Andy Schueler
Posts: 43
Joined: April 2013
|
| Quote (midwifetoad @ April 08 2013,10:21) | I don't see the problem. We can make nested hierarchies
from DNA with human families, even when three are for generations are alive.
At what point does this break down? Or am I missing something? |
There would still be a nested hierarchy within this thought experiment (although we might no longer be able to figure out how it looks like) so it is completely irrelevant for the point Joe G. tried to make in any case.
What would be different though, if all intermediate forms would still be around, is that the distinctions between the groupings we now use would become blurred.
You could still infer relationships (and infer that common descent is true) and arrange taxa by correlations of traits. But you could not pick a subset of taxa, assign it to a higher level classification, and claim that you have defined an objective group - no matter how you would try to assign taxa to groups, there would always be a very closely related form to members of your group that you could not objectively leave out.
Darwin phrased it like this:
| Quote | | Extinction, as we have seen in the fourth chapter, has played an important part in defining and widening the intervals between the several groups in each class. We may thus account for the distinctness of whole classes from each other--for instance, of birds from all other vertebrate animals--by the belief that many ancient forms of life have been utterly lost, through which the early progenitors of birds were formerly connected with the early progenitors of the other and at that time less differentiated vertebrate classes. There has been much less extinction of the forms of life which once connected fishes with Batrachians. There has been still less within some whole classes, for instance the Crustacea, for here the most wonderfully diverse forms are still linked together by a long and only partially broken chain of affinities. Extinction has only defined the groups: it has by no means made them; for if every form which has ever lived on this earth were suddenly to reappear, though it would be quite impossible to give definitions by which each group could be distinguished, still a natural classification, or at least a natural arrangement, would be possible. We shall see this by turning to the diagram: the letters, A to L, may represent eleven Silurian genera, some of which have produced large groups of modified descendants, with every link in each branch and sub-branch still alive; and the links not greater than those between existing varieties. In this case it would be quite impossible to give definitions by which the several members of the several groups could be distinguished from their more immediate parents and descendants. Yet the arrangement in the diagram would still hold good and would be natural; for, on the principle of inheritance, all the forms descended, for instance from A, would have something in common. In a tree we can distinguish this or that branch, though at the actual fork the two unite and blend together. We could not, as I have said, define the several groups; but we could pick out types, or forms, representing most of the characters of each group, whether large or small, and thus give a general idea of the value of the differences between them. This is what we should be driven to, if we were ever to succeed in collecting all the forms in any one class which have lived throughout all time and space. Assuredly we shall never succeed in making so perfect a collection: nevertheless, in certain classes, we are tending toward this end; and Milne Edwards has lately insisted, in an able paper, on the high importance of looking to types, whether or not we can separate and define the groups to which such types belong. |
The diagram he referred to is this one:
http://listoffigures.files.wordpress.com/2010....ife.jpg
|
Perhaps I am confusing the role of digital characters with 'analogue' ones? On the latter, a fossil record rich in transitionals would present them as 'still alive' in the 'where-to-put-'em' classificatory sense). 
