On speciation in Ice Age mammals, with special reference to cervids and caprids

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Abstract
Five types of species can be identified in large mammals. The evolution of three types, Ice Age giants, island dwarfs, and hybrids, can be explained, but not that of tropical food specialists and continental paedomorphs. Ice Age giants, which arose while colonizing latitudes (altitudes) with increasingly seasonal climates and productivity pulses, are characterized by ornate social organs, large bodies, and ecological plasticity. Colonizing landscapes with decreasing seasonality appears to conserve (or re-evolve) primitiveness, producing paedomorphs. Island dwarfs appear to be shaped by efficiency selection in the absence of predators. The explanation of mammalian Ice Age evolution hinges on the sensitivity of mammals to environmental factors, in particular nutrition. Extremes in food abundance generate extremes in phenotypes and selection regimes. Abundance is linked to colonization and selection for new social and ecological adaptations; scarcity is typical of settled areas and maintenance regimes. These select for efficiency in the procurement, processing, and use of food. Rapid speciation is predicted during colonization, followed by a gradual, continuous fine tuning of the ecology of the new form. Neither the punctuated nor the gradualistic model of speciation adequately explains evolution in large mammals. Early predictions of the "dispersal hypothesis" of mammalian evolution have now been tested for caprids. Results from cytogenetic, electrophorectic, and immunodiffusion studies support the dispersal hypotheses.

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