Now that we've established common descent we have admitted the principle of change. The ancestor of the dog and horse had to change, somehow, into a dog and horse. Common descent implies that species are not locked in to a single pattern throughout the biological time-line.
The next thing that is fairly intuitive is that change has happened. Modern mammals didn't exist during the vast swaths of time when there were dinosaurs. It is obvious that there has been change (i.e. evolution) because of the fossil record itself. Species are born and die just like individuals.
But how does it happen? First of all, descent implies generations, the "long bead chain of repeated birth," in the words of Robert Frost. If individuals just stayed around forever and never reproduced, you couldn't have change. Secondly, you need a certain variability within a single generation, a single population. If every individual were the genetic clone of every other one, you wouldn't have much to work with. What's not intuitive here to the layman is how this mutability can produce such wide variation. Even creationists admit biological variability and small changes, after all. They know that dog breeders have produced breeds of dogs with traits different from other dogs. They just won't accept larger changes. They still believe that species are essentially locked into an immutable pattern. They can't accept that a dog and a wolf are related, or a wolf and a sabre-tooth tiger, and so on up the chain. They will tell you, sure, micro-evoluation exists, but not macro.
But once you get someone to admit a wolf is related to a dog, once you admit that species arose or became extinct, then it's hard to draw a line saying that biological change has a fixed barrier at the level of species. It would be like saying Spanish can change, can have dialects and regional differentiations, but it will never actually change into a completely different language.
The next logical step might be to look at mechanisms of change: selection, adaptation, and the so-called "survival of the fittest."
(Once again, I'm trying to reason all this out just with my extremely limited knowledge. I think people confuse themselves by knowing too much and ignore the intuitive simplicity of some of this. Also, though, I'm trying to understand why evolution is counterintuitive to people who don't readily accept it.)
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The point at which this story becomes hard to believe is that these modifications actually lead to each species being adapted to its environment. The story is something like: individuals with favorable mutations (favorable as in better-adapted) produce more surviving offspring; therefore over time an initially rare mutation becomes increasingly common, etc. This seems fanciful, because there is no intuitive reason to believe in a large reproductive edge for a slightly favorable trait, and is different from the idea that if you deliberately only let individuals with the desired traits have offspring they will eventually take over. A lot of good scientific ideas seem absurd, but this is precisely why they're hard for lay readers to believe.
(It is sometimes claimed that the fittest are precisely those who survive; this dodge has the unsatisfactory consequence of making the "theory" a tautology: if "fitness" is not a predictable trait the theory has no explanatory power, it would be just as consistent with an entirely different set of traits having evolved. But of course the naive version of the theory -- the fittest, in some easily predetermined sense, survive -- is incorrect. The truth is somewhere in between...)
Dawkins makes the interesting point (in his "Greatest Show on Earth", though surely elsewhere as well) that every organism has always been in the same species as its father/mother (or other type of parent, for other types of reproduction), but that as you go back far enough from a particular living organism today, eventually there is an ancestor with whom that given organism would not be able to reproduce.
"This seems fanciful, because there is no intuitive reason to believe in a large reproductive edge for a slightly favorable trait, ..."
It doesn't have to be large, and having a slightly favorable trait doesn't mean that that organism will reproduce. It's playing the odds, over deep time.
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