"feel" and expertise of taxonomic specialists on given groups.* Rigorous and quantitative studies of tempo (or pattern) can lead to inferences about modes (or processes). Scientific theory is, essentially, the attempt to explain nature's processes. By using uniquely paleontological data about pattern to infer the unseeable processes of long temporal spans, paleontology may be an active purveyor of evolutionary theory.

This strategy of using uniquely paleontological data about tempo to infer mode, and thus to develop theory directly from the domain of macroevolution, pervades Simpson's book and underlies all his examples. To cite just two cases:  

  1. Designation of the three modes. Simpson's last, and best-known, chapter (1944, pp. 197–217) uses data of tempo to propose a fundamental division of evolutionary processes into three modes, each with different meaning: speciation for a low-level process of iterating diversity, with no significant input to trends or other larger-scale patterns; phyletic evolution for the ordinary style of directional change, leading to evolutionary trends and accounting for some 90% of paleontological data; and quantum evolution for rapid and rare, but efficacious, "all-or-nothing" transitions from one adaptive zone to another through an inadaptive phase (a process analogized with Wright's model of genetic drift).
  2. The theory of horotely, tachytely, and bradytely. This fascinating and brilliant, if ultimately flawed, theory has been widely misunderstood by people who do not grasp Simpson's central strategy of using tempo to infer mode. Many critics have stated that Simpson only invented some arcane, Greek-based jargon to divide the ordinary continuum of evolutionary rates into slow (brady), ordinary (horo), and fast (tachy). Not at all. Simpson was trying to identify separate peaks (modes in the statistical sense) in the distribution of tempos in order to specify distinct modes (in the ordinary sense) of evolution. Thus, horotely is not the central tendency of a single distribution of rates (with tachytely as the right tail, and bradytely as the left tail, as in the  

*  

Among leaders of the second phase of the synthesis, only Simpson was well trained mathematically, and only he could read the primary source material of the first phase with full understanding. (Dobzhansky, for example, often stated that he adopted a "father knows best" approach in his collaborations with Sewall Wright—that is, he simply accepted Wright's verbal interpretation because he could not understand Wright's equations in their own joint papers!) Simpson was mathematically adept and a particularly fine statistician. His textbook, Quantitative Zoology, written with his wife Anne Roe, was a standard source for decades, and remains unmatched for clarity and well-chosen examples. How ironic that words built the bridge to the second phase, while formulae constructed the pillars and anchor of the first phase—so that, with Simpson's exception, the crucial linkage rested upon faith.



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