Contemporary presentations of evolutionary theory seem always to incorporate cladograms, sometimes at the species level and sometimes relating groups of organisms. Cladistics is being used to chart the course of evolutionary history. Yet, since it was first developed as a methodology for phylogenetics, it has routinely attracted the critical attention of philosophers of science. Some years ago, Colin Patterson brought these debates to a head when he championed transformed cladism to free cladistics from the charge of circular reasoning. One of the famous controversies about this is discussed here. However, although the unransformed version of cladistics continues to be widely used, the problems remain.
"All life on Earth is united by evolutionary history; we are all evolutionary cousins - twigs on the tree of life. Phylogenetic systematics is the formal name for the field within biology that reconstructs evolutionary history and studies the patterns of relationships among organisms." (source here)
Lars Vogt has contributed a thoughtful analysis of contemporary thinking in a paper entitled: "The unfalsifiability of cladograms and its consequences". Crucial background theory comes from the philosopher Karl Popper:
"As Popperian falsificationism takes in such a central role within the respective theoretical discussions I will take in the Popperian point of view throughout this paper. [. . .] I will infer and discuss the consequences of consistently applying falsificationism to phylogenetics. Therefore, this paper represents a thought-experiment: let's assume Popperian falsificationism is really the only reasonable and justifiable way to do empirical research - What would this imply for phylogenetics?"
The paper is technically demanding and there's not a lot to be gained by trying to digest it to a paragraph or two here. But we do need to note the conclusion:
"Consequently, sensu Popper's demarcation criterion of falsifiability, seeking phylogenetic trees does not represent a scientific endeavour and cladograms represent no scientific but metaphysical hypotheses."
Putting this in more popular language, cladists have adopted a variety of rationales to justify giving weight and credence to their evolutionary trees, but these rationales do not survive critical scrutiny if the test is Popper's demarcation criterion for science.
This disquieting outcome leads Vogt to point to a resolution that makes a distinction between empirical and historical science and limits Popper's criterion to the former:
"All the conclusions presented here rest on the premise that Popperian falsificationism is the only valid scientific theoretical approach. This is highly questionable and might turn out to be wrong, especially with respect to historical sciences - and phylogenetics represents a historical science. The reason for my doubts refers to the fact that it is in principle impossible to predict future observations to test historical hypotheses, which represents the initial idea of Popper's hypothetico-deductive setting - to test necessary predictions against future observations. This rationale is obviously designed for sciences that use experiments in which they control a set of critical conditions in order to generate the deductively predicted necessary effect in order to test a universal causal hypothesis. Perhaps it is time that phylogeneticists develop their own philosophy, a philosophy of phylogenetics that meets the specific requirements of our scientific field, instead of trying to apply a philosophy like Popper's falsificationism that has been developed for experimental sciences such as physics, which is seeking universal laws and regularities instead of the reconstruction of particular historical events."
Crucial to Vogt's point is the distinction between "experimental science" and "historical science". Many scholars involved in the Origins debate have been arguing for some time that there are significant methodological differences between physics and chemistry (empirical sciences) and historical geology and evolutionary biology (historical sciences). It has been noted on numerous occasions that many evolutionists like to champion empirical science and then claim, using Popper's criterion) that the thinking of their opponents is unjustifiable. Consequently, Vogt's contribution makes a really interesting contribution to debate, because he demonstrates in a compelling way that evolutionary biologists face the same charge of adopting unfalsifiable methodologies. He shows that the cladistic approach rests on metaphysical foundations and the outputs of the method are heavily dependent on the assumptions input by the researchers.
Anyone familiar with origins debates will realise that cladistics is a special case of a general principle. So-called proofs of evolutionary transformation are only convincing to those who presuppose common ancestry. The debates within evolutionary circles are always about specifics: the broader issues are not debated because they have an axiomatic status. So, evolutionary theorists do not have the mental tools that would allow them to disprove common ancestry, or whether design inferences are warranted. Consequently, it is not unreasonable to conclude, from the perspective of empirical science, that proposed evolutionary scenarios represent not "scientific but metaphysical hypotheses".
The unfalsifiability of cladograms and its consequences
Cladistics, 24(1), February 2008, 62-73 | doi 10.1111/j.1096-0031.2007.00169.x
Abstract: Popper's falsificationism provides the normative reference system in recent discussions regarding theory and methodology of systematics. According to Popper, the falsifiability of a hypothesis represents a necessary precondition for its corroborability. It is shown that cladograms, independent of "strict", "methodological" or "sophisticated" falsification, are not falsifiable in principle. No present observation is prohibited by any tree hypothesis and, thus, no Popperian test of cladograms exists. It is shown that the congruence test, which is commonly said to represent a Popperian test of cladograms, instead tests sets of apomorphy hypotheses. Three different strategies that have been proposed to circumvent this problem are discussed and refuted: (1) referring to Popper's convention to renounce ad hoc maneuvers; (2) referring to Popper's treatment of probability hypotheses; and (3) decoupling corroboration from falsification. As a consequence, within a Popperian framework the unfalsifiability of cladograms implies that cladograms cannot explain any present day observation and, thus, represent metaphysical hypotheses.
However, Popper's falsificationism has been criticized and questioned by many philosophers before and it seems to be about time that phylogeneticists develop their own philosophy of phylogenetics that meets their specific requirements of a historical science that is not seeking for universal laws and regularities, but instead reconstructing particular historical events.
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