EvolutionEvolution is the process in which inherited traits become more or less prevalent in a population over successive generations. Over time, this process can lead to speciation, the development of new species from existing ones. All extant organisms are related by common descent, having evolved over billions of years of cumulative genetic changes from a single ancestor.
The basic mechanisms that produce evolutionary change are natural selection and genetic drift; these two mechanisms act on the genetic variation caused by mutation, genetic recombination and gene flow. Through genetic drift, the frequency of heritable traits changes randomly. Through natural selection, organisms with traits that help them to survive and reproduce will have more offspring, passing these beneficial traits on to the next generation. This leads to advantageous traits becoming more common in each generation, while disadvantageous traits become rarer. Given enough time, this passive process can result in varied adaptations to changing environmental conditions.
The theory of evolution by natural selection was first propounded in detail in Charles Darwin's 1859 book On the Origin of Species. In the 1930s, Darwinian natural selection was combined with the theory of Mendelian heredity to form the modern evolutionary synthesis, also known as "Neo-Darwinism". The modern synthesis describes evolution as a change in the frequency of different versions of genes, known as alleles, within a population from one generation to the next. With its enormous explanatory and predictive power, this theory has become the central organizing principle of modern biology, relating directly to topics such as the origin of antibiotic resistance in bacteria, eusociality in insects, and the biodiversity of Earth's ecosystem.
The Great Chain of Being
The great chain of being or scala naturæ is a classical and western medieval conception of the order of the universe, whose chief characteristic is a strict hierarchical system.
It is a conception of the world's structure that was accepted, and unquestioned, by most educated men from the time of Lucretius until the Copernican and Darwinian revolution and the ultimate flowering of the Renaissance. The chain of being is composed of a great number of hierarchal links, from the most basic and foundational elements up through the very highest perfection, in other words, God, or the Prime Mover.
God, and beneath him the angels, both existing wholly in spirit form, sit at the top of the ladder. Earthly flesh is fallible and ever-changing: mutable. Spirit, however, is unchanging and permanent. This sense of permanence is crucial to understanding this conception of reality. One does not abandon one's place in the chain; it is not only unthinkable, but generally impossible. (One exception might be in the realm of alchemy, where alchemists attempted to transmute base elements, such as lead, into higher elements, either silver, or more often, gold—the highest element.)
In the natural order, earth (rock) is at the bottom of the chain; these elements possess only the attribute of existence. Moving on up the chain, each succeeding link contains the positive attributes of the previous link, and adds (at least) one other. Rocks, as above, possess only existence; the next link up, plants, possess life and existence. Beasts add not only motion, but appetite as well.
Man is a special instance in this conception. He is both mortal flesh, as those below him, and also spirit. In this dichotomy, the struggle between flesh and spirit becomes a moral one. The way of the spirit is higher, more noble; it brings one closer to God. The desires of the flesh drag one down. The Christian fall of Lucifer is especially terrible, because that angel is wholly spirit, who yet defies God, the ultimate perfection.
Theory of Catastrophism by Georges Cuvier
In geology, the doctrine that at intervals in the earth's history all living things have been destroyed by cataclysms (e.g., floods or earthquakes) and replaced by an entirely different population. During these cataclysms the features of the earth's surface, such as mountains and valleys, were formed. The theory, popularly accepted from the earliest times, was attacked in the late 18th cent., notably by James Hutton, who may be regarded as the precursor of the opposite doctrine of uniformitarianism.
Catastrophism, however, was more easily correlated with religious doctrines (e.g., the Mosaic account of the Flood) and remained for some time the interpretation of the earth's history accepted by the great majority of geologists. It was systematized and defended by the Frenchman Georges Cuvier, whose position as the greatest geologist of his day easily overbore all opposition. In the 19th cent., it was attacked by George Poulett Scrope and especially by Sir Charles Lyell, under whose influence the contrary doctrine gradually became more popular. Recent theories of meteorite, asteroid, or comet impacts triggering mass extinctions can be interpreted as a revival of catastrophism.
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Use and Disuse by Jean Lamarck
The Lamarckian evolutionary idea that if an animal often used a particular trait or characteristic, that trait or characteristic would become more pronounced. Alternately, if a trait went unused, it would become less pronounced.
Natural Selection by Charles Darwin
Natural selection is the biological theory that explains why living creatures seem to match their environmental niches so well – the process by which individual organisms with favorable traits are more likely to survive and reproduce than those with unfavorable traits. Insofar as there is genetic variability for the trait under selection, the genotypes associated with the favored traits will increase in frequency in the next generation. Given enough time, this passive process results in adaptations and speciation (see evolution).
Natural selection is one of the cornerstones of modern biology. The term was introduced by Charles Darwin in his 1859 book The Origin of Species, by analogy with artificial selection, by which a farmer selects his breeding stock.
Labels: evolution
In geology, the doctrine that at intervals in the earth's history all living things have been destroyed by cataclysms (e.g., floods or earthquakes) and replaced by an entirely different population. During these cataclysms the features of the earth's surface, such as mountains and valleys, were formed. The theory, popularly accepted from the earliest times, was attacked in the late 18th cent., notably by James Hutton, who may be regarded as the precursor of the opposite doctrine of uniformitarianism.
Natural selection is the biological theory that explains why living creatures seem to match their environmental niches so well – the process by which individual organisms with favorable traits are more likely to survive and reproduce than those with unfavorable traits. Insofar as there is genetic variability for the trait under selection, the genotypes associated with the favored traits will increase in frequency in the next generation. Given enough time, this passive process results in adaptations and speciation (see evolution).