Debates and the biosphere

Macroevolution

Evolutionary theory began with Charles Darwin and Alfred Russel Wallace. The key arguments of their joint 1858 paper were, briefly: all organisms produce more offspring than their environment can support; abundant variations of most characters occur within species; competition of limited resources creates a struggle for life or existence; descent with heritable modification occurs; and, in consequence, new species evolve (Kutschera and Niklas 2004). Neo-Darwinism began when August Weismann (1892) proposed that sexual reproduction (recombination) creates in every generation a new and variable population of individuals. Starting in the late 1930s, by fusing advances in the fields of genetics, systematics, and palaeontology with neo-Darwinism, the sextumvirate of Theodosius Dobzhansky, Ernst Mayr, Julian Huxley, George Gaylord Simpson, Bernhard Rensch, and G. Ledyard Stebbins forged the synthetic theory or evolutionary synthesis. Their basic conclusions were twofold (Mayr and Provine 1980, 1). First, gradual evolution results from small genetic changes ('mutations') and recombination, with natural selection ordering the genetic variation so produced. Second, the observed features of evolution, especially macroevolutionary processes and speciation, are explicable by known genetic mechanisms. Germane to the discussion latter in the book are their views that speciation proceeds gradually, and that macroevolution (evolution above species level) is a gradual, little-by-little process and an extension of microevolution (the evolution of races, varieties, and species). Proposers of the synthetic theory allowed that, as the fossil record reveals, rates of evolution vary considerably (e.g. Mayr 1942), but they were adamant that microevolution and macroevolution proceed in tiny steps. In short, they prosecuted a gradualistic system of evolution. Only when Niles Eldredge and Stephen Jay Gould proposed the model of punctuated equilibrium in 1972, did a serious challenge to gradualism arise. The basis of punctuated equilibrium is that large evolutionary changes condense into discontinuous speciational events (punctuations) that occur very rapidly, and after a new species has evolved it tends to remain largely unchanged for a relatively long period. Although very contentious, punctuated equilibrium has generated a lot of research. Both schools - gradualism and punctuated equilibrium - can find supporting evidence in the fossil record.

Not all biologists and palaeontologists accept a smooth continuity between microevolution and macroevolution. Richard Goldschmidt (1940) discriminated between micro-

evolution (evolution within populations and species) and macroevolution (evolution within supraspecific taxa). He did not use the terms descriptively, but as a means of labelling two distinct sets of evolutionary processes. Microevolution encompassed natural selection, genetic drift, and other forces acting in accordance with neo-Darwinian and synthetic theories. Macroevolution encompassed the appearance of new species and higher groups owing, not to the sifting of small variations within populations, but to macromutations. Discussion of macroevolution petered out after the early 1950s, but resurfaced in the 1970s when, after the recognition that speciation may be punctuational, some palaeontologists insisted that macroevolution and microevolution are different processes, with macroevolution governed by macroevolutionary laws (e.g. Stanley 1979). The outcome of this line of thinking was the decoupling of macroevolution from microevolution by some researchers while recognizing the 'grand analogy' between the two (p. 100).

A related debate concerns the possibility of macromutations. The idea of macromutations stems from the work of Charles Victor Naudin, Hugo de Vries, Otto H. Schindewolf, and particularly Goldschmidt (1940), who famously argued for chromosomal rearrangements producing 'hopeful monsters' in a swift evolutionary jump. Some biologists have toyed with the idea of macromutations, but the topic is still very much the underdog to micromutational studies. A most promising line of enquiry at present is the role of Hox genes. These regulatory genes might provide a microevolutionary means of producing macroevolutionary changes.

An important debate focuses on the continuity (or lack of it) between microevolution and macroevolution. Gould (1985) proposed a three-tier, hierarchical view of evolution involving ecological moments, normal geological time, and periodic mass extinctions, with different 'rules and principles' governing each tier. The rationale for this hierarchical view of evolution rests on the inability of creatures to ready themselves for mass extinctions spaced over tens of millions of years or more. Third-tier catastrophes often overturn, override, and undo first-tier accumulations of adaptations, although species adaptations in the ecological moment may provide them with exaptations (characters acquired from ancestors and co-opted for a new use) that help them survive later catastrophes. However, if the work of Andrew M. Simons (2002) should prove to be correct, then there may be continuity between microevolution and macroevolution. Such continuity, suggested by a bet-hedging strategy, would help to solve the contradictions displayed by evolutionary trends over different time-scales, which often go in diverse directions and seem to indicate a lack of coupling between microevolution and macroevolution.

Life crises

The earliest 'geological' studies of the modern era reported seashells on mountaintops, which scholars interpreted as evidence for Noah's Flood (see Huggett 1989b). Over the following centuries, explanations of Earth history recognized several catastrophes or revolutions, each marking a huge or even total loss of life (see Huggett 1997b). The advent of Charles Lyell's uniformitarian system of Earth history silenced these catastrophist views, which were in fact a perfectly credible interpretation of the fossil record (Gould 2002, 484). Interest in biotic crises or mass extinctions, as they became known, grew again in the 1950s and 1960s. Schindewolf (1954a, 1954b, 1958, 1963) noticed that abrupt biotic changes occur in fairly complete sequences over a large part of the Earth, and indicate episodes of greatly increased rates of extinction and evolution (see also Newell 1956). During the 1960s, Norman D. Newell published several papers on crises and revolutions in the history of life (Newell 1962, 1963, 1967). He bemoaned the fact that many geologists still followed Lyell in thinking of geological changes as smooth and gradual, uniform and predictable, rather than episodic, variable, and stochastic. To him the stratigraphical record supplied abundant evidence that geological and biological processes have fluctuated greatly in extent and rate in the past, that environments have always changed, and that biological reactions to the changing environments have varied (Newell 1967, 64). He was convinced that 'the evidence requires the conclusion that many significant episodes in geologic history took place during comparatively brief intervals of time and that some of these probably involved unusual conditions for which there are no modern close parallels' (Newell 1967, 65). As to the causes of these biotic crises, Newell looked to sea-level changes, arguing many transgressions and regressions have affected much of the world in short spans of time.

Improved data, especially on marine invertebrates, led to a better appreciation of species origination, extinction, and diversity through the Phanerozoic. It became apparent that several mass extinctions had indeed befallen the world biota. Two issues arose: what caused these extinction events; and were they random or periodic? As to there first question, there is a choice of catastrophes - bolide impacts, volcanism, sea-level change, and many more -that has formed the basis of heated arguments, especially since evidence of a huge impact at the close of the Cretaceous emerged in 1980. Suggestions that mass extinctions are periodic, following a galactic or geological timetable, have fuelled equally heated exchanges of views.

Time's arrow

Lyell was adamant that the world was in a steady state, displaying no overall direction in its history. It did change, but only about a mean condition. In the face of evidence indicating that some geological climates were cold and some hot, he devised an ingenuous explanation based on the distribution of land and water that squared with his steady-state view. His argument was that, were the land all collected round the poles, while the tropical zone were occupied by the ocean, the general temperature would be lowered to an extent that would account for the glacial epoch. Conversely, were the land all collected along the Equator, while the polar regions were covered with sea, this would raise the temperature of the globe considerably. So precious to Lyell was his uniformity of state that for most of his career he maintained that, since the Creation, life displayed no overall direction, to the extent that he believed one day a fossil Silurian rat would turn up. Eventually, the burden of proof for directionality in the geosphere and biosphere became so overpowering that Lyell conceded directional change in life history.

Since the nineteenth century, more and more evidence of directional change has amassed so that no scientist would now attempt to uphold Lyell's steady-state interpretation. The evolving states of the atmosphere and the evolving states of sedimentary rocks bear out directionality in the geosphere. In the biosphere, an increase in the complexity of life, an increase in the size and multicellularity of life, and an increase in the diversity of life all bespeak directionality. The increasing diversity of life has not followed a smooth, monotonic progression. The fossil record seems to show, as the early geological cata-strophists maintained, periods of relatively stable species composition broken by short periods of species change. Moreover, the communities each side of the periods of change commonly possess convergent forms or ecomorphs. The pattern of stasis and change is variously styled coordinated stasis, repeating faunas, pulse-turnover, and chronofaunas. It is an extension of the idea of punctuated equilibrium to whole communities. Some researchers question the reality of coordinated stasis. Its causes are also the subject of deliberation. Even more controversial perhaps is the assertion that there have been cycles in diversity through the Phanerozoic. The latest analysis of an extensive dataset of marine invertebrates compiled by Jack Sepkoski revealed hitherto unrecorded, and somewhat mysterious, cycles of 62-million and 140-million years.

Gaia

The relationship between life and its environment has a much longer history than is sometimes realized. Speculation on the interdependencies between natural phenomena and on the essential unity of all living things is possibly as old as the human species. By Classical times, Herodotus and Plato thought that all life on Earth acts in concert and maintains a stable condition. Plato envisaged a balance of nature in which the organisms are seen to be parts of an integrated whole, in the same way that organs or cells are integrated into a functioning organism itself (e.g. Plato 1971). As a theme of enquiry, the holistic unity of Nature re-emerged in the mediaeval period and through the Renaissance. The idea of holism, with Nature seen as an indivisible unity, has waxed and waned with the relentlessness of lunar tides throughout the modern period. Holistic views were fashionable in the late eighteenth and early nineteenth centuries. Johann Reinhold Forster (1778) presented the natural world as a unified and unifying whole, and attempted to weave into a coherent pattern the physical geography and climate of places with their plant life, and animal life, and human occupants (including agricultural practices, local manufactures, and customs). Gilbert White, author of the celebrated The Natural History of Selbourne (1789), studied Nature as an interdependent whole rather than a series of individual parts. Several German philosophers, including Friedrich Wilhelm Joseph Schelling and Georg Wilhelm Friedrich Hegel, embraced and elaborated the idea of an organic planet (Marshall 1992, 289-94).

Modern scientists usually credit James Hutton (1785, 1788, 1795) as the great-grandfather of the Gaia hypothesis. Inspired by Isaac Newton's vision of planets endlessly cycling about the Sun, Hutton saw the world as a perfect machine that would run forever through its cycles of decay and repair, or until God deemed fit to change it. Hutton offered a revolutionary and comprehensive system of Earth history that involved a repeated, four-stage cycle of change - what geologists now call the geological or rock cycle - that keeps the Earth habitable. His four stages were: the erosion of the land; the deposition of eroded material as layers of sediment in the oceans; the compaction and consolidation of the sedimentary layers by heat from the weight of the overlying layers and from inner parts of the Earth; and the fracturing and uplift of the compacted and consolidated sedimentary rocks owing to heat from within the Earth. He realized that the water cycle had a crucial role to play in this schema by maintaining a flux of material from the continents to the oceans. Taken together, the four stages produce a cycle or 'a circulation in the matter of the globe, and a system of beautiful economy in the works of Nature' (Hutton 1795, vol. II, 562). Hutton likened the Earth to a superorganism, but he used this similitude as a metaphor and did not imply that life contributed materially to the geological cycle (Lovelock 1989). Rather, God created the geological cycle to serve life. Moreover, Hutton saw the world as an organic whole, floating the interesting notion, not without its precursors, that the rock cycle is comparable to the life cycle of an organism: the circulation of blood, respiration, and digestion in animals and plants having their equivalents in terrestrial processes.

Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck, presented a unified system of Nature, in which life and its physical environment constantly interacted. He believed that the study of the Earth should include considerations of the atmosphere (meteorology), the external crust (hydrogeology), and living organisms (biology). In particular, he maintained that a full appreciation of the science of life (biology) demanded the incorporation of the Earth's crust and the atmosphere: living phenomena, for him, did not stand in isolation; they are part of a larger whole that we call 'Nature'. Only by recognizing the constant interaction between the living and non-living worlds, therefore, could sense be made of living things (see Jordanova 1984, 45). Alexander von Humboldt, famed for his concept of climatic zonality and its influence on vegetation, possessed a grand, holistic vision of Nature. Early acquaintance with Johann Wolfgang von Goethe, the great Romantic philosopher and poet, and knowledge of the philosophical ideals of Immanuel Kant's universal science no doubt prompted him to think in this way. During the nineteenth century, Humboldtian holism was a common theme in biological, geographical, and geological discourse. Mary Somerville (1834) emphasized the connections of the physical sciences and sought to integrate the diverse elements of the organic and inorganic worlds into an ordered whole. Karl Ritter expressed the Zusammenhang, or 'hanging-togetherness', of all things. To Ritter, the Earth was not a dead, inorganic planet, but one great organism with animate and inanimate components (Ritter 1866). Ritter's pupil, Arnold Henri Guyot (1850), also suggested a similar notion of the world as an organism. The geologist Bernhard von Cotta, in his Die Geologie der Gegenwart (1846, 1874, 1875), made an important connection between the development of the organic and inorganic worlds. He opined that the rise of organisms was a further step in geological development because new materials were taken from the atmosphere by life and later deposited (Cotta 1874, 199); in its turn, geological development, especially the growing diversity of climate with its diversifying influence on the Earth's surface, affected the development of the organic world (Cotta 1874, 203).

Then, at the end of the nineteenth and beginning of the twentieth centuries, a few Russian scientists put forward interrelated ideas on the coevolution of life and the environment. Andrei G. Lapenis (2002) integrated the ideas of Piotr Alekseevich Kropotkin (1842-1921), Rafail Vasil'evich Rizpolozhensky (1847-1919), and Vladimir Ivanovich Vernadsky (1863-1945), and Vladimir Alexandrovich Kostitzin (1886-1963) and showed that they formed a concept of directed evolution of the global ecosystem. Like the Gaia hypothesis, this concept predicted the evolution of the global ecosystem toward conditions favourable to organisms; unlike the Gaia hypothesis, it contended that this evolution stemmed from local and regional, rather than global, forces (Lapenis 2002).

Some authors do credit Vernadsky with being the first scientist to demonstrate the important functions of the Earth's biosphere in influencing the composition of the modern atmosphere and hydrosphere. Some even credit him with being the source of the Gaia hypothesis. However, as this short discussion has demonstrated, the interdependence of life and its environment has been a rich source of ideas and debates since Classical times. A modern debate on this theme concerns the rival Hadean and Gaian hypotheses, although middle-of-the road positions are popular, too. Some critics were quick to point out the Gaia hypothesis is not subject to refutation. However, ground-breaking work on maximum entropy production in the Earth system, which uses testable hypotheses, shows that life helps to keep the atmosphere-ocean system in a state that benefits living things.

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