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RE: The role of morphology in systematics

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Posted by: CKing at Fri Nov 5 11:24:34 2004  [ Report Abuse ] [ Email Message ] [ Show All Posts by CKing ]  
   

Most systematists, even most molecular systematists, would agree that morphology cannot be ignored. The question is how one deals with conflicting data sets. Some people combine molecular data and morphological data and do a single analysis. Such an approach may seem objective, but if the data set is made up of mostly morphological characters, the signal from the molecular characters may be drowned out. Personally I do not favor such an approach. Like it or not, morphological characters, because they tend to be adaptive, are often less informative of phylogeny than neutral nucleotide substitutions. One need to look no further than Darwin's "Origin" to get a glimpse of how good systematists of the past try to search for good (informative) characters while avoiding the pitfall of bad (uninformative) characters. Darwin, for example, explained how rudimentary organs can often be the most informative of phylogeny. Unfortunately, many practicing systematists (especially the cladists and pheneticists who claim that their approaches are 'obejective') pay far less attention to character goodness than Darwin and his contemporaries and followers. Ignorance of character goodness is not objectivity.

I agree that molecular approaches are not bullet proof, but I believe they are better than morphological approaches if the systematists are careful about choosing outgroups and good characters. I have noticed a few recent molecular studies in which a member of the ingroup was chosen as the outgroup. For example, in a recent mtDNA study of the chorus frogs (Pseudacris, Hylidae), two members of the North American Hyla clade were chosen as the outgroup to this same clade. The result is an unreliable phylogeny that shows a number of anomalies. In another mtDNA study of relationships among Eurasian ratsnakes of the genus Elaphe, a derived descendant of Elaphe (namely Lampropeltis) was chosen as the outgroup of Elaphe! Again, the results from this study is anomalous as well. Choosing Lampropeltis as the outgroup is like picking Homo sapiens as the outgroup when one analyzes the relationship among the apes. Such silly mistakes often result in synapomorphies being mistaken as sympleisomorphs and vice versa. The result can be disastrous, as S. J. Gould pointed out on page 214-215 of his book "Wonderful Life." Symplesiomorphs that are mistakenly thought to be synapomorphies can result in the recognition of polyphyletic groups.

One way to detect silly mistakes made by molecular systematists is by using morphological data. Utiger et al., in a recent analysis, concluded that the American species Senticolis triaspis is a member of the ratsnake clade (genus Elaphe 'sensu lato') even though there is poor statistical support for that node. Morphologically, Senticolis lacks the intrapulmonary bronchus that is possessed by all American members and some Eurasian members of the ratsnake clade. Morphologically, prior studies also fail to establish a close relationship between Senticolis and any other colubrid snake. Without corroboration, Utiger et al.'s analysis could have been uncritically accepted.

It appears that most systematists do agree that morphological data cannot be ignored when attempting to construct a phylogeny. Systematists, however, may disagree strongly on how and how much morphological data should be incorporated.

Unfortunately, there are also a large number of systematists, namely the cladists, who will totally ignore morphological data once they have constructed a phylogeny. To them, the classification is the branching diagram. To them, morphological disparity has no role in ranking organisms. It is quite ironic that many cladists may use morphological data almost exclusively when constructing a phylogeny and then totally ignore the data when trying to classify organisms. It is the cladistic practice of ignoring morphological disparity when ranking organisms that the Darwinians and I find most objectionable.


   

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