Over at The Skeptical Zone Elizabeth Liddle quotes me regarding the circular reasoning that would be necessary to suppose that cladistics establishes common descent:
It does not take a genius to know that cladistic techniques do not establish common descent; rather they assume it. But I bet if one asked, 9 out of 10 materialist evolutionists, even the trained scientists among them, would tell you that cladistics is powerful evidence for common descent. As Johnson argues, a lawyer’s training may help him understand when faulty arguments are being made, sometimes even better than those with a far superior grasp of the technical aspects of the field. This is not to say that common descent is necessarily false; only cladistics does not establish the matter one way or the other.
In response to this Liddle calls me out and charges me with making two errors, which I will address in turn:
First Liddle writes that I have
. . . confused the assumption of common descent with the conclusion of common descent, and thus detected circular reasoning where there is none.
Where did I do such a thing? Boiling that paragraph down I made the following claims:
- Common descent is not necessarily false.
- But Cladistics does not establish common descent one way or the other.
- Instead, cladograms are constructed ASSUMING common descent.
- It is circular reasoning to conclude that a technique establishes that which it assumes in the first place.
- Therefore, anyone who says that cladistics establishes the fact of common descent has used faulty reasoning and is mistaken.
- There are in fact people who make that mistake.
To establish beyond doubt point 6, Glen Davidson kindly jumps into Liddle’s own combox with this:
Barry: “This is not to say that common descent is necessarily false; only cladistics does not establish the matter one way or the other.”
Glen: “Of course it does. What a ridiculously ignorant dweeb.”
All six assertions seem to me to be on solid ground. Not only are they true, they are not even controversial. But for Liddle’s charge to be correct, at least one of the points I made must be false. OK Liddle, which of the six totally non-controversial points I have made do you disagree with? If the answer is “none,” then the only gracious thing to do is to withdraw your claim.
Secondly, Liddle says I have
. . . confused the process of fitting a model with the broader concept of a hypothesised model . . .
The analogy here with cladistics is: choosing to fit a tree model does not entail the assumption that a tree model will fit. What is tested is the null of “no tree” . . .
So my second point is that when a palaeontologist fits a tree model to her data, she is a) testing the null hypothesis that the data are not distributed as a tree . . .
I take it that Liddle’s point is that cladistics does not always assume common descent but also “tests” the assumption of common descent.
This assertion is risible and betrays a profound misunderstanding of how cladistics works. As a matter of simple logic, a technique cannot test that which it assumes to be true in the first place. The assumption of common descent in cladistics is pervasive from beginning to end.
But don’t take my word for it. This is what that bastion of conservatism and design theory the University of California, Berkeley Museum of Paleontology says in its Journey into Phylogenetic Systematics:
There are three basic assumptions in cladistics:
- Any group of organisms are related by descent from a common ancestor.
- There is a bifurcating pattern of cladogenesis.
- Change in characteristics occurs in lineages over time.
The first assumption is a general assumption made for all evolutionary biology. It essentially means that life arose on earth only once, and therefore all organisms are related in some way or other. Because of this, we can take any collection of organisms and determine a meaningful pattern of relationships, provided we have the right kind of information. Again, the assumption states that all the diversity of life on earth has been produced through the reproduction of existing organisms.
The same site says that cladistics has three uses: (1) it is a system of classification; (2) it helps make predictions about properties of organisms based on the assumption of common descent; and (3) it helps in the testing evolutionary mechanisms.
I invite readers to go to that site and read it in full. It says nothing about Liddle’s proposed fourth use of cladistics – testing (as opposed to assuming) common descent to begin with.
For goodness sake, Liddle, even uber-Darwinist Nick Matzke agrees that cladistics cannot establish common descent. He wrote:
. . . phylogenetic methods as they exist now [cannot] rigorously detect . . . direct ancestry, and, crucially, . . . this is neither a significant flaw, nor any sort of challenge to common ancestry, nor any sort of evidence against evolution.
Certainly Nick is right* that cladistics’ inability to establish common ancestry does not mean that common ancestry is necessarily false. But that is exactly what I said in the part Liddle quoted: “This is not to say that common descent is necessarily false; only cladistics does not establish the matter one way or the other.”
Liddle is simply wrong when she says that cladistics tests, as opposed to assumes, the claim of common ancestry.
Liddle knows this as well as anyone I suspect, and explains why in the very same post she walks back on her initial claim when she writes:
Of course palaeontologists aren’t seriously testing the null hypothesis that the data are distributed as a tree – we know, from countless cladistics studies that they are, and it isn’t even disputed by anyone.
Again, as Matzke says, all of this does not necessarily mean that common descent is false. I made no assertion regarding that matter one way or the other. It does not mean that cladistics cannot simultaneously assume and test common descent. Simple logic.
So Liddle’s attempt to show that a lowly lawyer has nothing useful to say has blown up in her face. Far from establishing that, by using faulty logic and reasoning – things that as a lawyer I am trained to detect – she has actually established that which she set out to refute.
*Bovina Sancta! Can I actually be agreeing with Nick about something? I suppose it is true that even a blind squirrel finds and acorn now and then.