Is biodiversity aging? Heuristic questions on the taxonomic diversity in the Phanerozoic

Abstract

The trend of numerosity component of diversity was analysed from palaeontological data, according to various time intervals and taxonomical ranks. Taxa numerosity of lower (with respect to family), with a nearly exponential increase, vs. higher ranks (from order to macrotaxon), with mainly a logarithmic trend, confirms to follow quite different patterns over time. This dataset seems to fit with a more assortative hypothesis, for higher taxa and a more divisive one for lower taxa. Then, a model was built to quantify the relative weight, over time, of the above hypothetical evolutionary components of various taxa ranks; such ranks were identified, in a palaeontological approach, by the maximum number of recorded taxa or, in a phyletic approach, by the time duration based on genetic data. The trends obtained by this model agree with observed records and the hypothesis, to be verified, of a quite different evolutionary origin of macro-(phylum, class, order ranks) and micro-taxa (genus and species), during the transition between two main time phases: the first (evidently more assortative), mainly linked to the lateral sharing of characters, the second (evidently more divisive), mainly influenced by the ever growing morpho-physio-genetic isolation even for the protection of complex adaptations, as in the present, "modern species". INTRODUCTIONSince evolutionary theory was accepted as the only possible rational explanation of the life diversification in the biosphere -e.g., Gould (2002); see also Mayr (1982), evolutionary processes have received many and different interpretations. Two main question points have been particularly investigated: (i) the basis of the biological variability, and (ii) the factors orienting its evolution: see, e. g., Gould (2002). Moreover, genetics has provided experimental evidence of the "transition" from a species into another species, within a framework that has been defined "microevolution".However, there has been a contextual difficulty in explaining the great differences in morpho-physiological models, occurring among high-rank taxa like phyla, classes, and orders by means of merely micro-evolutionary processes (Stanley, 1979).Therefore, Stanley (1979) hypothesized a "decoupling" of selective ranks, between microevolution (selection among individuals) and macroevolution (species as individual units of selection, in macro-evolution, in analogy with specimens in micro-evolution). KEY WORDSEvolutionary models; genetic timing; macroevolution; taxonomic diversity."Iamque adeo fracta est aetas effetaque tellus vix animalia parva creat quae cuncta creavit saecla …" (Lucretius).