In this lesson, we wish to ask: How did observations in nature lead to the formulation of the theory of evolution?
In the Prefacehe begins: A modern evolutionist turns to Darwin's work again and again. This is not surprising, since the roots of all our evolutionary thinking go back to Darwin. Our current controversies very often have as their starting point some vagueness in Darwin's writings or a question Darwin was unable to answer owing to the insufficient biological knowledge available in his time.
But one returns to Darwin's original writings for more than historical reasons. Darwin frequently understood things far more clearly than both his supporters and his opponents, including those of the present day.
In both scholarly and popular literature one frequently finds references to "Darwin's theory of evolution", as though it were a unitary entity.
In reality, Darwin's "theory" of evolution was a whole bundle of theories, and it is impossible to discuss Darwin's evolutionary thought constructively if one does not distinguish its various components. A better understanding of the meaning of this term is only one reason to call attention to the composite nature of Darwin's evolutionary thought.
One Darwinism and natural selection cogent reason why Darwinism cannot be a single monolithic theory is that organic evolution consists of two essentially independent processes, as we have seen: The two processes require a minimum of two entirely independent and very different theories.
I consider it necessary to dissect Darwin's conceptual framework of evolution into a number of major theories that formed the basis of his evolutionary thinking.
For the sake of convenience, I have partitioned Darwin's evolutionary paradigm into five theories, but of course others might prefer a different division. The selected theories are by no means all of Darwin's evolutionary theories; others were, for instance, sexual selection, pangenesis, effect of use and disuse, and character divergence.
However when later authors referred to Darwin's theory thay invariably had a combination of some of the following five theories in mind: This is the theory that the world is not constant or recently created nor perpetually cycling, but rather is steadily changing, and that organisms are transformed in time.
This is the theory that every group of organisms descended from a common ancestor, and that all groups of organisms, including animals, plants, and microorganisms, ultimately go back to a single origin of life on earth.
This theory explains the origin of the enormous organic diversity. It postulates that species multiply, either by splitting into daughter species or by "budding", that is, by the establishment of geographically isloated founder populations that evolve into new species.
According to this theory, evolutionary change takes place through the gradual change of populations and not by the sudden saltational production of new individuals that represent a new type. According to this theory, evolutionary change comes about throught the abundant production of genetic variation in every generation.
The relatively few individuals who survive, owing to a particularly well-adapted combination of inheritable characters, give rise to the next generation. Let's look at some of the implications of Mayr's analysis.
Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they do spread, if discovered in a geological formation, they will appear as if suddenly created there, and will simply be classed as new species. That is, despite the distortions of some anti-evolutionists, Darwin explictly did not think that evolution proceeds by the production of "hopeful monsters" -- Darwin himself never proposed that a fully-dinosaur parent gave birth to fully-bird progeny.
Rather, the change took place in a series of intermediate, perhaps nearly insensible, steps in successive generations.
Note that change over a thousand generations of any species appears as "sudden" or "abrupt" change in the fossil record, because a thousand generations is such an infinitesimally small fraction of Earth's history.
After Darwin, some biologists distorted the theory of natural selection into the doctrine of "strict adaptionism", in which every feature of every organism was held to be produced by natural selection and thus some explanation of why the feature is adaptive was required.
But Darwin didn't say that all selection is natural adaptive selection -- only that natural selection is the source of some change, and can explain why adaptive change occurs. Modern biologists have proposed other mechanisms for change -- neutral selection, genetic drift, the "founder effect", etc.These are the basic tenets of evolution by natural selection as defined by Darwin.
The following is a quote from Darwin. "Variation is a feature of natural populations and every population produces more progeny than its environment can manage.
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