Alleles, individuals, and species are all examples of entities possessing variation in the properties that underlie natural selection: branching (reproduction), persistence (survivorship), and heritability of characters. This suggests that the logic embodied in the theory of natural selection can be abstracted from its usual application to the level of individuals to encompass selection operating among any biological entities for which these essential properties can be meaningfully defined. This approach leads to a unified perspective of adaptation, selection, and fitness at all levels. Expanded versions of the Price covariance selection equations provide a convenient and useful conceptual vehicle for this discussion. The advantages of a hierarchical approach are twofold: it permits exploration of concepts and ideas across levels by analogy, and it focuses attention upon the mechanisms that account for different evolutionary dynamics at each level rather than obscuring these biologically unique properties with argument by extension from a single “special” level.We point out that the choice of a single measure of evolutionary change restricts the context in which “other level” processes will be perceived. We illustrate the limited forms in which higher and lower level selection can be recognized from the unique perspective provided by any given level through extensions of Price's formula.An exploration of the implications of such an approach leads us to the assertion that the development of a unified theory of evolution demands the recognition and incorporation of hierarchical structure as a conceptual foundation.
A literature-based compilation of phylogenetic relationships and biometric measurements of 342 Cenozoic planktonic foraminiferal species suggests that the group shows a net increase in size through the Cenozoic, thus appearing to follow Cope's Rule of phyletic size increase. However, when the data are corrected for size-related biases, they do not support the hypothesis that this apparent trend is driven by an organismal adaptive advantage of larger size.When the planktonic foraminifera return to their “primitive” globigerine morphology during the Eocene-Oligocene transition, there is no indication of size-dependent origination or extinction; however, when the extinction signal is decomposed into pseudoextinctions and true lineage terminations, a differential pulse of pseudoextinction is observed among the smaller forms. This observation suggests that smaller bodied species, rather than surviving stressful times with static morphologies, may evolve their way through times of crisis and go on to found lineages which, by virtue of their initial small size, are stochastically likely to increase in mean size during subsequent diversification. Thus, one general explanation for Cope's Rule might be that smaller bodied species are more adaptively responsive due to their tendency to have shorter generation times. During times of stress, this adaptive responsiveness may give them an advantage that is correlated with, but causally unrelated to, their size.
Age-dependent extinction is an observation with important biologicalimplications. Van Valen's Red Queen hypothesis triggered three decades of research testing its primary implication: that age is independent of extinction. In contrast to this, later studies with specieslevel data have indicated the possible presence of age dependence. Since the formulation of the Red Queen hypothesis, more powerful tests of survivorship models have been developed. This is the first report of the application of the Cox Proportional Hazards model to paleontological data. Planktonic foraminiferal morphospecies allow the taxonomic and precise stratigraphic resolution necessary for the Cox model. As a whole, planktonic foraminiferal morphospecies clearly show age-dependent extinction. In particular, the effect is attributable to the presence of shorter-ranged species (range Ͻ 4 myr) following extinction events. These shorter-ranged species also possess tests with unique morphological architecture. The morphological differences are probably epiphenomena of underlying developmental and heterochronic processes of shorter-ranged species that survived various extinction events. Extinction survivors carry developmental and morphological characteristics into postextinction recovery times, and this sets them apart from species populations established independently of extinction events.
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