Creationists often hail the lack of transitional fossils as damning evidence, if not the death blow for the theory of evolution. Such arguments are often countered by pointing out that the fossil record is bound to be incomplete, is not the only evidence, is in fact a minor part of the evidence, etc, etc. Also, evolutionists explain, absence of evidence is not evidence of absence.
But I think a very important point if often overlooked when considering the argument of transitionals, when in fact it could be a very powerful weapon in the debate, if it is properly understood. What I am talking about is that the concept of 'transitionals' is a bit misleading. It is misleading in that it forces upon us - but mostly upon the undiscerning creationist mind - the distinction between "finished" and "unfinished" creatures.
Thus the finished creatures belong to a species, while the unfinished creatures are deemed somewhere "in between" species. Also, a transitional creature is supposed to exhibit useless features, or even defective features, because they are not the "finished product". But this is a false picture. The reality is that
all species are transitional. There is no such thing as a "finished species". It's a mirage, for two reasons.
First, the term 'species' is an arbitrary, man-made concept. The system of classification in biology, of which 'species' is a layer, has been revised more than once, to accomodate new insights in the relatedness of newly found creatures to ones already known. Also, there are several definitions of the term 'species'. One definition takes interbreeding as its criterion. Ernst Mayr defines 'species' as:
quote:
"groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups"
But interbreeding assumes sexual reproduction. So what about all those creatures that reproduce asexually? How are they classified into species? Enter the concept of a "morphologial species", that is to say, when two creatures are similar enough, we call them a species, and when they differ too much, they are "clearly" two species. But what is "similar enough"? When do they "differ too much"? All in all, it seems obvious that the concept of 'species' is a bit problematic.
Second, the term 'finished' presupposes a plan. Something can only be called 'finished' if it is known in advance what the thing is going to look like. But evolution advances by random-mutation and natural selection, making it logically impossible for there to be a plan. And if there is no plan, then a species cannot rightly be called 'finished' or 'unfinished'. It simply is what it is: a species in its own right.
To illustrate the point that all species are transitional, here's a thought experiment. Let's create a row of numbers, by following a simple rule: we start with the number 1 and create each following number by adding 1 to the preceding number. Let's repeat this until we have reached 1 million.
Now we have a long row of numbers, with small numbers at one end, and large numbers at the other. Everyone will probably agree that 1 is a small number, and a million is a large number. The number 2 is also a small number, as are 3, 4 and 5. At the other end there are more large numbers: 999,999 is large, so are 999,998 and 999,997. However, the row is continuous, so the question is: where in the process that created the row did a small number give rise to a large number? Whatever boundary is taken as the point of transition, the numbers on both sides of the divide differ by only one, so it is a bit difficult to maintain that the one is a small number and the other is a large number. A solution could be to introduce 'intermediate' numbers, which are neither small nor large, but, obviously, intermediate.
But is that really a solution? Actually, no. Because now we have not one, but two boundaries to determine. When is a number still small, and when is it intermediate? What intermediate number gave rise to a large number? We find we still have the same problem, only twice. And introducing still finer gradations just adds to our problems, until...
Until we come to the point where each number is in its own division, and the dillemmas disappear all of a sudden. But now our divisions no longer provide us with more information about the numbers than the numbers themselves do, effectively making the divisions obsolete. This means that there are no discrete dividing lines to be drawn. The transition takes place along the entire range of numbers. Each number is itself a transitional, from small to large.
What we need to realise is that we are trying to impose an arbitrary system of discrete divisions on a contiuous set of elements. With species it is no different. If you translate the example of the numbers to species, you can imagine a continuous sequence of intermediates from any ancestor you'd care to start with, right up to yourself. You are the same species as your parents, and they the same as theirs. At the other end of the line, the ancestor you started with is the same species as its offspring, and they are the same species as their offspring.
But if you went back far enough in the lineage so that the ancestor you start with is a tree-dwelling primate, then obviously this ancestor is not the same species as you. There must be transitionals. But wherever you look in the lineage, locally you cannot pinpoint any real transitions. That's because the transition takes place all over the lineage. Each and every one of your ancestors is a transitional. And if you have children or plan on having them, you are a transitional yourself.
The next time a creationist asks "Where are the transitionals?", you can answer "Look around you."
{Up for debate: "All species are transitional", forum "Biological Evolution"}
™ Version 4.2