When I was a student, I remember reading a book about the (short) history of the SDP. It intriguingly stated that the only Tory MP to defect to the party had crossed the floor "literally", which I took to mean that he had reached the end of a speech, announced he was defecting, and theatrically walked across to the opposite side of the Commons chamber and sat down with his new colleagues. I'm not sure whether that actually happened, and even if it did it's effectively lost to posterity, because there were no TV cameras allowed in those days. So it was quite a privilege to actually be in the hall today for something equally theatrical - Alex Salmond announcing at the end of his leader's speech that Ash Regan had just joined Alba, and Ms Regan then immediately appearing on the platform to rapturous applause. If the BBC and STV weren't there to film such a moment of high drama, they have no-one but themselves to blame - although I'm sure Alba will be happy to share their own footage.
So what effect will this have? Above all else, credibility. Alba are now in the Scottish Parliament, which has become a six-party chamber. That will be reflected at least to some extent in media reporting from Holyrood, with Alba's voice being heard occasionally. Arguably this represents the same degree of credibility boost, albeit of a different type, that Alba spurned by not putting up a big name candidate in the Rutherglen by-election and seeking an electoral breakthrough. Some may even suggest that the whole reason for the Rutherglen decision was that the leadership privately knew Ms Regan's defection was coming and that they didn't want to take any risks with its potential impact - although ultimately defections can only take you so far, and true credibility will only flow from success at the ballot box.
Alba have also just bought themselves some time. Although they'll obviously do their best to hold Neale Hanvey's and Kenny MacAskill's seats, those are really difficult constituencies to defend - that would have been true for the SNP as much as it's true for Alba. There was a big danger that Alba would cease to have any elected representation at all after next year's general election, but that will no longer be the case, because Ash Regan is in place until 2026.
There will be opportunities going forward for the new Alba MSP to harry the First Minister on lack of progress towards independence, and on independence strategy, at FMQs. (She won't have automatic leader's questions due to Alba only having one seat, so she'll have to wait her turn, but the chances will come up occasionally.) That would have been one of the big prizes if Alba had won seats in 2021, so it's good that it's happening belatedly.
Paul Hutcheon affected weariness a few hours ago and suggested the SNP wouldn't be that bothered about losing Ash Regan. If he really believes that, he's a fool. This is a potential 'genie out of the bottle' moment - there was a good reason why the SNP were so euphoric about shutting Alba out completely in 2021. Now that Alba have their foot in the door of Holyrood, it becomes much easier to imagine them staying there.
The big question now is how many of the thousands of SNP members who voted for Ash Regan in the leadership election in March will follow her across to Alba. I must admit I'm a bit conflicted about that, because it was only with the second preferences of those people that Kate Forbes came so close to stopping Yousaf. If the SNP are ever to be reclaimed from the ruling clique, the votes of the more radical members will probably be needed. But I suppose every member will just have to make an individual decision about whether the SNP can be saved and is worth saving, or whether the greater impact can be made by joining Alba.