quote:
no arbitary barriers kind of implies that all living things should be capable of cross breeding...ie horse with cow or rabbit with wombat
what do you mean exactly.
What I mean is that if I got all things that have ever lived together in a room and pointed at one of them I could, in principle, point to something else that is the same species as that one. Then I could point to something else that was the same species as the second one. And I could keep pointing at things which are the same species as the last one moving from one organism to the next until I've moved from an ostrich to an orchid.
It's probably more helpful to think of Dawkins's "hairpin" analogy from
The Greatest Show on Earth. (And if I had my copy here with me at work I'd be able to quote it exactly.)
Imagine starting with a single living organism. For the sake of this example let's make it a goldfish. Now go back a generation to its parent, which looks pretty close, but not exactly like its progeny. (Probably easier for the goldfish to tell the difference that it is for us.) Go back another generation, and again the parent is similar but not exactly the same as the progeny. Keep going back generation after generation, seeing minor changes in the DNA as we go, and eventually we end up with something that doesn't look at all like the modern goldfish we started with. Let's take it all the way back to the first chordate. Now here's the hairpin turn. Go forward one generation from this organism, but to a different descendant. (For the sake of the argument, we can start here with one of our chordate's close relatives, a cousin or someone else of the same generation.) The descendant is again going to look similar and have similar DNA, but will not be an exact copy. Moving forward generation after generation, traveling down a
different line of descent, a different evolutionary pathway, we can find ourselves ending up with a descendant way far away on the genetic map from the goldfish we started with, say an anthropoid named Charlie or Peg.
As long as you grant that living beings can change in any degree, however small, from one generation to the next, and as long as you grant that changes are inheritable, then common descent can indeed demonstrate a kinship between any two organisms, from ostrich to orchid. You just have to go back far enough.
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