A test for claimed knowledge of how macroevolution occurs

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But remind me again: how do they recognize it as a mutation? I mean what about the DNA sequence makes it a mutation?

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Somethig messed up my post and I'm not up to repeating it.

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These are the salient results of the van’t Hoff et al paper:

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The gene where the change resides that made the light-colored moths dark is called cortex, and is known in Drosophila to be involved in cell division of the egg in females. It wasn’t known, at least in flies, to be involved in color.

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The association between cortex and color was found by “association mapping” in B. betularia. The researchers took a bunch of moths of both the carbonaria and typica forms (and insularia as well), and sequenced DNA in the region where the mutation was known to reside, looking for a consistent change in the DNA that would distinguish the various forms with near-perfect ability.

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The change was found not to be a single-base mutation in the DNA, but an insertion of a “transposable element” (or “transposon”)”a bit of DNA that can move around in the genome”into the carbonaria form. The whole inserted region comprises 21,925 nucleotides, and involves a single element present in 2.3 copies that has moved as a unit into the cortex gene.

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The transposable element actually activated (rather than silenced) the cortex gene, increasing the amount of its product in a manner we don’t understand. Curiously, the increased gene activity in the dark form was far more pronounced in larvae (caterpillars, which the paper calls “crawlers”) rather than pupae or adults, presumably because the precursors for scale development and their color are being formed at this stage of development.

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The association between the transposon and color was nearly perfect, but not 100% so. Every one of the typica and insularia moths lacked the element, while 105 of 110 black carbonaria forms had the element. This means that other genes besides cortex may influence color”or developmental/environmental effects that aren’t genetic”but that the insertion in cortex is likely the most important one: the one that produced the fuel for natural selection on color.

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By looking at the DNA around the transposon, the researchers could estimate the age of the mutation. If it arose recently in a single individual, most carriers of the mutation would have similar sequences nearby, as recombination wouldn’t have time to put the transposon next to varied DNA from other individuals. If the mutation was old, the surrounding regions of the carbonaria form would be more diverse, as recombination would have combined the inserted gene with other nearby genes from different individuals. Using simulations, the researchers gave the most likely date of the mutation as 1819, shortly before it was first seen in the wild. The “interquartile range” for the dates, which I take to be the range of dates between the 25% and 75% likelihood that it originated, is 1681-1806. Below is the graph showing the probability density of when the mutation to carbonaria originated (i.e., when the transposon moved). You can see that all the dates are fairly recent, so this mutation did not occur thousands of years ago. The highest probability is at 1819, not far from when it was first seen in the wild (1848; dotted line). A new variant butterfly would be found pretty quickly by the Brits, who are avid butterfly and moth collectors!

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The mutation occurred only shortly before the environmental change”pollution”that caused the evolution of the color difference. That’s interesting, but it wasn’t necessary, for mutations like this occur continuously, and can hang around permanently. That’s because, although natural selection weeds such genes out of populations (dark moths would be at a disadvantage before the Industrial Revolution), mutation keeps putting them back in, so there is a reservoir of low-frequency mutations hanging around that could be the basis for a new adaptation should the environment change. (This is called a “mutation/selection equilibrium.”) Remember, though, that those mutations aren’t hanging around for the purpose of providing future adaptive evolution. Errors in DNA happen randomly, cannot arise to anticipate the organisms’s future needs, and sometimes, but not usually, turn out to be useful.

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OK, and how do you know that this transposition was the result of a mistaken in replication rather than a sequence that was already there?

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Biston betularia [the peppered moth] is found in China (Heilongjiang, Jilin, Inner Mongolia, Beijing, Hebei, Shanxi, Shandong, Henan, Shaanxi, Ningxia, Gansu, Qinghai, Xinjiang, Fujian, Sichuan, Yunnan, Tibet), Russia, Mongolia, Japan, North Korea, South Korea, Nepal, Kazakhstan, Kirghizstan, Turkmenistan, Georgia, Azerbaijan, Armenia, Europe and North America.

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