Macroevolution versus microevolution

Macroevolution encompasses a variety of patterns and processes involving species and larger clades. Some of these patterns can plausibly be described as the result of microevolutionary processes extended across the great expanses of time and space provided by the fossil record. . . . But discontinuities have been documented at a variety of scales, from the punctuated nature of much speciation, to patterns of community overturn, the sorting of species within clades by differential speciation and extinction, and finally mass extinctions. These discontinuities impart a hierarchical structure to evolution, a structure which impedes, obstructs, and even neutralizes the effects of microevolution. As is so often the case in evolution, the interesting question is not, is macroevolution distinct from microevolution, but the relative frequency and impact of processes at the various levels of this hierarchy.

In the nineteenth century, a number of German palaeontologists, such as Johann Christoph Matthias Reinecke, Friedrich August von Quenstedt, and Wilhelm Waagen, separated evolutionary processes within closely related groups of organisms from biological processes occurring on a grander scale (see Hoffman 1989, 88-9). However, the term macroevolution is a product of the twentieth century. The Russian biologist and geneticist Iurii Aleksandrovich Filipchenko coined it in 1927. Richard Goldschmidt popularized it in his The Material Basis of Evolution (1940), wherein he distinguished between microevolution (evolution within populations and species) and macroevolution (evolution within supraspe-cific taxa). George Gaylord Simpson (1902-1984) used Goldschmidt's terms in his Tempo and Mode in Evolution (1944), but included speciation within microevolution. He also coined the term megaevolution for evolutionary phenomena at the level of the family and above. Simpson used the prefixes micro, macro, and mega simply as descriptive devices: he thought that the same processes cause evolution at all levels. In this he was supported by

Figure 6.3 Selected character changes in Eurasian mammoth lineage. (a) Plate count of third upper plus lower molars. (b) Hypsodonty index of third upper molars. Open bars: European samples (shaded, Voigtstedt); filled bars and italic names/ages: north-eastern Siberian samples; cross-hatched bars: African sample (the earliest known mammoths, M. subplanifrons, from southern and eastern Africa, with low plate numbers and shallow crowns). Solid vertical lines connect samples of equivalent age. Dotted lines traverse groups of samples (or subsamples in the cases of Taman' hypsometry index and West Runton and Marsworth plate number) at similar evolutionary level. Asterisks indicate conventional significance levels (two-tailed tests; * p = 0.05; ** p = 0.01; *** p = 0.001) between successive, whole European samples only (i.e. bimodal samples are treated as a whole, and Siberian samples are ignored). The hypsodonty index of M. subplanifrons is shown as mean ± 1 standard error and 1 standard deviation. Sample sizes (P, HI) in brackets are after sites names in the central gutter. Source: Reprinted with permission from A. M. Lister and A. V. Sher (2001) The origin and evolution of the woolly mammoth. Science 294, 1094-7. Copyright © 2001 AAAS.

Bernhard Rensch (1947) and other modern synthesizers, though Rensch would substitute the terms intraspecific and trans-specific for microevolution and macroevolution. Contrary to Simpson, Goldschmidt used the terms microevolution and macroevolution to distinguish two distinct sets of evolutionary processes, rather than as mere descriptors. On the one hand are natural selection, genetic drift, and other forces acting in accordance with the synthetic theory; and, on the other hand, are the appearance of new species and higher groups owing, not to the sifting of small variations within populations, but to macromutations producing 'hopeful monsters', the appearance of which is necessary for evolution to occur (pp. 102-3). Similar views were taken by Otto H. Schindewolf (1936, 1950a, 1950b) and, more recently, by Pierre-Paul Grasse (1973, 1977).

The notion of macroevolution went out of vogue for twenty years or so after Simpson's (1953) deciding to drop the term lest it should confuse and mislead biologists and describing quantum evolution as merely a rapid form of phyletic evolution. Its second birth occurred in the 1970s. In process of being reborn, the term itself evolved and came to mean different things to different people. Today, palaeobiologists define it in many ways. Antoni Hoffman (1989, 91) has picked out the common denominator of all the definitions: 'they all entail phenomena that can be described using species and higher taxa, rather than individual organisms or genotypes, as entities'. Thus, macroevolution is the temporal and spatial patterns of supraspecific phenomena (Hecht and Hoffman 1986). It includes the origin of new basic body plans and rates of species (or genera, family, and so on) origination and extinction. Megaevolution is a subset of macroevolutionary phenomena, specifically those encompassing the grandest possible biological scales - the entire biosphere or at least a substantial realm of life (Hecht and Hoffman 1986). The big question is whether macroevolutionary and megaevolutionary patterns can be explained by microevolutionary processes, as the modern synthesizers maintain, or whether they can be explained only by macroevolutionary and megaevolutionary laws 'describing the action of evolutionary forces complementary to, or superimposed upon, those envisaged by the genetical theory' (Hoffman 1989, 91-2). The architects of the synthetic theory of evolution regard macroevolution as a tiny-step-by-tiny-step process that is an extension of microevolution. As Mayr (1942, 298) had it, 'All the available evidence indicates that the origin of higher categories is a process which is nothing but an extrapolation of speciation. All the processes of macroevolution and the origin of higher categories can be traced back to intraspecific variation even though the first steps of such processes are usually very minute'. He held the same view throughout his long career. Stebbins (1977) was equally adamant that higher categories evolve by the same processes that bring about the origin of races and species.

Despite the influential opinions of Dobzhansky and company, the debate over microevolution and macroevolution is still running. Many palaeobiologists doggedly maintain that macroevolution and microevolution are different processes, and some claim to have uncovered macroevolutionary laws. They do not deny that the modern synthesis explains microevolution and the origin of races, but they are convinced that it cannot explain macroevolution, which theory accounts for patterns of species origination, existence, and extinction, and the corresponding patterns of stasis and change of phenotypic features (cf. Eldredge 1985, 203). Particularly interesting is Gould's 'grand analogy' between the microevolution of organism and macroevolution of species (Table 6.1). Such ultra-Darwinians as Richard Dawkins object vociferously to this kind of hierarchical schema. For Dawkins (2004, 498), 'macroevolution (evolution on the grand scale of millions of years) is simply what you get when microevolution (evolution on the scale of individual lifetimes) is allowed to go on for millions of years'. To bring his point home he uses the parallel of

Table 6.1 Simplified version of Gould's 'grand analogy' between microevolution of organisms and macroevolution of species.

Feature

Organism level

Species level

I TRIAD OF STRUCTURE Individual Part

Collective

Organism Gene, cell Deme, species

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