surely Brian can find somebody to explain to him the problems with letting people off because God might have done it (and you haven’t recreated the murder in every detail to prove otherwise).

Thomas’ “argument” – such that it is – is also hypocritical. Earlier on in his article he said this (the first sentence of which is inaccurate, I might add):

The scientists discovered through rigorous experimentation that all 18 insects had certain amino acids substituted at numbered positions 111 and 122 of the gene that codes for the cellular pump. Very few positions are altered in this gene. Since most alterations would diminish or halt its vital effectiveness, creatures tolerate very few such changes.

He does not seem to realise that by concluding that “creatures tolerate very few such changes” to the gene on the basis that few changes have been made to it he is having to accept evolutionand especially that the Earth is old – as a premise. If God created the gene as-is, or if Brian’s imaginary and unspecified mechanism is operating, then the evidence is insufficient to conclude that the gene is resistant to mutations. Yet this gets a pass, but the merest mention of convergent evolution – which he goes so far as to call “conceivable” – does not. I’m forced to conclude that creationists are first and foremost allergic to the word “evolution,” and only secondarily to the ideas behind it.

Thomas concludes:

These researchers conducted a rigorous study, to their credit. However, there was no scientific reason for them to have excluded origins possibilities that are at least equally valid.

His proposed “possibilities” have no mechanism, entirely lack supporting evidence, and fail Occam’s razor. Unless the scientists at the Institute for Creation Research can somehow rectify these problems Brian’s whining is not worth anyone’s consideration.

*The type of convergent evolution we tend to be more familiar with is when the same morphology – eg in the case of the thylacine and the placental wolf – is created by different underlying structures and genetics. The lower-level variety in this paper still falls under the same term, but is less well-studied.

One thought on “Convergent Evolution

  1. “we know how convergent evolution works”.
    Then please can you explain how it works? No need to get too technical, but if there are particular scientific papers discussing the mechanism of convergent evolution, it would be great to have those references!


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