Lecture 2 - Darwin's Theory On his return to England...

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Darwin’s Theory On his return to England, Darwin began to pull together a great deal of information from the voyage and other sources, notably a reading of Thomas Robert Malthus’ book On Population. He was able to arrive at a mechanism for all the changes that he had observed. He proposed that evolution has taken place through natural selection. Summary of Darwin’s theory: 1. Species are mutable. Darwin’s observations on the Galapagos made it clear to him that species can alter over time, and can give rise to new species. This was part of Lamarck’s theory too, but Darwin realized as Lamarck did not that it is the environment in which organisms find themselves that determines what kinds of organisms will survive there. 2. Natural selection can give rise to new adaptations and new species . The principle of Malthus (see graph below) applies to natural populations. Because unchecked reproduction leads to exhaustion of resources, not all individuals are able to survive and reproduce. The result is a struggle for existence , leading to an increase in the probability that the best - adapted individuals will survive and reproduce. It is these individuals that are more likely to pass their characteristics on to the next generation. This process is one of natural selection , and it operates on the individuals of each generation. Darwin was led to this idea by the writings of Thomas Robert Malthus (1766-1834), who pointed out that because populations increase exponentially while their resources increase at the most arithmetically, populations will always tend to outstrip their resources.
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1 10 6 1.5 10 6 2 10 6 2.5 10 6 3 10 6 3.5 10 6 1 10 6 1.5 10 6 2 10 6 2.5 10 6 3 10 6 3.5 10 6 0 20 40 60 80 100 120 Population Resources (hectares of agricultural land) Time (years) The contrast between the exponential growth of a human population (red) and the arithmetic growth of its resources (blue). 3. Natural and artificial selection are very similar but not identical . Darwin realized that one of the most convincing demonstrations of the power of selection comes from the artificial selection used by plant and animal breeders to produce rapid changes over short periods of time in a wide variety of domesticated breeds. He was particularly fascinated by pigeon breeding, and carried out experiments of his own that convinced him of the remarkable and swift effects of artificial selection on these birds.
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Some of the many varieties (variants produced by artificial selection) of pigeons that have been produced by pigeon-breeders over a span of thousands of years, at least since the time of the ancient Mesopotamians (pigeons appear on coins dated from 4500 B.C.). All of these varieties are descended from an ancestral species still living in the wild, the blue rock dove Columbia livia (above).
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