I can but try. Take the two species of chimps: Chimpanzees (Pan troglodytes) and Bonobos (Pan paniscus). Their native habitats are on opposite banks of a major river, the Congo. At some point in the past all chimp ancestors were in one species and lived on one bank of the river. Then, for some reason a breeding population crossed the river and landed safely on the other bank. It could have been as small as two individuals, or even a pregnant mother. Perhaps they were swept away in a flood, managed to climb onto a floating log and reached the opposite bank?
There were then two different breeding populations on either bank of the river. Each population accumulated their own mutations, but both started with the same chimp-ancestor DNA (to within founder effect). Neither side had to develop wholly new systems; both had the same original systems to start with.
Humans are the same. There is no part of the human body that is not also present in both species of chimpanzee. The proportions are different: we have shorter arms, longer lags and bigger brains, but all the pieces we have are present in chimps. That is part of the evidence for us sharing a common ancestor with chimpanzees.
We see similar effects from geographical isolation in many species. Islands are particularly prone to this. For instance, Dodos were a species of pigeon. A breeding population arrived and evolved in isolation from other pigeons. They became flightless, because there was little need to escape predators, and became larger. Of course once man and the predators he introduced arrived…
The general point is that new species do not just appear, they develop from previously existing species. The only exception was that very first just-about-alive proto-cell 3.7 billion years ago.
rossum
