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Revision as of 10:56, 5 June 2005 by Lexor (talk | contribs) (→1920s-1940s: the modern evolutionary synthesis)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)The history of evolutionary thought has a long history since the idea of biological evolution has existed since ancient times, but the modern theory wasn't established until the 18th and 19th centuries, with scientists such as Jean-Baptiste Lamarck and Charles Darwin. Darwin greatly emphasized the difference between his two main points: establishing the fact of evolution, and proposing the theory of natural selection to explain the mechanism of evolution.
Pre-Darwin
Pre-Darwinian evolutionary ideas include Lamarckism and orthogenesis.
1850s - early 20th century: Darwin's theory
Main articles: Inception of Darwin's theory, Development of Darwin's theory, Publication of Darwin's theory, Reaction to Darwin's theory
While transmutation of species was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species which provided the first cogent mechanism by which evolutionary change could persist: his theory of natural selection. Darwin was motivated to publish his work on evolution after receiving a letter from Alfred Russel Wallace, in which Wallace revealed his own discovery of natural selection. As such, Wallace is sometimes given shared credit for the theory of evolution. However, Wallace himself backed away from claiming too much credit, admitting that Darwin's formulation of the theory and his work on evolution went far beyond Wallace's conjectures in scope and explanatory power (he would later, to Darwin's great disappointment, back away completely from the idea that humans were evolved by natural means as he began to turn towards spiritualism).
Wilkins identifies Darwin's into seven categories:
- Transmutationism
- Common descent
- Struggle for existence
- Natural selection
- Sexual selection
- Biogeographic distribution
- Heredity
Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life (and indeed resulted in a small social revolution), could not explain several critical components of the evolutionary process. Namely, he was unable to explain the source of variation in traits within a species, and he could not provide a mechanism whereby traits were passed faithfully from one generation to the next. Darwin's theory of pangenesis, while relying in part on the inheritance of acquired characteristics, proved to be useful for statistical models of evolution developed by his cousin Francis Galton and the "biometric" school of evolutionary thought. It was, however, found to be of little use to biologists.
While the scientific community generally accepted that evolution had occurred, many disagreed that it had happened under the conditions or mechanisms provided by Darwin. In the years immediately following Darwin's death, evolutionary thought fractured into a number of interpretations, include neo-Darwinism, neo-Larmarckism, orthogenesis, Mendelism, the biometric approach, and mutation theory. Eventually this boiled down to a debate between the Mendelians (discrete variation) and the biometricians (continuous variation), which were assembled into the modern evolutionary synthesis by the 1930s.
1920s-1940s: the modern evolutionary synthesis
Main article: Modern evolutionary synthesis
These questions of interpretation were not settled until the early 20th century, beginning with the work of an Austrian monk named Gregor Mendel in the late 19th century, who outlined, through a series of ingeniously devised experiments, a model for inheritance of traits based on the fundamental unit of the gene. Mendel's work was unappreciated at the time and largely ignored by the biological community. When it was "rediscovered" in 1900, it led to a storm of conflict between Mendelians (Charles Benedict Davenport) and biometricians (Walter Frank Raphael Weldon and Karl Pearson), who insisted that the great majority of traits important to evolution must show continuous variation that was not explainable by Mendelian analysis.
Eventually, the two models were reconciled and merged, primarily through the work of the biologist and statistician R.A. Fisher. This combined approach, applying a rigorous statistical model to Mendel's theories of inheritance via genes, became known in the 1930s and 1940s as the modern synthesis of Darwin's theory.
1940s-1960s: developments following molecular biology
In the 1940s, following up on Griffith's experiment, Avery, McCleod and McCarty definitively identified deoxyribonucleic acid (DNA) as the "transforming principle" responsible for transmitting genetic information. In 1953, Francis Crick and James Watson published their famous paper on the structure of DNA, based on the research of Rosalind Franklin and Maurice Wilkins. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process: the mutation of segments of DNA.
During this era of molecular biology, it also became clear that a major mechanism for variation within a population is the mutagenesis of DNA. In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, firmly establishing the importance of genetic drift as a major mechanism of evolution.
1960s-1980s: Williams revolution, punctuated equilibrium
Coined after the evolutionary biologist, George C. Williams, the Williams revolution is a paradigm shift which occurred in evolutionary biology in the mid-1960s in which verbal arguments, couched in terms of "survival of the species" (essentially group selection arguments) were largely replaced by a gene-centered view of evolution, epitomised by kin selection. Models of the period showed that group selection was severely limited in its strength, although these models have since been shown to be too limited and newer models do admit the possibility of significant multi-level selection.
Other debates have continued within the field. One of the most prominent outstanding debates is over the theory of punctuated equilibrium, a theory propounded by Niles Eldredge and Stephen Jay Gould to explain the paucity of transitional forms between phyla.
1970s-2000s: evolutionary biology as a discipline
Evolutionary biology as an academic discipline in its own right emerged as a result of the modern evolutionary synthesis in the 1930s and 1940s. It was not until the 1970s and 1980s, however, that a significant number of universities had departments that specifically included the term evolutionary biology in their titles. In the United States, as a result of the rapid growth of molecular and cell biology, many universities have split (or aggregated) their biology departments into molecular and cell biology-style departments and ecology and evolutionary biology-style departments (which often have subsumed older departments in paleontology, zoology and the like).
Microbiology has recently developed into an evolutionary discipline. It was originally ignored due to the paucity of morphological traits and the lack of a species concept in microbiology. Now, evolutionary researchers are taking advantage our extensive understanding of microbial physiology, the ease of microbial genomics, and the quick generation time of some microbes to answer evolutionary questions. Similar features have led to progress in viral evolution, particularly for bacteriophage.
Recent developments in evolutionary theory
Daniel Dennett (1995) argues in Darwin's Dangerous Idea that natural selection is an algorithmic process applicable to many circumstances besides biological evolution. This conception of evolutionary has been dubbed "universal Darwinism".
Symbiogenesis
Main article: Symbiogenesis
Another extension to the standard modern synthesis, advocated by Lynn Margulis, is symbiogenesis. Symbiogenesis argues that acquisition and accumulation of random mutations or genetic drift are not sufficient to explain how new inherited variations occur in evolution. This theory states that species arise from the merger of independent organisms through symbiosis. Symbiogenesis emphasizes the impact of co-operation rather than Darwinian competition. This commonly occurs in multigenomic organisms throughout nature.
Neo-structuralist themes in evolutionary theory
In the 1980s and 1990s there was a renewal of structuralist themes in evolutionary biology by biologists such as Brian Goodwin, that incorporates ideas from cybernetics and systems theory, and that emphasizes the role of self-organized processes as being at least as important as the role of natural selection. Some extreme variants consider natural selection as the result of biological evolution and not its cause, though most neo-structuralist biologists would not go this far.
The evolution of altruism
Main article: Altruism
Altruism has been one of the last (and most deeply embedded) thorns in the side of evolutionary theory, but recent developments in game theory have suggested explanations with an evolutionary context. If humans evolved, then so did human minds, and if minds evolved, then so does behaviour - including, according to these models, altruistic tendencies.
Theories of eusociality and the undoubted advantages of kin selection have made good progress in this direction, but they are far from unproblematic. Some writers have pointed out that the conscience is just another aspect of our mental behaviour, and propose an evolutionary explanation for the existence of conscience and therefore altruism. One recent suggestion, expressed most eloquently by the philosopher Daniel Dennett, was initially developed when considering the problem of so-called 'free riders' in the tragedy of the commons, a larger-scale version of the Prisoner's Dilemma.
An interesting example of altruism is found in the cellular slime moulds, such as Dictyostelium mucoroides. These protists live as individual amoebae until starved, at which point they aggregate and form a multicellular fruiting body in which some cells sacrifice themselves to promote the survival of other cells in the fruiting body.
Unconventional extensions to evolutionary ideas
De Chardin's and Huxley's theories
Pierre Teilhard de Chardin and Julian Huxley formulated theories describing the gradual development of the Universe from subatomic particles to human society, considered by Teilhard as the last stage. (see Gaia theory). These are not generally recognized as scientifically rigorous.
Nine levels of development are described in their scheme. Stages one through five are grouped into the Lithosphere, also called Geosphere or Physiosphere, where the evolution of the structure of organisms is ruled by mechanical laws and coincidence. Levels six, seven, and eight are the classical biological stages. Stages six through eight are collectively called the Biosphere, where the progress of the structure of the organisms is ruled by genetic mechanisms. The actual stage, stage 9, is called the Noosphere, where the structure of human society is ruled by psychological, informational and communicative processes.
References
- Peter J. Bowler, Evolution: The history of an idea, Revised Edition (Berkeley, CA: University of California Press, 1989).