The tempo of evolution

The fossil record yields up evidence of evolution. A big debate surrounds the pattern of evolution - is it slow and stately (gradualism) or does it proceed by rapid change followed by periods of stasis (punctuated equilibrium).?

Gradualism versus punctuated equilibrium

Gradualism

Othenio Abel (1929) and George Gaylord Simpson (1953) distinguished between phyletic change (anagenesis) and phylogenetic change (speciational change or cladogenesis). Phyletic change occurs within a single lineage, whereas phylogenetic change occurs between different lineages, of a clade. Traditionally, biologists tend to be evolutionary gradualists, adhering to Darwin's dictum that 'Nature never progresses by leaps' and subscribing to the view that evolution proceeds by the gradual accumulation of small genetic changes (micromutations). As Michael Ruse (1982, 210) put it: 'A mile is simply 63,360 inches, end to end, and the evolution of mammals from fish is simply a multitude of small random variations, sifted by selection, end to end'. Richard Dawkins (1996) makes the same kind of point in his parable of Mount Improbable, the lofty peaks of which represent such pinnacles of evolutionary achievement as the eye, ears, hearts, and wings. On one side of the mountain is a sheer and seemingly impossible climb to the top of the towering peaks. However, round the far side of the mountain is a gradual ascent representing 'the slow, cumulative, one-step-at-a-time, non-random survival of variants that Darwin called natural selection' (Dawkins 1996, 70).

Many modern palaeobiologists are unhappy with the strict gradualism urged by the ultra-Darwinians, as Niles Eldredge (1995, 4) dubbed them, as an explanation of evolutionary changes. They would doubt if the paths on the far side of Mount Improbable lead to the top, concurring with Verne Grant (1977, 305) that the gradualism of extreme micromuta-tionism is too slow to account for, and is inconsistent with, the observed changes in the fossil record. George C. Williams (1992, 127-35) disagreed with this contention. He accepted that the observed changes in fossils are inconsistent with gradualism, but did not accept that this inconsistency is because the rates of change are hopelessly slow. Rather, after pointing out that virtually all empirical data on evolutionary rates signal speedy change, he turned the argument upside down, suggesting that organisms have done far less evolving than would be expected.

An influential sub-school of middle-of-the-road micromutationists allows the reorganization of the genotype within relatively few generations, and sees such periods of relatively fast genetic change as the possible seat of macroevolutionary changes. The source of this idea was Sewall Wright's (1931) classic paper on the adaptive landscape, in which it was shown that a colonial population system is the most favourable set up for radical evolutionary changes by ordinary micromutational genetic changes (a combination of drift and selection). Many evolutionists do believe, on theoretical grounds, that colonial-type population structures have been involved in the bouts of evolution giving rise to new major groups such as the mammals and angiosperms (Grant, 1977, 292). An espouser and developer of this view was George Gaylord Simpson. In his Tempo and Mode of Evolution (1944), Simpson convincingly described how a population might shift from one adaptive peak to either a new or a previously unoccupied adaptive peak. The shift usually involves a small population evolving at unusually rapid rates. This kind of macroevolutionary change he styled quantum evolution, a process that may give rise to new organisms at any taxonomic level from species upward. The idea of rapid and large changes being associated with small, colonial populations peripheral to a parent population was also explored by Ernst Mayr (1954), who established the 'founder principle', and by Verne Grant (1963). Clearly, all these ideas on rapid speciation shift the emphasis away from gradual changes, in the strict sense employed by Lyell and Darwin, and place it squarely in the punctuationalists' court. However, Simpson and the others envisaged evolution as a continuous process that speeded up during speciation; they did not invoke saltatory changes of the kind proposed by Richard Goldschmidt (p. 103); nor did they reject the conventional model of allopatric (geographical) speciation. In this light, the term 'quantum' was an inappropriate choice as it implies discontinuous change from one state to another. The picture of speciation envisaged by Simpson and Mayr does not go against Darwin's dictum; it merely bends it a bit and is a sort of 'slow, slow, quick, quick, slow' process.

Punctuated equilibrium

Niles Eldredge and Stephen Jay Gould combined phyletic change (change within a lineage or anagenesis) and phylogenetic change (speciational change or cladogenesis) in the much-debated theory of 'punctuated equilibrium' (Eldredge and Gould 1972; Gould and Eldredge 1977, 1993). Their theory stands in antithesis to the phyletic gradualism prosecuted by promulgators of the synthetic theory (Figure 6.1). Punctuated equilibrists, as Michael Ruse (1982) dubbed them, do not deny phyletic change, but they do relegate it to a minor role. Punctuated equilibrium concerns the 'origin and deployment of species in geological time' (Gould 2002, 765). According to Eldredge and Gould (1977), large evolutionary changes condense into discontinuous speciational events (punctuations) that occur very rapidly; after a new species has evolved it tends to remain largely unchanged. This view seems to explain a pattern of change commonly found in the fossil record that has become evident now that the absolute dating of fossiliferous strata is reasonably precise: 'species typically survive for a hundred thousand generations, or even a million or more, without evolving very much' (Stanley 1981, xv). The conclusions are that 'most evolution takes place rapidly, when species come into being by the evolutionary divergence of small populations from parent species', and that after their rapid origination 'most species undergo little evolution before becoming extinct' (Stanley 1981, xv).

The implications of punctuated equilibrium, and the furore it stirred up, are too wideranging to be rehearsed in full here (see Hecht and Hoffman 1986; Hoffman 1989;

(a) Punctuational model (b) Gradualistic model

(a) Punctuational model (b) Gradualistic model

Figure 6.1 Styles of evolutionary change. (a) Punctuational change. (b) Gradualistic change.

Eldredge 1989, 1995; Gould 2002). As far as the tempo of organic change is concerned, the chief implication of the theory is that the continuous and gradual changes, modulated by the mild accelerations and decelerations advocated by such modern synthesizers as Simpson, should be replaced by geologically discontinuous and catastrophic (punctuational) changes as the prevailing pattern in macroevolution (Figure 6.1). These relatively swift speciation events would take something around 5,000-50,000 years to complete, or about one thousandth of the average species' lifetime (Eldredge 1995, 99). Nowhere do Eldredge and Gould suggest that macromutations create new species in a single stroke (see Eldredge 1995, 100). Nevertheless, some critics took punctuated equilibrists to be macro-mutationists, thus stirring up considerable confusion. John R. G. Turner, a British geneticist, understood punctuated equilibrium to be a macromutational-based model and cleverly, if erroneously and perhaps a little rudely, described it as 'evolution by jerks' (see Eldredge 1995, 100). Richard Dawkins (1996, 94-5) asked if punctuational events are simply spells of rapid gradualism or if they are saltations? Dawkins, being an ultra-Darwinian, did not object to the idea of rapid gradualism, but hated the notion of macromutation. Williams (1992), another ultra-Darwinian, made a strong case in favour of rapid gradualism, a view supported by then current papers describing examples of 'rapid evolution' in the past and at present (e.g. Chapin et al. 1993; Sanz and Buscalioni 1992; Stewart and Baker 1992). Indeed, by advocating the fluidity of biological forms, he faced the problem of explaining apparent stasis in the fossil record, which he did rather nicely by invoking normalizing clade selection (Williams 1992, 132). Given that no side seriously entertains macromutation as a potent evolutionary process (but see p. 104), the arguments against punctuated equilibrium rest largely in the choice of terms to describe evolutionary patterns. The ultra-Darwinians are happy with punctuationalism as rapid gradualism. But is rapid gradualism the same thing as slow catastrophism? How big must a departure from uniformity be before it becomes non-uniform or catastrophic? Such linguistic niceties may seem trivial, but they account for some misunderstanding between opposing factions.

Evidence of slow and fast changes

Over the past decades, numerous studies have shown that evolutionary transitions are gradual, although the rates of phylogenetic developments may vary. It follows that evo lution is both gradual and occasionally more or less 'punctuated' . . . At any rate, the conflict between gradualist and punctualist interpretation of the fossil record is no longer an issue, i.e., evolutionary rates can and do vary, often appreciably . . . The real issue is whether 'rapid' evolution as gauged by geological time scales is evidence for the absence of microevolutionary modification of genomes as gauged by reproductive time scales. Although the debate lingers on, the evidence that the mechanisms underlying macroevolution differ from those of microevolution is weak at best.

(Kutschera and Niklas 2004, 266)

The empirical studies required to satisfactorily demonstrate any pattern of speciation are time consuming, but in lineages where multiple characters have been studies with sufficient rigor . . . punctuated equilibrium is common . . . suggesting the presence of a discontinuity between intraspecific, adaptive evolution and the processes that influence species formation.

The gradualists and punctuated equilibrists still do battle with each other. Research since the mid-1990s supports both schools of thought (Figure 6.2). This would suggest gradual changes and rapid changes are both characteristic of evolution. Conversely, the detection of gradual changes in living species and the fossil record does not invalidate the tenets punctuational equilibrium, but the confirmation of punctuational changes does tend to crack the foundations of gradualist thinking.

To complicate matters further, the fossil record reveals several other patterns of specia-tion. An analysis of 58 studies on speciation patterns in the fossil record published between 1972 and 1995 revealed the widespread occurrence of stasis and a mixture of speciational patterns (Erwin and Anstey 1995). The organisms included radiolarians, foraminifera, ammonites, and mammals, and ranged in age from Cambrian through to the Neogene. The patterns were punctuated anagenesis (stasis and rapid change without branching), gradualism (gradual morphological divergence), and gradualistic anagenesis (constant directional evolution without branching), as well as cladogenesis. Stasis was surprisingly prevalent, occurring in 71 per cent of the studies, and associated with anagenesis in 37 per cent of the cases and with punctuated patterns in 63 per cent of the cases. Michael J. Benton and Paul N. Pearson (2001) noted that different groups of species tend to display different modes of speciation. Radiolaria, diatoms, and foraminifera and other microfossils of pelagic plank-

Figure 6.2 Examples of gradual speciation and punctuated speciation in the fossil record. (a) Gradual speciation in the diatom Rhizosolenia. Judging by the height of the length of the apical process, the hyaline area, and the width of the valve measured 8 mm from its apex, speciation occurred about 3.1 million years ago, when one population split into two morphologically distinct populations. The distinction is visible in all three measured parameters. In all cases, the parental species, R. bergonii (open circles), remains largely unchanged, and the daughter species, R. raebergonii (closed circles), diverges. R. raebergonii later invaded the Indian Ocean where it appears suddenly in the sediment record. (b) Punctuational speciation in the bryozoan Metrarabdotos. Metrarabdotos lineages seem to have evolved rapidly and in a decidedly punctuational manner. Speciation was notably rapid from 7-8 million years ago, when nine new species appeared. Owing to sampling quality in the preceding interval, there are questions over the origins of the nine basal species, but a punctuational origin for the remainder (tenue, n. sp. 10, and n. sp. 8) seems highly likely. Source: (a) Adapted from Sorhannus et al. (1988) and Sorhannus et al. (1991); (b) Adapted from Cheetham (1986).

ton often show gradualistic patterns of evolution and speciation. Marine invertebrates from continental shelves tend to exhibit punctuational patterns. Terrestrial vertebrates, at least where the fossil record allows speciation patterns to be identified, present a variety of patterns, but no records of gradual speciation events exist.

Age (million years)

Age (million years)

Age (million years)

Passerine birds and ratites

One way of establishing the relative roles of gradual evolutionary changes and abrupt shifts associated with speciation events is to look at morphological diversity through time. Under anagenesis, morphological variance increases with time (Raup and Gould 1974); under cladogenesis, it increases with the logarithm of the number of species (Foote 1996). Species diversity and time are themselves correlated, but multiple regression analysis can pick out their relative contributions. One such study investigated the effects of time and species number on morphological diversity within clades of 106 passerine bird species (Ricklefs 2004). The results demonstrated unequivocally that the number of species wields a strong influence upon morphological variance independently of time, and that time has no unique effect. They led to the conclusion that morphological evolution in the passerine bird species studied is associated with cladogenesis. However, it is important to note that the strong association between species numbers and morphological diversity does not automatically mean that punctuated equilibrium played a role in passerine bird evolution. Speciation may foster evolutionary diversification in a roundabout way by establishing selection for divergent evolution in evolutionary independent lineages. Therefore, divergent evolution could occur gradually over long periods after lineage splitting, and not necessarily abruptly as new species from in small and peripherally isolated populations that perforce rapidly reorganize their gene pools.

A study of ratite evolution used permutational multiple phylogenetic regression to compare phyletic and phylogenetic models (Cubo 2003). Jorge Cubo measured bone characters of humerus, femur, ulna, radius, and tibiotarsus for three greater rheas (Rhea americana), three lesser rheas (R. pennata), three ostriches (Struthio camelus), two emus (Dromaius novaehollandiae), two southern cassowaries (Casuarius casuarius), one dwarf cassowary (C. bennetti), two northern cassowaries (C. unappendiculatus), three brown kiwis (Apteryx australis), and one little spotted kiwi (A. owenii). For the range of morphological features investigated, evolutionary change tends to have been speciational rather than gradual. The speciational model explained morphological variation in humerus shape, ulna shape, radius shape, and the variation of the wing-length-leg-length ratio.

Other studies find no evidence of cladogenesis in particular lineages. The evolution of cranial capacities in the genus Homo, using 94 cranial samples for the period 1.8 million yeas ago to 50,000 years ago, shows no signs of punctuated equilibrium (Lee and Wolpoff 2003).

European mammoths

Establishing the rate of evolutionary change within a lineage requires abundant samples that have a finely resolved stratigraphy, that are accurately dated, and that correlate across a broad geographical area (Jablonksi 2000). Most recent work on adaptive evolutionary changes in lineages meeting these requirements reveals a complex pattern of local morphological innovation, migration, and extirpation. This is the case with the mammoth lineage in Eurasia (Lister and Sher 2001). Conventionally, palaeontologists recognize three chronospecies of European mammoth (Mammuthus) - Early Pleistocene M. meridionalis (about 2.6 to 0.7 million years ago), early Middle Pleistocene M. trogontherii (about 0.7 to 0.5 million years ago), and the late Middle and Late Pleistocene woolly mammoth (M. primigenius) (about 0.35 to 0.01 million years ago). During this sequence, several important changes took place in characters of the specimens, including a shortening and height ening of the cranium and mandible, a heightening of the molar crown (hypsodonty), an increase in the number of enamel bands (plates) in the molars, and a thinning of the enamel. Palaeontologists argue that a shift from woodland browsing to open grassland grazing triggered the dental changes, which produced teeth more resistant to abrasion.

Using well-dated samples from across the mammoth's Eurasian range, Adrian M. Lister and Andrei V. Sher (2001) plotted plate count (the raw number of plates on complete third upper and lower molars) and hypsodonty index (HI) (the ratio between the maximum height and maximum width of the crown, including cement, for third upper molars), against time (Figure 6.3(a) and (b)). Although the data show a broadly incremental pattern of change in the two characters, they also reveal substantial intervals of stasis, indicated by the dotted lines and, at the two intervals of important transition between chronospecies, bimodality. The first transition occurred around 1,000,000 to 700,000 years ago, where samples from the Taman' Peninsula, Azov Sea, have some M. trogontherii characters and indeed are classed as an intermediate form between M. meridionalis and M. trogontherii -M. meridionalis tamanensis. The 700,000-year-old West Runton and Voigtstedt samples are also bimodal. By 600,000 years ago, M. trogontherii is the only mammoth species in Europe. It persists in stasis through to 190,000 to 150,000 years ago. Starting with researchers in the first quarter of the twentieth century, it is a common view that mammoths dating from 450,000 years ago are woolly mammoths, because the lamellar frequency of the third upper molars increased at around that time. Lister and Sher (2001) show that this increase in lamellar frequency of molar teeth (defined as the number of enamel plates in a 10-cm length of crown) is partly the consequence of a simple reduction in mammoth size and it does not necessarily indicate an evolutionary increase in the number of plates in the crown. The end of M. trogontherii marks the second transition and samples again display bimodality, with some specimens having characteristics of M. trogontherii and some having characteristics of M. primigenius. This is evident at Marsworth in England.

Intriguingly, the Siberian mammoth sequence shows the same morphological transitions as the European sample, but the changes occur consistently ahead of those in Europe. This led Lister and Sher (2001) to propose that advanced forms of mammoths similar to later European M. trogontherii evolved in north-eastern Siberia, presumably from an eastern M. meridionalis population, and later dispersed to Europe and in time ousted the indigenous M. meridionalis form. They noted that the first occurrence of M. trogontherii in Europe was in the east (on the Taman' Peninsula) tends to support this view. They also conclude that, given the complexity of variation in the European mammoths between 1,000,000 and 500,000 years ago, the contemporary European populations must have received some genetic input from the Siberian immigrants during that period. Lister and Sher (2001) reinterpret the second transition - from M. trogontherii to woolly mammoth - as statis followed by sympatry and then replacement, rather then gradual evolution. They believe that the M. trogontherii evolved phyletically into M. primigenius in north-eastern Siberia and then dispersed to Europe, where it interbred with some indigenous M. trogontherii populations. This hypothesis would explain the rarity of relict M. trogontherii morphology in Late Pleistocene Siberia and the persistence of trogontherii-like variation within Late Pleistocene woolly mammoths. The shifting pattern of Pleistocene palaeoenvironments probably drove these changes, with the early initiation and persistent advancement of grazing adaptations in Siberian mammoths probably linked to the earlier advent and greater severity and continuity of periglacial conditions in that region compared to Europe. Therefore, Siberia provided a continuing source of grazing-adapted mammoths that acted as a repeated source of evolutionary advancement into periodically glaciated Europe (Lister and Sher 2001).

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