Interesting new study that has some relevance to the discussion had with @Cornelius_Hunter and @Swamidass. Not that I want to get that whole thing going again. Really, I don’t, I think both sides showed grace and civility and the exchange ended laudably. But this does, nonetheless, show how a single point mutation changed how this gene was spliced and it had big consequences for the expansion of the neocortex.
Really nice study. Great example of what I’ve explained to @billcole.
The story starts with a duplication of a gene (this is an example of drift). Then a single nucleotide change changes the splicing. The novel spliced protein subtly changes development to increase the number of brain cells. This then gets selected for (an example of positive selection), and is one contributor to our increased brain size, and likely our intelligence too.
That is the really surprising things about Biology. A tiny change at the genomic level can cause a big change in molecular phenotypes (e.g. splicing) which in turn can cause a massive selective advantage at the organism level. Also, we see how neutral drift (gene duplication in this case) is a first step that enables the beneficial mutation to arise. The complete story, we see, is not strictly Darwinian. It includes a non-Darwinian mechanism too.
It wasn’t just a change in splicing though, the single point mutation also prematurely ended exon 5, causing a frameshift which created a completely novel exon 6. (Shown here in green)
Thesting in Mice showed that there is something about ARHGAP11B which made it effective when ancARHGAP11B was not effective. This could be for any one of these reasons:
Exon 5 was shortened a little
The original exon 6 was missing
A completely novel exon 6 was created due to the frameshift
Mere origin of novelty…it takes lots more to make a successful species out of one genetic mutation. How come it took so long for that mutation to kick in and get us to speaking french and all. There must have been divine protection to keep it from dying out. Most families die out. So…we have a good example of a change but is that change evolutionary or is it revolutionary. I’m thinking…I’m thinking.
If a single change to the sex chromosome can make the creature unable to breed with any population peer other than its own descendants or cousins, you have the basis of a new population that will drift in its own unique path.
Though the novel exon is because of the splicing change. But this draws attention to an important detail I neglected.
Because this extra 267 amino acids is required for the function of the new protein, it is another example of the complexity springing from non-coding DNA de novo, without any prior history as a protein sequence. This is also an important example of “exaptation,” the adaptation of one structure (a non-coding DNA frame) to another (a coding DNA frame) with an important function.
If functional sequences are extremely rare in sequence space, this should be impossible. Yet here we see it happen, all do to a single point mutation change.
Beautiful story. It is great for teaching. Keep posting examples like this.
Of course, we are not arguing that this single mutation is all that is required to produce a human. As @Cornelius_Hunter writes…
A single mutation? Here we have evolution reductio ad absurdum. A single mutation essentially worked the magic to create humans. How lucky we are.
Rather, we think it was a few thousand mutations like this that were cumulatively selected. Many more could have been functional, but not beneficial enough to be selected. The whole point of this study is that a single point mutation is enough to create > 250 amino acids of functional sequence. That is surprising fact about biology. It does not take much genetic change to produce a huge phenotypic effect.
As always, while @Cornelius_Hunter and Biologos have their differences, I do appreciate him engaging our conversation.
I have to admit I found the article quite a bizarre read. The fact that the study I linked somehow lead to that article is confusing to me. Also, to think that this one mutation, or even a few rare mutations, just turned us into modern humans is nonsensical. I’ve never read or thought that was the case. The reason I posted the study was to show that a tiny change genetically can produce a drastically different phenotype. It’s not really that much of a surprise with what we know at this point though.
