Darwin's Four Postulates

Minami Sakuma

Darwin's Postulates

  1. Individuals within a species are variable

  2. Some of these variations are passed along to offspring

  3. In every generation, more offspring are produced than can survive

  4. Survival and reproduction are not random; those with favorable variations survive and go on to reproduce.

These four postulates cause natural selection.

Postulate 1

The first postulate of Darwin is individuals within a species are variable. By observing variety of species, Darwin knew that there are variations between individuals in species, but he did not know how this variations is produced. With technological advances, now we know that it is because of genetic differences. As we learned, there are several sources that produce variations in genes of individuals. The first one is by mutations. Mutations alter existing genes to form new alleles, caused by copying errors during DNA replication, DNA damage, and repair or recombination during cell division. The second source of variation is by sexual reproduction. During the sexual reproduction, new combinations of DNA are produced with the independent assortment of genes.

In order to prove this first postulate, Darwin gathered a number of morphological variables from populations of Geospiza fortis, and measured its beak width, depth and length. As the graph shows, he found that there the beak depths of Geospiza fortis varied individuals.

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Postulate 2

The second postulate of Darwin is some of these variations are passed along to offspring. In 1856, when Darwin researched about the natural selection, he did not know about DNA and existence of genes. Therefore, he did not understand in details but he was sure that variations within species are transmissible from parent to offspring. We know can understand that variations are caused by differences in genes and some genes are passed onto offspring. Therefore, we have some similar traits with our parents. We also learned that genes are inherited to offspring independently (independent assortment).

To prove this postulate, Darwin compared the similarity in the beaks between pairs of relatives of finches. As a result, he found that there is a strong connection between them. The graph shows the heritability of beak depth in medium ground finches. The result from the two years, 1978 and 1976, are consistent, which show a strong relationship between beak depth of parents and their offspring. This means that parents with deep beaks tended to have offspring with deep beaks.

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Postulate 3

The third postulate of Darwin is in every generation, more offspring are produced than can survive. The good example of this postulate is fish. Many fish lay hundreds or thousands of eggs at once, but most of them will be eaten by its predators before they can reproduce its own offsprings and inherit its genes. From this, we know that by producing many offsprings, the chance of survival increase, because even though many individuals could not survive with limited food supply or pressure of natural selection, if even small portion of species survived, they would be able to continue to pass their genes onto next generation.

To prove this postulate, Darwin examined the size of beaks of finches before and after the severe drought of 1977. He discovered that in about 20 months, 84% of the medium finch population disappeared. This was because of starvation since there were scarcity of seeds available for the finches with medium beaks. The graph shows the changes in a number of the finches with medium beak corresponding to the change in a number of seeds that is available for them. This indicates that only a part of the population survived to reproduce.

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Postulate 4

The fourth postulate of Darwin is survival and reproduction are not random. As the third postulate explains, more offspring are produced than can survive, so some individuals within a species must die. This postulate explains that those who can adapt to the environment, where they live, are more likely to survive and reproduce their offsprings than those who do not adapt to the environment well. This adaptation is also called fitness of individuals. On genetic level, a specific allele is responsible for a trait that helps individuals to increase their fitnesses. Therefore, those who have the allele, which increases fitness, can survive and can pass this allele to the next generation. This shows that survival and reproduction are not random since only individuals with greater fitness can survive.

To prove this postulate, Darwin examined the traits of the finches who survived the drought, and he determined that only those finches with the deepest beaks had survived. This is because the finches with deep, narrow beaks were better able to crack and eat the most prevalent source of fruit in the area. The graph shows the result of Darwin’s experiment.

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