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Lecture notes Darwin's evidence for evolution: Classification
I. The organization of the hierarchical classification system (groups within groups) is not arbitrary (though taxonomic categories are arbitrarily applied)
The peculiar, nonrandom, nonarbitrary, hierarchical structure of the classification system is different from classification systems that arise out of arbitrary groupings like constellations. Because of this structure, original developers of the modern system (e.g., Linnaeus) thought it resulted from some master plan. (But if so, what does this plan predict about patterns of variation we should see? And why this structure and not another? )
II. The rules of classification are entirely explained by descent with modification
A. Adaptive characters are NOT used for reliable classification
B. Physiological importance does NOT determine a character's importance for classification
C. Successful classification systems depend on using aggregates of characters
D. Although adults play the most important role for a species, embryonic features are sometimes more important than adult features for systematists
E. Chains of "affinities" link species that otherwise share few common characters (e.g. Arthropods)
F. Finally, the "comparative value" (= taxonomic category) of the various groupings of species seem arbitrary, but not the hierarchical organization itself
All these "Rules" are explained if the characters showing "affinity" (=similarity) are those that are inherited ("like begets like", p. 420)
III. The importance of descent in the classification is also shown by: A. The same kinds of rules are used for classifying varieties, whose pedigrees (= genealogies) are known (and where the pedigree is the classification)
B. Grouping males and females in the same species, even when they differ greatly (males and females of the same species are related by ancestry)
C. Grouping larval stages and adults in the same species, even when they differ widely (adults are intimately related to the embryos they produce or from which they arise by ontogeny)
D. Thus (p. 425), if the same rules are used for grouping varieties and different sexes, etc., and these groups represent known genealogies, it's likely that higher orders of classification also represent genealogy (an extrapolation by uniformitarianism)
IV. Apparent difficulties to the hypothesis of descent with modification regarding the rules of classification:
A. Why are there some very distinct ("aberrant", monotypic) forms?
B. There are cases in which one species in one group shows a certain similarity to another species in a quite distinct group, which would seem to refute any scenario of descent. How does Darwin fit such cases into his theory?
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