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AntonThe idea of 'links in a chain' is not... never has been... a part of the theory of evolution. it's simply not needed, not to mention just plain wrong. It seems to be something that's found mainly in creationist indoctrination pamphlets. They often contain that ridiculous image of some sort of monkey slowly morphing into another figure which morphs into yet another figure, ending with a human. Of course, that has nothing to do with evolution. It’s a specious straw-man.
In short... that's simply not how evolution works. As you will be aware, different species share common ancestors, so that there is a series of divergences between any two species... and the evidence for that process is overwhelming.
Let’s begin with species, and once we’re clear about what we mean by that word, we can better discuss what evolution is, and how it works.
When talking about the concept of species, the first problem that seems to pop up is Essentialism. This is a hangover from Plato, who thought that every triangle (for example) was but an imperfect shadow of some essential triangle that existed in some or other conceptual space.
Ernst Mayr has pointed out that this same thinking seems to appear when people think about species… as if there’s some quintessential rabbit, against which it can be assessed whether or not any given organism is, or is not, a rabbit.
But a species should never be seen as representing some gigantic and sudden leap from something to something else. There is no magical point in time where biological differences allow separate species classifications.
If you don’t understand this, then you’ll be be unable to understand evolution. (This appears to be the source of the creationist error that asks ‘where are the transitional fossils?’ without any awareness that
all fossils are transitional fossils).
The closest we can come to the quintessential rabbit would be a specimen that sits in the centre of a vast number of bell-shaped distributions… a vast number because those distributions can address so many features (number of paws, size of paws, number of ears, size of ears, ability to leap, mechanics of leaping, tendency to leap, ability to digest grass, presence of whiskers, number of whiskers, nature of whiskers, muscular control of whiskers, etc… hundreds of thousands of such distributions would only be scratching the surface)
These distributions shift with time. That’s important, so I’ll repeat it...these distributions shift with time.
Over a large number of generations the distribution of ear lengths may (will) change, with ear lengths gradually becoming longer and longer (as an example… they may well move in the other direction). Eventually, the new distribution may not in any way overlap with the previous distribution… the longest ear length of the previous distribution will still be shorter than the shortest ear length of the current distribution.
Here’s the question. How many distributions need to change, and to what degree, before the cloud of distributions we thought of as a ‘quintessential’ rabbit now forms a different ‘quintessential’ something else?
