On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 5 : Laws of Variation | Summary

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Summary

In Chapter 5 Darwin is concerned with describing the laws of variation, although he admits "our ignorance ... is profound." He begins by noting environmental conditions—such as food and climate—play a role in variation, and likely natural selection. However, the degree of this role is unclear. He argues the relative amount of use or disuse of a part results in changes due to natural selection. He cites numerous examples, including cave animals who, due to the lack of use of their eyes, have gone blind. He also notes there are changes in species and variations resulting from acclimatization. Different species within the same genus can often be found in very different climates. Darwin attributes some of this variation to changes related to natural selection.

Darwin notes there are multiple competing factors at play that are likely the result of patterns he sees in species variation. Some of these factors include: the "tendency to reversion," the "tendency to new variations," and the "power" of maintaining stasis within a species. He argues the large degree of random variation makes sense only in the context of natural selection, in contrast to the traditional explanation of "each species having been independently created."

Darwin goes on to address the variability in secondary sexual characteristics. A high degree of variation in these traits may occur within the same species, and independent species may develop analogous traits. He points to the domestic pigeon, noting multiple breeds develop similar traits seemingly independently. He ends the chapter by stating the laws of variation should become clearer through the descriptions of various genera and species.

Analysis

In this chapter Darwin is attempting to determine the mechanism for variations and natural selection. He admits he is unsure of the fundamental cause, but presents a wide variety of examples and descriptions in an attempt to elucidate some general laws. Interestingly, the cause of these patterns was determined at approximately the same time On the Origin of Species was initially published in 1859. In 1865 Gregor Mendel published his famous experiment involving crossbred pea plants that confirmed the mechanism of natural selection to be genetic inheritance. However, it would not be until after 1900 that scholars would rediscover Mendel's work and combine it with Darwin's theory of natural selection.

It is now known the variations Darwin describes in this chapter are all controlled by genes through a combination of processes: the law of segregation, the law of independent assortment, and the law of dominance. Together, these "Laws of Heredity" explain a large proportion of the variation Darwin describes in this chapter. For example, the reemergence of traits that have seemingly fallen out of the gene pool of a particular species could be due to the phenotypic expression of recessive traits.

In this chapter Darwin also discusses the "law of compensation." Mathematician Johann Goethe and naturalist Étienne Geoffroy Saint-Hilaire formulated this law, which states changes in one structure of an organism are correlated with a reduction in the function of another structure. This argument places a great deal of emphasis on the form of a structure in contrast to its function. Not only does Darwin argue his theory of natural selection goes against the traditional view of function over form, but he offers evidence the traditional view of independent creation does not explain the variation inherent in the natural world. These two ideas exemplify some of the reasons why Darwin was hesitant to publish on this topic—they were at the core of some of the arguments against the theory of natural selection after the initial publication of On the Origin of Species.

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