M Shpak, University of Texas at El Paso, El Paso, TX, USA © 2008 Elsevier B.V. All rights reserved.

Neo-Darwinism and the 'New Synthesis' Paradigm Punctuated Equilibrium Species Selection

Mass Extinction and Other Challenges from the Fossil Record

Adaptive Radiations Evolutionary Novelty and Innovation Concluding Remarks Further Reading

The term 'macroevolution' refers, broadly speaking, to evolutionary changes above the species level. The term can be used to refer to phenomena such as the origin of morphological (or biochemical) novelty, the origin and subsequent diversity dynamics of higher taxa due to changes in origination or extinction rates, and the role of regulatory genes and developmental constraints on the nature and rate of morphological change within and between clades.

There have been two schools of thought on the nature of macroevolution. While nearly all biologists recognize macroevolutionary phenomena as real patterns observable in the fossil record and from the reconstruction of phylogenies, it has been debated from the time of Darwin and the rediscovery of Mendel's laws whether the microevolutionary processes at the population level extrapolated over time account for all macroevolutionary patterns, or whether there are special evolutionary mechanisms, fundamentally different from those acting within species and populations, responsible for the major events in the history of higher taxa.

Historically, this debate has taken several forms since the resurrection of Mendelism in the early twentieth century. The first was the conflict between the 'gradualist' Darwinian school and the 'mutationist' school represented by geneticists such as H. de Vries and later R. Goldschmidt. While the work of R. A. Fisher reconciled the alleged conflict between Mendelian genetics and the quantitative traits studied by the biometricians, it left open the debate of whether the most important traits in evolution were polygenic quantitative traits (nearly normally distributed due to the small effects of many genes) or discrete traits under the control of small numbers of genes. The macromutationist view emphasized the importance of mutations with large effects for the origin of the type of evolutionary novelties that define higher taxa and allow organisms to invade new regions of eco-space, limiting the importance of gradual Darwinian adaptation to phenomena at the intraspecific population level.

Since that time, 'mutationism' in the form advocated by de Vries and Goldschmidt has fallen into disrepute due to the simple empirical observation that almost every observed macromutation severely reduces the fitness of an organism, while potentially beneficial macromutations are sufficiently rare to be of limited evolutionary importance. Nevertheless, the spirit of mutationist thinking, which emphasizes the importance of rare, drastic changes in phenotype and the importance of internal genetic and developmental factors rather than molding by selection, lives on as an active source of debate.

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