Speciation + Evolution = More Diversity

I'm sure RAZD will do a better job than I will but I'll have a go at it:It used to be thought that only if a population was separated geographically could a speciation event occur. This is called "allopatric" speciation. In this case it might be "driven" by lots of things. Great distance might do it if the genes can not flow easily throughout the range. Note the ring-species case: if part of the ring is killed off (by whatever) then there are two (or more) separated populations. Once that is true the the accumulating changes to the now separate gene pools will eventually (and this appears to take perhaps, millions of years for mammals) will be completely incompatible. (E.g., lions and tigers have been separated for that kind of time and are nearly completely separate).We now know that many things can drive speciation. E.g., there is a parasite of (I think) fruit flies (don't remember too well) that changes them so only infected animals can breed with each other. It is an immediate separation of populations. There have been cases (many in plants, fewer in animals) where a chromosome number change separates the populations. This can produce nearly "instantaneous" speciation.A mutation changing something like a birds song can cause a separation of a population into those without is and those with it.And so on... No. They don't "know". Some survive some don't.In the specific case of ring species both daughter species may be well suited to the environment. Let's take a case where there are only 3 sub-species in the "ring".A in the middle and X on the east and Y on the west. We'll take it that A is the parent. The range of the populations is long and narrow (like around a mountain range). The differences in X and Y from A and from each other maybe small. But if the individuals don't travel far relative to the range then there may be enough separation to stop the population from being exactly like one well-mixed gene pool. So there is a possibility of drift occurring.If the environment where X and Y live is very different then there will be a high selection pressure on them and they may diverge more rapidly.Now if the remaining gene flow is disrupted from A to X and A to Y and if X and Y are already different enough on the far side of the ring so they don't interbreed (much or at all) we now have room for new species to arise.It may be that the disruption is by a environmental change (a big city appearing?) in the home range of the parent sub-species and it goes extinct or it maybe that the range is disrupted enough to reduce the gene flow to very, very low levels. In general, no. There are too many variables for a general answer most times . It is possible to have a speciation event and have both parent and daughter populations do just fine in the same area. There are a lot of niches available. Both. It doesn't matter. Some animals select a mate on song, color or behavior. If any change enough they may "choose" not to mate but it amounts to being unable to mate really. It is entirely possible that we may be interfertile with chimpanzees but we choose not to mate with them (though I have heard rumours ). If we can't get interested then we can't mate.

I think you are still a bit mixed up (perhaps my fault). There are lots of ways that a single gene pool (an interbreeding population) can be separated so the gene flow is reduced. It seems as we learn more we find that there are a big bunch of ways that a gene pool can be split. It is as simple as that.Once it is split then mutations will happen differently to the two pools. Moreover, even if the same mutation occurs in both pools it may be selected for in one pool and against in the other.All of this depends on the exact circumstances: How different are the environments; what is causing the split (geography, mutation, niches) and so on. On what I think are rare occasions a common mutation to a few individuals can split them off instantly. For plants this can be one individual. The parent can be one species and the many seeds can be a different one, never to meet again in a common gene pool. Again: It depends. Over and over this will be the answer. If they compete for exactly the same resources then yes one will probably be eliminated. (or there will be some quasi-stable state reached). But if the split happens because they occupy slightly different niches then no both may survive in close proximity. Sticklebacks in lakes in this province have speciated in not very big lakes with one living mid lake and one near shore (IIRC).