Ancient Distribution Patterns And Changing Earth Surface

Changing Concepts

Widely disjunct distributions of older origin than discussed so far are revealed when taxa of higher rank, for example, families, are considered. Mainly in the Southern Hemisphere, ranges of close relatives may be separated by wide oceans. Explanations proposed for these patterns changed in accordance with the developing understanding of animal evolution and of changes occurring on the surface of the earth.

First, scientists proposed the former existence of numerous land bridges in early times to explain disjunctions. Most of the proposed land bridges never existed, but a few indeed did occur beyond those entirely caused by Pleistocene sea level fluctuations. Greenland, for example, was long connected with North America; the so-called DeGeer route connected northern Greenland with the extreme northwest of the then larger European continent. To the south, the Thule bridge connected Iceland, the British Isles, and the rest of Europe.

Later, and for as long as continents were believed to have been stable, insect dispersal was the favorite explanation for disjunctions. Routes and corridors along which animals might have moved, and bottlenecks or filters allowing only the passage of selected taxa, were discussed. Dispersal over long distances and across major hazardous obstacles along so-called sweepstake routes, where only few taxa would succeed, and largely by chance, also was considered. Actually, some dispersal can never be excluded, but the phenomenon is by itself insufficient to explain many insect distributions. This is also obvious from the studies of L. Croizat in 1958, who developed a method that he called pan-biogeography. Croizat connected ranges of related animals by lines (or tracks) and observed similar patterns in quite different groups of animals, suggesting the existence of "generalized tracks." Some of these tracks connected landmasses across oceans that all the many taxa with different dispersal capacities could not possibly have crossed; however, no general explanation of wide transoceanic disjunctions was offered.

Major advances in both animal systematics and in the earth sciences profoundly changed the situation, and this occurred only a few decades ago. On the zoological side, the work of

FIGURE 4 Midoceanic ridges and continental plates: A, Arabic plate; Ca, Caribic plate; Co, Cocos plate; S, Somalian plate. Arrows indicate directions of plate movements. Double lines represent midoceanic ridges; transverse lines across them are faults. Subduction and compression zones are mainly along deep-sea rift valleys (dotted lines) or mountain chains (crosses). Figures and stippling identify million years of seafloor spreading. [Modified after Thenius, E. (1979). "Die Evolution der Säugetiere." UTB 865. Gustav Fischer Verlag © Spektrum Akademischer Verlag, Heidelberg.]

FIGURE 4 Midoceanic ridges and continental plates: A, Arabic plate; Ca, Caribic plate; Co, Cocos plate; S, Somalian plate. Arrows indicate directions of plate movements. Double lines represent midoceanic ridges; transverse lines across them are faults. Subduction and compression zones are mainly along deep-sea rift valleys (dotted lines) or mountain chains (crosses). Figures and stippling identify million years of seafloor spreading. [Modified after Thenius, E. (1979). "Die Evolution der Säugetiere." UTB 865. Gustav Fischer Verlag © Spektrum Akademischer Verlag, Heidelberg.]

Willi Hennig was instrumental in the development of modern zoogeography. Henning showed how the degree of phyloge-netic relationship, or closeness of common ancestry (as opposed to some vague relatedness) between taxa can be recognized and reflected in the animal system. He also explained that postulating former land connections is logically justified only if sets of phylogenetically related taxa (i.e., branched sections from the hierarchical animal cladogram) exhibit similar disjunctions. Otherwise, relic distribution from once wider ranges or, alternatively, chance dispersals, are no less probable, even among widely disjunct individual sister taxa.

In the earth sciences, Alfred Wegener in 1912 suggested that continent positions are not stable but change over time. Evidence presented in support of continental displacements included the good fit of continental shelf lines, as well as observations of areas with particular deposits or minerals, traces of paleozoic glaciations, and particular mountain chains on separate continents. However, as long as no mechanism providing the power for movements of continents could be identified, this evidence remained unconvincing.

The situation changed a few decades ago when midoceanic ridges on the seafloors (Fig. 4) were recognized as sources of magma from the fluid interior of the earth; ridges form a network delimiting the continental plates. As magma appears at the surface, it pushes sideward, and the seafloor is spreading. Magnetic particles in the magma become uniformly oriented in the global magnetic field. This orientation is preserved when the magma cools and hardens. Bands of seafloor differing in magnetic orientation (or in paleomagnetism) extend parallel to the midoceanic ridges; evidently, the global magnetic pattern is at times reversed. Several centimeters of new seafloor is produced per year. In combination with measurements of paleomagnetism, the age of seafloors was estimated and found to increase with distance from midoceanic ridges, from contemporary at the ridge to only about 65 mya at the distant sites. Seafloors are generally young, the most ancient ones are only about 200 mya old. Seafloor spreading provides the power that shifts the continental plates, which because of overall differences in elemental composition, are of lower specific weight than seafloor. Therefore, most of the continental material remains afloat while essentially excess seafloor is subducted back into the fluid center of the earth. Such subduction zones occur in deep-sea valleys, mostly along continental edges. Subduction zones coincide with arcs of major vulcanism and earthquake activities. Floating continents may slide past each other along friction zones, continents may collide and cause upfolding of mountains, or they may merge or break up.

Overview of Continental Drift Pattern

Once the mechanism driving continental movements had been recognized, continental drift was widely accepted. Continental plates are moving, merging, and breaking up since their formation. Using all available evidence, paleogeography can describe past changes of the earth's surface in fair detail (Fig. 5). The origin of life in general and

FIGURE 5 Pictorial summary of continental drift, from approximately 330 mya to present times: (A) 330 mya, (B) 300 mya, (C) a single landmass, Pangaea, 280—200 mya, (D) separation of Gondwanaland and Laurasia (180 mya), (E) 40 mya, (F) the Americas reunited, 25 mya. [From Vickery, V. R. (1989). The biogeography of Canadian Grylloptera and Orthoptera. Can. Entomol. 121, 389-424.]

FIGURE 5 Pictorial summary of continental drift, from approximately 330 mya to present times: (A) 330 mya, (B) 300 mya, (C) a single landmass, Pangaea, 280—200 mya, (D) separation of Gondwanaland and Laurasia (180 mya), (E) 40 mya, (F) the Americas reunited, 25 mya. [From Vickery, V. R. (1989). The biogeography of Canadian Grylloptera and Orthoptera. Can. Entomol. 121, 389-424.]

also of several insect orders dates back farther than the formation of a single supercontinent, Pangaea, but the methods of zoogeography cannot provide insight into earlier events. Zoogeography deals mainly with subsequent changes, in particular, the breakup of Pangaea into the northern and southern continents, Laurasia and Gondwanaland, respectively, and their further fates.

Consequences for Biogeography

Knowledge of seafloor spreading, plate tectonics, and continental drift has profoundly affected zoogeography and increased the relative importance of its alternative approaches. The so-called vicariance approach, based on evolutionary theory and on phylogenetic systematics in combination with information on continental drift, gained great explanatory power. Closely related taxa inhabiting separate ranges are called vicariant. Splitting up of populations with disruption of gene flow, which are called vicariance events, becomes the starting point of divergent evolution, eventually leading to differences in species. Over geological time, different vicariant sister clades may evolve from the ancestral species.

Today, one can understand how the breakup of supercontinents led to wide disjunctions and induced separate evolution of related taxa, on separate continents. Continental drift actually provided for means of transport, and insects can now be seen rafting on drifting continents instead of dispersing between them, across wide oceans. Vicariance biogeography seeks for congruences between the evolution of landmasses and the evolution of animals living on them and envisions the first process driving the second. There are now elaborate methodological considerations as well and they are described in works by Humphries and Parenti, and Wiley.

Ancient Disjunctions in Northern Hemisphere

Intra-American faunal differences provide evidence that contemporary insect distributions are almost always the result of a variety of causes that were effective at different times. Most disjunctions between related groups inside North America occur along a line that runs through the central plains, from northwest to southeast. This separation line results from present ecological differences between the mountains that support mainly the arboreal biome and the essentially eremial plains, from the past existence of a midcontinental seaway in the area of the present plains, and from past affiliations of the mountainous eastern and western halves of North America with other continents.

The areas adjacent to the present Bering Strait support tundras, and so did the ice-free areas on and around the former Bering bridge, but forest-dwelling insects had no access to this land bridge. Nevertheless, among the more southern arboreal insects, genera are often shared between eastern Asia and North America; species tend to differ between continents (Fig. 6). These disjunctions date back much further than the

Pleistocene. Because the Angara shield was separated from the Canadian shield only about 2 mya, by the opening of the northern Pacific Ocean, range disjunctions at the specific or generic levels between the Asian Far East and western North America are observed in many orders.

The northern continents formed through fusion of the ancient Canadian (or Laurentian), Fennoscandian (or Baltic), and Angara continental shields that were subsequently again divided and reunited, until the present pattern appeared. North America and Europe were connected until the opening of the Atlantic Ocean, through seafloor spreading, about 70 mya ago. Close phylogenetic relations between various insect groups in eastern North America and Europe are evidence of the past unity (Fig. 7). Europe was to the east long separated from Siberia by the Turgai Strait east of the Ural Mountains, which explains differences in the European and Asian faunas, despite the present continuity of land

Southern Hemisphere Case Study

The stepwise disintegration of Gondwanaland caused some of the most striking disjunctions and long unexplained "transantarctic" or "amphinotic" relations between animals living in Andean South America, Australia, and New Zealand. These landmasses are now known to have long remained connected with, or closely adjacent to, the then forested and inhabitable Antarctic continent.

The fundamental change in views, from dispersalism to continental drift, is recent; the Plecoptera (or stoneflies) provide an example. In 1961 stonefly evolution was still explained entirely by long-distance dispersal involving transgressions of the equator, two in each of the two then recognized suborders. An initial movement of primitive taxa from south to north was assumed, followed by the return of evolu-tionarily advanced forms to the Southern Hemisphere. In 1965 elements of cladistics and continental drift were added to this scenario. A few years later, a cladistic approach to

Plecoptera systematics led to a widely accepted revised system, suggesting that continental drift steered stonefly evolution. The breakup of Pangaea into Laurasia and Gondwanaland seems to have caused the separation into distinct Southern and Northern Hemisphere suborders, the Antarctoperlaria and Arctoperlaria, respectively. When Gondwanaland fell apart, the ranges of the suborder Antarctoperlaria and its families became disjunct, distinct representatives that evolved on each of the distant landmasses (Fig. 8).

However, continental drift alone can probably not explain all of the present Plecoptera distribution. Ecology and also dispersal remain important. Antarctoperlaria must have been present on Gondwanaland before Africa and India broke away from it. Ecological change, perhaps past dryness, is thought to have caused their disappearance from these lands. More difficult are two arctoperlarian families of which subordinate endemic groups are present also in the Southern Hemisphere. Contrary to widespread belief, not all Plecoptera are cool adapted. The Australian Gripopterygidae, Eustheniidae, the arctoperlarian Leuctridae, and the Nemouridae include many tropical species; they are most numerous in the large family Perlidae. The many Neoperla in the Ethiopian region are clearly of northern origin, but the origin of the diverse South American Perlidae is uncertain. Most problematic, however, is the so-called family Notonemouridae. Its mono-phyly is doubtful; it may represent independent early branches of the Nemouridae. Nevertheless, all notonemourids live in temperate parts of South America, Australia, New Zealand, South Africa, and Madagascar, but not in India. Dispersal seems to have contributed to these distributions that, admittedly, remain essentially unexplained. A practical test using the methodological refinements of vicariance biogeography proposed by Humphries and Parenti would require a better understanding of phylogenetic relationships among Plecoptera than is presently available.

FIGURE 7 American—European disjunctions. Solid lines: range of the stonefly genus Leuctra (Plecoptera: Leuctridae); bold figures are total numbers of species per continent, figures in italics are regional numbers of species. A single species, L. fusca, occurs all over Europe and extends through Siberia to the southern portion of the Russian Far East. Broken lines: ranges of the extant ants Ponera pennsylvanica (America) and P. coarctata (Europe); P. atavia (black square) is an amber fossil (Hymenoptera: Formicidae). [Range information after Noonan, G. R. (1988). Faunal relationships between Eastern North America and Europe as shown by insects. Mem. Entomol. Soc. Can. 144, 39-53.]

FIGURE 6 America—Asian relations, and the distinctness of the European fauna: distribution of the genera of Chloroperlinae (Plecoptera: Chloroperlidae). The ranges of five genera (Alloperla, Haploperla, Plumiperla, Suwallia, and Sweltsa) indicated by 5 in northeastern Asia and northwestern America, largely overlap. Numbers of genera of this group decline east- and westward; figures in italics are numbers present in the respective areas. The other genera are A, Alaskaperla; B, Bisancora; C, Chloroperla; I, Isoptena; P Pontoperla; R, Rasvena; S, Siphonoperla; T, Triznaka; X, Xanthoperla.

FIGURE 7 American—European disjunctions. Solid lines: range of the stonefly genus Leuctra (Plecoptera: Leuctridae); bold figures are total numbers of species per continent, figures in italics are regional numbers of species. A single species, L. fusca, occurs all over Europe and extends through Siberia to the southern portion of the Russian Far East. Broken lines: ranges of the extant ants Ponera pennsylvanica (America) and P. coarctata (Europe); P. atavia (black square) is an amber fossil (Hymenoptera: Formicidae). [Range information after Noonan, G. R. (1988). Faunal relationships between Eastern North America and Europe as shown by insects. Mem. Entomol. Soc. Can. 144, 39-53.]

FIGURE 8 Phylogenetic system and distribution of the Plecoptera. The range of the family Gripopterygidae is shown on a map of Gondwanaland at the end of the Cretaceous, with dates of last possible faunal exchange (mya). Different shading indicates that each of the disjunct areas has an endemic fauna; no genus is shared. The Eustheniidae and Austroperlidae are distributed in the same way but have narrower ranges, the five species of the Diamphipnoidae are all South American. [Map based on Crosskey, R.W. (1990). "The Natural History of Blackflies," copyright The Natural History Museum, London.]

FIGURE 8 Phylogenetic system and distribution of the Plecoptera. The range of the family Gripopterygidae is shown on a map of Gondwanaland at the end of the Cretaceous, with dates of last possible faunal exchange (mya). Different shading indicates that each of the disjunct areas has an endemic fauna; no genus is shared. The Eustheniidae and Austroperlidae are distributed in the same way but have narrower ranges, the five species of the Diamphipnoidae are all South American. [Map based on Crosskey, R.W. (1990). "The Natural History of Blackflies," copyright The Natural History Museum, London.]

It is not common for the first step in the breakup of Pangaea to be clearly reflected in the phylogenetic system; otherwise, however, the Plecoptera have many parallels among other insects. Southern Hemisphere disjunctions suggesting a Gondwanian origin are widespread among aquatic (e.g., Ephemeroptera, Odonata, various dipteran midges) and terrestrial insects, for example, in the Hemiptera, Neuroptera, Mecoptera, and Coleoptera, to name a few. Some of these disjunct groups also comprise African representatives. The phylogenetic relationships within several of these groups of insects appear to reflect the proposed sequence of the disintegration of Gondwanaland. The breakup provided series of vicariance events enabling phylogenetic divergence.

See Also the Following Articles

Biodiversity • Fossil Record • Introduced Insects • Island Biogeography and Evolution • Population Ecology

Further Reading

Briden, J. C., Drewry, G. E., and Smith, A. G. (1974). Phanerozoic equal-

area world maps. J. Geol. 82, 555-574. Cranston, P. S., and Naumann, I. D. (1991). Biogeography. In "The Insects of Australia," Vol. 1, pp. 180-197. Melbourne University Press, Melbourne.

Croizat, L. (1958). "Panbiogeography," Vols. 1, 2a, 2b. Caracas.

Downes, J. A., and Kavanaugh, D. H., eds. (1988). Origins of the North American insect fauna. Mem. Entomol. Soc. Can. 144, 1-168.

Hennig, W. (1960). Die Dipteren-Fauna von Neuseeland als systematisches und tiergeographisches Problem. Beitr. Entomol. 10, 221-329.

Humphries, C. J., and Parenti, L. R. (1986). "Cladistic Biogeography." Oxford Monographs on Biogeography 2. Clarendon Press, Oxford, U.K.

Malicky, H. (1983). Chorological patterns and biome types of European Trichoptera and other freshwater insects. Arch. Hydrobiol. 96, 223-244.

Morgan, A. V., and Morgan, A. (1980). Faunal assemblages and distributional shifts of Coleoptera during the late Pleistocene in Canada and the northern United States. Can. Entomol. 112, 1105-1128.

Morrone, J. J., and Crisci, J. V. (1995). Historical biogeography: Introduction to methods. Annu. Rev. Ecol. Syst. 26, 373-401.

Platnick, N. I., and Nelson, G. (1978). A method of analysis for historical biogeography. Syst. Zool. 27, 1-16.

Raven, P. H., and Axelrod, D. I. (1974). Angiosperm biogeography and past continental movements. Ann. Missouri Botan. Garden 61, 539-673.

Taberlet, P., Fumagalli, L., and Wust-Saucy, G. (1998). Comparative phylogeography and postglacial colonization routes in Europe. Mole. Ecol. 7, 453-464.

Tarling, D. H., and Tarling, M. P. (1975). "Continental Drift." 2nd ed. Doubleday, Garden City, NY.

Wiley, E. O. (1988). Vicariance biogeography. Annu. Rev. Ecol. Syst. 19, 513-542.

Zwick, P. (2000). Phylogenetic system and zoogeography of the Plecoptera. Annu. Rev. Entomol. 45, 709-746.

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