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Geobios
32, 205-221 (1999) Jerzy Dzik Abstract.
In the fossil record, distinction between the two basic aspects of evolution,
i.e. branching of evolutionary trees (cladogenesis) and evolutionary
transformations along lineages (phyletic evolution, ‘anagenesis’),
is especially well visible. The process of allopatric speciation, presumably
the most common, if not the only, way to split a lineage, cannot be recognised
directly in any single geological section, as it proceeds in a geographic
dimension. It has to be inferred from data preserved in many places, which
makes the inference vulnerable to all the limitations of rock correlation. On
the contrary, the course of phyletic evolution is potentially observable in a
single geological section, with application of the Steno’s rule of
superposition as the only guide to the time distribution of evolutionary
events. However, only rarely and in extremely stable environments can a
complete time range of a morphologically recognisable segment of the evolution
(chronospecies) be established. The stratophenetically documented phyletic
evolution of both pelagic (Ordovician and Carboniferous conodonts) and benthic
(Ordovician ostracods) organisms from such unusual sites provides evidence
that ranges of chronospecies observed in particular sections represent
generally only a minor part of their actual durations. As a result, rates of
evolution estimated by counting reported ranges of taxa must appear much
higher than they really were. Moreover, biometrically proven rates of
evolution invariably appear to be much slower than expected. Ranges of fossils
have thus little to do with the evolution of species they represent. In the
studied cases, with a good geological time control, there is no evidence of
any direct relationship between the speciation rate (or migration events) and
the rate of morphological change. Generally,
numerous slowly evolving lineages may develop in a varied environment, and a
uniform environment may remain inhabited by a few, although fast evolving,
lineages. Rates of speciation and extinction mostly reflect features of the
physical environment and its transformations, being thus rather unreliable
measures of the rate of biological evolution. Perhaps they may offer a measure
of the evolution of complexity of ecosystems and/or effectiveness of their
biocoenoses in exploiting resources. Nevertheless, they can hardly be used in
empirical studies as neither speciations nor extinctions are directly
observable in the fossil record and their identification requires a complex
inference based on not obvious assumptions. |