How many generations does speciation take?

It is clearly the case that you can not assign any such numbers. A speciation event can be "instantaneous", exceedingly gradual or anywhere in between. It can be a single mutation or whole bunch of them. It can be due to the possibility of a viable hybrid at the genetic level, the fact that one population doesn't happen to like the "looks" of another (including different bird songs) or who knows what factor.Another thing that might be true (but I'm guessing again) is the you don't have a "population" until you have a speciation event. Until that happens you have gene flow all over. I don't know pop. genetics or the math here but that strikes me as a nonsense number right off the top. It would seem to me that the time taken for fixation would depend on too many things for there to be any such single number. The population size is one obvious one, the degree that a change is selected for (how beneficial is it), how it relates to other genes and so on. It stikes me that no such single number would say anything meaningful at all. Perhaps a geneticist can help here.

Well, you should be able to do some playing with numbers yourself.One thing that we might want to know is just how many "tries" at a different phenotype have there been.For starters, how many different individual multicellular (to keep the number only astronomical) organisms have lived on the planet in the last 600 million years?How many were born (but many were a bad try and died immediately or were still born)?Each of these is a more input material for selection to work on? How many have there been?

But you're right there is some more tinkering that needs to be done.Clearly there have been one hell of a lot more the 350k species. An extra input assumption you need is the average duration of a species. This could be a bit tricky even to define over time since one will frequently blend into another. But why not pick a number as a starting point. Call it 1 million years (seems high for beetles but I've read the 5,000,000 is the average for larger animals).Remember that life is a bush of species. So once you have a new twig (species) you get a new opportunity for a branch. I think that means that if you get to 10,000 species you want to ask how many of them wil l produce a speciation event per year.I would say that the rate of a new species arising from one species every 757 years seems high (though critters like these seem to be able to speciate in decades) However, as noted above, we aren't dealing with one species once we get going.Let's look from the current diversity forward , how many new species may arise from 350 K species in a set number of years? How many have to speciate to maintain diversity if we all 350K of them will be gone in a million years? We need 3 new species a year don't we. But this one has to arise out of 350,000 species.Continueing to think out loud: That suggests a speciation event from one in 100,000 species each year will do it. That seems to work out to an event from 1 species each 100 kyr period which is plenty long enough right?This is fun! But it is going to take a lot of reworking to arrive at anything which makes any sense at all.