Indeed you are. Now can you tell me how a part of the group (species) becomes geographically isolated from the rest of the group, substantially changes, becoming different in kind (i.e. "reproductively isolated") from the greater number of the same group (species) that didn't evolve? After all, by definition, one group must have mutated faster than the other group.
How could the "daughter" species produce creatures "reproductively isolated" from the "parent" species without being "reproductively isolated" from the other members of the "daughter" species, unless some members of the "daughter" species mutated favorably in the same way in the same generation?
If you want to propose evolution by small mutation to explain the emergence of variation in "daughter" species, then you have brought forward as a solution the problem that punk eek was brought forward to explain: the lack of transitional forms in the fossil record.
Isolation is not a necessary condition. It only increases the chances that a beneficial mutation spreads to the whole population by reducing the size of the 'reproductively available' population.
Okay. First things first. Geographical isolation happens all the time. Mountains may rise in the middle of a species' range; a river might split a population in twain; the isthmus of Panama even did a number on quite a few species.
Once the two or more populations no longer have contact with each other, any mutations which arise in one population stay in that particular population -- they do not get shared with the other populations and the populations become increasingly genetically divergent.
None of the groups is "mutating faster than the others" -- they are simply mutating differently. If, for instance, the environment inhabited by the lion's share of the population hasn't significantly changed for a long time, the population isn't likely to be too different from its ancestral population -- but the small group that got stuck in a rather different environment would have to change rapidly or face extinction. In this case, you'd have the mother and daughter species coexisting. Of course, when the population was split, the different subpopulations might find themselves in rather untenable environments and all the populations would be forced to adapt to local conditions or die off. In this case, the mother species disappears and is replaced by one or more daughter species.
The reason speciation occurs more rapidly if the isolated part of the species is small is that the genetic mutations move more rapidly through a small population than a large one, because the individual with the mutation represents a larger chunk of the breeding population.
Now note, none of this is going on in the blink of an eye -- It still takes generations for the genetic differences to render the resulting populations unable to interbreed. Only when looking back through the telescope of time do we get the impression it happened rapidly. The reality is it still took longer than most civilizations have existed to affect the changes that lead to speciation.
Okay, I don't think we're making it as clear as we can. When a population splits initially, the two groups could still interbreed if they could just get together. If a mutation pops up in any of the subpopulations initially, it is not going to prevent the owner of the mutation from breeding with others lacking that mutation -- if it did, the mutation would disappear in that one generation. Therefore you do not need two organisms with the same mutation to produce viable offspring. Over succeeding generations the mutation will spread through the population as those without it die off and are replaced by those who have it. It is the accumulation of many mutations that eventually render one population incapable of interbreeding with another population -- it is not something that happens overnight. Remember, we are talking about MILLIONS of years; human civilization is only about 10,000 years old. A million years is 100 times this (humans could have gone from living in caves to walking on the Moon 100 times in this span of time). And we are talking about many millions of years. That is an incredibly long time during which, at any given moment changes are but teeny-tiny increments.
One group is perhaps under more pressure. Let's take a hypothetical group of apelike creatures in East Africa. Their habitat is pretty much all jungle. Let's suppose some sort of barrier cuts part of it off from the rest.
Let's say a fault line shearing creates an increasingly high line of cliffs that leaves one population (to the west) sort of high and wet, in rain-forest rather like the original conditions.
East of the cliffs, the climate changes. The rain forest breaks up into pockets, with open grassland in between. For the forest apes that live in this section, it's a crisis of shrinking habitat. Every few generations, more trees die and more food supply disappears. Famine culls the eastern apes again and again.
West of the line, not much is happening. At any rate, not much fossilizes in highland jungle so we'd hardly know if it did.
In the east, the desperate apes experiment with wandering into the grass to scavenge, chasing the vultures from rotting meat. It's not much like the fruity diet they prefer, but it beats starving.
They quickly learn that large groups can chase the vultures or even a leopard off far more easily than just one. It also helps to carry sticks. Dragging the meat home can be a pain, but it helps if you can walk with just two legs.
Oddly enough, the smaller your group, the easier it is for you to adapt. Say, there are only a few in your little inbred pocket of forest cover. One gets a mutation that works. It's a scant few generations before everyone else has it.
Not all such little pocket populations on the edge of extinction can be expected to fight back, of course. Most probably go over the edge, but the ones that don't simply take over. When a population finds a really successful new adaptation, re-radiation takes place.
Meanwhile, west of the cliffs, the old population is still there and little changed. But now they look pretty different from the gracile, taller, upright-walking new population.
I suppose it couldn't really happen, of course, but it's a good hypothetical example of the kind of thing punk-eek is talking about.
Actually, it was hypothesized to explain the RELATIVE SCARCITY, not LACK of transitional fossils. There is a world of difference between the two concepts.