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bivalves

Figure 5.4 Fine-scale evolution in fresh-water snails and bivalves in Lake Turkana, Kenya, through the last 4 myr. The volcanic tuff beds allow accurate dating of the sequence. Major speciation events seem to take place at times of lake-level change: are these examples of punctuational speciation, or merely ecophenotypic shifts? (Based on Williamson 1981.)

without gaps, abundant fossils throughout and good dating.

The problems in testing became evident early on, because sampling was generally not extensive enough. Williamson (1981) attempted to counter this problem in one of the most enormous sampling exercises ever. He studied hundreds of thousands of specimens of snails and bivalves in sediments deposited in the Lake Turkana area of Kenya from 1.3 to 4.5 Ma (Fig. 5.4). Lake Turkana lies in the East African Rift Valley, on a tec-tonically active line where the continent of Africa is unzipping to form two major plates. Lake muds and sands accumulated in thick deposits as the rift opened, and volcanic ash (tuff) beds occur sporadically throughout the sequence.

Williamson recorded changes in 19 species lineages, and found that stasis was the normal state of affairs, but that rapid morphological shifts had taken place three times, two of which corresponded to substantial lake level rises (Fig. 5.4). He interpreted this as evidence for the punctuated equilibrium model, arguing that rapid environmental changes had caused evolutionary shifts and speciation events. The new species were short-lived, he argued, because the parental stock had survived in neighboring unstressed lakes, and returned to colonize Lake Turkana after the lake-level changes had taken place.

However, even this enormous study aroused controversy. Critics pointed out that the sequence of sediments was not complete enough to be sure that all fossils had been found: there were gaps of 1000 years or more, and a great deal of gradual evolution could take place in that time. Second, Williamson's critics noted that the environmental stress of lake-level change induced only short-term changes in shell shape, and when the stress was over, the shell shapes apparently reverted to normal (Fig. 5.4). Hence, they proposed that speciation had not taken place, and that the shells had merely changed shape ecophe-notypically. This means that the changes happened during the animals' lifetimes, in response to particular stresses as they grew in size, and these changes were not genetically coded, and hence were not evolutionary.

Most recently, in a thorough re-study of this work, Bert van Bocxlaer and colleagues (2008) have suggested that Williamson got it wrong. They studied mollusks from several of the African great lakes, revised the taxonomy, and argue strongly that the three apparent speciation shifts (Fig. 5.4) are invasion events, when flooding episodes allowed bivalves and gastropods to enter the lakes from nearby rivers. As water levels subsided, the faunas returned more or less to their pre-flooding condition. So, they argue, this classic example of punctuated equilibrium might be better interpreted as an example of repeated climate change and migration. The new study casts serious doubt on a classic case of supposed punctuated equilibrium, but does not, of course, reject the whole concept.

Consensus on speciation in the fossil record

This debate might have led paleontologists to despair of ever finding a convincing case to assess the two models of species evolution. Now, after more than 30 years of debate, and hundreds of case studies, there seems to be a consensus (Benton & Pearson 2001). Both modes of evolution happen in different situations, punctuated equilibrium particularly among sexually reproducing species that live in ever-changing environments where barriers may be established, and phyletic gradualism is seen among asexual organisms, such as microorganisms that live in the surface waters of the oceans, where evolution is slow and barriers non-existent. So, it seems that Williamson (1981) mistook migrations for punctuations, but doubtless his snails were evolving by punctuational speciation, had the evidence been clearer.

The fossil record demonstrates the widespread occurrence of stasis. In a review by Erwin and Anstey (1995) of 58 published studies on speciation patterns in the fossil record, with organisms ranging from radio-laria and foraminifera to ammonites and mammals, and stratigraphic ages ranging from the Cambrian to the Neogene, 41 (71%)

showed stasis, associated either with gradualism (15 cases; 37%) or with punctuated patterns (26 cases; 63%). It seems clear then that stasis is common, and that had not been predicted from modern genetic studies.

Microfossil groups such as the single-celled foraminifera, radiolaria and diatoms (see pp. 209, 211 and 229, respectively) commonly show gradual patterns of evolution and speciation. The microscopic skeletons of pelagic (open ocean) plankton can often be recovered in large numbers from sedimentary deposits that can be shown to have accumulated continuously over vast periods of time. A study by Sorhannus and colleagues in 1998 on the diatom Rhizosolenia (Box 5.3) is probably the most detailed recent work on speciation in planktonic organisms.

In this case, speciation is evidently sympat-ric (happening on the same spot), because the same splitting event is seen in most of the rock cores from around the equatorial belt of the Pacific. There is no evidence of an invasion of one species from an isolated population elsewhere; indeed, it is difficult to imagine where that population might have hidden and yet remained viable. Second, it is clear that most morphological evolution was not associated with speciation, but occurred afterwards, over about 500,000 years after the morphological distinction first becomes visible. Third, one of the new biological species evolved more rapidly than the other, becoming gradually smaller and evolving a markedly diminished hyaline area, whereas the other retained a morphology more like the ancestral species. Finally, the two species must have evolved

Box 5.3 Gradual speciation in radiolarians

Rhizosolenia is a planktonic diatom that occurs today in huge abundance in the highly productive waters of the equatorial Pacific. The siliceous valves of this genus rain on to the seafloor, where they accumulate in thick piles, mixed with other types of sediment. The morphological evolution of Rhi-zosolenia can be traced by sampling cores of this sediment, which have been taken in several places in the equatorial current system. Relative depths within each core provide a relative chronology, and this chronology can be tied to an absolute age scale using magnetic field reversals in the sediment. Ulf Sorhannus and colleagues from the University of Pennsylvania used this technique to study several million years' worth of evolution of Rhizosolenia, which encompasses a well-marked speciation event (Fig. 5.5).

Figure 5.5 Phyletic gradualism and speciation in the planktonic diatom Rhizosolenia. Today there are two distinct species, R. bergonii and R. praebergonii, that do not interbreed and that differ in the height of the hyaline area. When tracked back through the past 3.4 myr, the species can be seen to have diverged through a span of up to 500,000 years, from 3.2 to 2.7 Ma. The plot shows samples taken from deep-sea boreholes in the central Pacific, and each measurement of the height of the hyaline area is based on a large sample of hundreds of individuals; the means and 95% error bars for each sample are shown. The rock succession is dated by reference to the magnetostratigraphic scheme of normal (black) and reversed (white) polarity. (Courtesy of Ulf Sorhannus.)

Figure 5.5 Phyletic gradualism and speciation in the planktonic diatom Rhizosolenia. Today there are two distinct species, R. bergonii and R. praebergonii, that do not interbreed and that differ in the height of the hyaline area. When tracked back through the past 3.4 myr, the species can be seen to have diverged through a span of up to 500,000 years, from 3.2 to 2.7 Ma. The plot shows samples taken from deep-sea boreholes in the central Pacific, and each measurement of the height of the hyaline area is based on a large sample of hundreds of individuals; the means and 95% error bars for each sample are shown. The rock succession is dated by reference to the magnetostratigraphic scheme of normal (black) and reversed (white) polarity. (Courtesy of Ulf Sorhannus.)

The valves of Rhizosolenia are conical in shape, terminating in an apical process that is rooted in a structure known as the hyaline area. The valves are usually broken at their distal ends, but Sorhannus and his colleagues were able to measure three distinct biometric variables: the length of the apical process, the height of the hyaline area, and the width of the valve at an arbitrary 8 |im from its apex. The first two characters are related to the overall size of the valve; the third is a shape parameter related to both size and the conical angle of the valve. These measurements were conducted on 5000 specimens in a number of populations in eight different cores, spanning 2 million years of evolution and about 60° of longitude.

Planktonic diatoms generally reproduce asexually, but like many predominantly asexual organisms they occasionally produce sexual offspring, probably to counteract the buildup of deleterious mutations (see p. 200). This sexual reproduction means that the large populations of Rhizosolenia can be considered as biological species, and speciation must be effected by a permanent barrier to reproduction.

The morphometric data provide convincing evidence that speciation occurred at or before about 3 Ma. Prior to this, there is only one discernible population, but afterwards, two morphologically distinct populations occur, within which there is a range of intergrading variation, but between which there is a morphological gap. The distinction is visible in all three measured parameters. The descendant species (R. praebergonii) later invaded the Indian Ocean where it appears abruptly in the sediment record.

Read more about speciation and punctuated equilibrium at http://www.blackwellpublishing. com/paleobiology/.

slightly different environmental tolerances, for although their geographic ranges overlap for all their evolution, one of the two daughter species is entirely absent in one of the cores.

Sympatric speciation and gradual evolution are probably rarer among marine invertebrates and continental vertebrates, where there are many more possibilities for the establishment of physical barriers to interbreeding. Studies of lineage evolution among marine invertebrates from shallow waters suggest punctuated patterns of speciation. Such studies are much harder to make than those of deep-sea microfossils because continental shelf sediments accumulate sporadically, and this makes it harder to acquire information with high sampling precision. Nonetheless, immensely detailed studies have been carried out. For example, in long-term studies Alan Cheetham and Jeremy Jackson of the Smithsonian Institution have sampled various genera of bryozoans in the past 10 million years of sediments in the Caribbean, and their studies suggest punctuational patterns of speciation (Box 5.4).

Current evidence suggests that Eldredge and Gould (1972) were right to challenge the assumption that evolution always had to be slow and gradual; in some cases it seems clear that species can split off rather rapidly, and that is entirely consistent with Darwinian evolution. Paleontological studies have shown that species often remain unchanged for long periods - the new phenomenon of stasis that had not been predicted from genetics. Asexual planktonic microorganisms appear to speciate slowly, perhaps over intervals of 0.5-1 myr in a gradualistic way, and sexually reproducing animals that occupy divided and complex habitats perhaps tend to speciate rapidly, in a punctuated manner.

Species selection_

Steven Stanley (1975) argued that a punctua-tional model of evolution could imply a different kind of process, termed by him species selection, that occurred at the same time as, but separate from, natural selection. Stanley envisaged a process that sorted species, and ensured that some parts of the tree of life might diversify rapidly and others more slowly. He emphasized that if there was such a process as species selection, then the species-level characters must be distinct from the individual-level characters involved in natural selection. It is not enough to say, for example, that among African large cats, lions might survive certain kinds of competitive situations because they are larger than the other hunters. Being large is an individual-level character, and selection for size is through natural selection. Species-level characters must be irreducible to the individual level.

Possible species-level characters include the size of the geographic range of a species, the pattern of populations within the overall species' range, characteristic levels of gene flow among the populations of a species, and average species' durations. Some studies have suggested that species-level characters of these kinds may play a part in evolution. Geographically widespread species of gastropods, for example, tend to have longer durations than more localized species, and hence can be said to survive longer because of a species-level character. If species selection is a real force in evolution, then Darwinian evolution would have to be expanded to incorporate a hierarchy, or multilevel array, of processes.

A possible resolution of this issue is the effect hypothesis of Vrba (1984). She argued that some species-level characters may be reducible indirectly to the individual level. That is to say, a broadly based feature of the species actually depends on some other character that is under the influence of natural selection. She gave an example from her own work on the evolution of antelope over the past 6 myr (Fig. 5.7). About 5 Ma antelopes branched into two groups, one consisting of long-lived species that never became diverse, and the other of shorter-lived species that radiated widely. Species' duration in the first group was 2-3 myr and total species diversity through the Plio-Pleistocene was two; in the second group species' duration was 0.253 myr and 32 species evolved. Surely here, she argued, species selection was taking place: the character of short species' duration in the second group permitted great success, as measured by overall species diversity. Vrba noted, however, that the long-lived antelope had wide ecological preferences, while those in the second group were specialists. Hence, the whole pattern could be explained by natural selection at the level of individual antelope,

Box 5.4 Punctuated speciation in bryozoans

Metrarabdotos is an ascophoran cheilostome bryozoan (see p. 320) that is represented today in the Caribbean by three species. Coastal rocks on Dominica and other islands document the past 10 myr of sedimentation in shallow seas, and they yield abundant fossils of this bryozoan. The fossils show that Metrarabdotos radiated dramatically from 8 to 4 Ma, splitting into some 12 species, most of which then died out by the Quaternary. Studies by Cheetham and Jackson have established a variety of protocols for distinguishing species within Metrarabdotos, taking into account the genetics of related extant species, and their amount of morphological differentiation, and then extending comparable statistical tests of morphological differentiation to the fossil forms (they demonstrated highly significant correlations between genetic and morphometric differences among the modern forms). Based on 46 morphometric characters, the authors established a mechanism for distinguishing lineages among the fossils (Jackson & Cheetham 1999).

Lineage splitting in Metrarabdotos seems to have been rapid and punctuational in character (Fig. 5.6). Speciation was especially rapid in the interval from 8 to 7 Ma, with nine new species appearing. There is some question about sampling quality here, since sampling is poor in the preceding interval, and so some of these nine new species might have appeared earlier. However, the interval from 8 to 4 Ma, represented largely by information from Dominica, has been intensely sampled (DSI, Dominican sampling interval). So, although there are questions over the origins of the nine basal species within this interval, the origins of the remainder (tenue, n. sp. 10 and n. sp. 8) are more confidently documented as punctuated. The same kind of punctuated pattern of speciation has been found also in virtually all other studies on fossil marine invertebrates that have been carried out.

Read more about speciation and punctuated equilibrium at http://www.blackwellpublishing. com/paleobiology/.

tenue group unguiculatum group tenue group unguiculatum group

Figure 5.6 Punctuated evolution and speciation in the bryozoan Metrarabdotos in the Caribbean. Today, there are three species of this genus, but there have been many more in the past. Careful collecting throughout the Caribbean has shown how the lineages exhibited stasis for long intervals, and then underwent phases of rapid species splitting, especially in the time from 8 to 4 Ma, the Dominican sampling interval (DSI), where records are particularly good. (Courtesy of Alan Cheetham.)

Figure 5.6 Punctuated evolution and speciation in the bryozoan Metrarabdotos in the Caribbean. Today, there are three species of this genus, but there have been many more in the past. Careful collecting throughout the Caribbean has shown how the lineages exhibited stasis for long intervals, and then underwent phases of rapid species splitting, especially in the time from 8 to 4 Ma, the Dominican sampling interval (DSI), where records are particularly good. (Courtesy of Alan Cheetham.)

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