On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 4 : Natural Selection; Or the Survival of the Fittest | Summary

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Summary

In this chapter Darwin describes how the principle of natural selection applies not only to domesticated animals but also to wild animals. Darwin begins by defining natural selection, or survival of the fittest, as the "preservation of favorable individual differences and variations, and the destruction of those which are injurious."

He initiates his argument on the premise environmental change results in variability of species. Thus, in contrast to domestication, natural selection on wild animals can act on "the whole machinery of life." The correlation of traits—so that a change in one trait results in a corresponding change in another—contributes to the possibility of a large degree of variation.

Darwin describes how natural selection affects "the structure of the young in relation to the parent," thus altering the structure of a "whole community." Natural selection is a population-level process—individual variations result in large-scale changes. He emphasizes natural selection operates only to provide beneficial adaptations. Thus, while negative variations can emerge at the individual level, they will not persist to the population level unless they provide an adaptive quality. Darwin then provides a specific section on sexual selection, arguing it is similar to natural selection in its processes, but "less rigorous." He defines sexual selection as the situation where "one sex [is] modified in relation to the other sex."

Darwin provides two examples of the ability of natural selection to operate on wild animals: the hunting ability of wolves, and the coevolutionary relationship between bees and flowers. After describing these two examples in detail, Darwin notes his examples are susceptible to objections similar to those raised in Charles Lyell's work, Principles of Geology. However, he argues these objections have been mainly falsified for geological processes and thus should not be applied to biological processes.

Darwin then explains genetic variation in a species is advantageous, as "close interbreeding diminishes vigor and fertility." This serves to increase the amount of variation in a species and acts as a driving process for natural selection. Isolation also serves to drive natural selection through the decrease of genes in the gene pool. In contrast, a large area also serves to promote variation and natural selection processes. He concludes this section with the statement: "Natural selection generally acts with extreme slowness."

This "slowness" makes it possible to identify natural selection and extinctions in the geological record. Darwin describes in the following section how natural selection results in extinctions. This ties into the branch diagram at the end of the chapter Darwin uses to illustrate the creation of new species, genera, and families over time and space.

Following this section, Darwin touches on the idea of advancement in natural selection. He notes scientists have posited there is a "gradual advancement" of organisms in the world. However, he argues the evidence for this is not sufficient, based on the principles of natural selection or the organization of organisms.

Before providing a summary of the chapter, Darwin touches on two ideas: convergence and the issue of infinite species. The first, convergence, refers to a "similarity of structure" of distinct species. The second refers to the question Darwin first alludes to in a previous chapter: "What then checks an indefinite increase in the number of species?" Darwin argues it is the carrying capacity of the environment, and the rapid process of species extinction, in contrast to the relatively slow process of natural selection.

Analysis

This chapter contains the bulk of Darwin's argument natural selection is the driving factor behind species formation in not only domesticated animals but in wild animals as well. Darwin also explains what is meant by the term survival of the fittest. While this term was brought up in the previous chapter, it was not fully explained. In this chapter Darwin details how the term refers not just to individual survival, but how the survival of many individuals results in the relative survival of a population or species.

Darwin goes on to describe sexual selection as distinct from natural selection. However, today sexual selection is seen as a form of natural selection. Males who are able to attract a greater number of females are those who will pass on similar traits. These traits over time result in population-level changes, and can result in variations within a species.

Darwin briefly mentions objections raised in Lyell's work, Principles of Geology. Here he is referring to the objections raised by a group of scientists who promoted the idea "catastrophism"—massive cataclysmic events, such as volcanic eruptions, that cause species extinction—resulted in the geological formations seen today. Darwin argues these same large-scale processes can be argued for the speciation of animals (process of biological species formation), but, as has been proven to be the case with geological processes, speciation does not occur over these short time periods. Darwin ties this to the idea of species extinctions, arguing the processes of natural selection that result in the origination of new species are the same ones that result in species extinctions. He notes these long-term processes can be seen in the geological record.

Darwin also touches on the idea of "advancement" through natural selection. He argues natural selection is not based around the "advancement" of a species. However, this idea is one that would later be misrepresented as "Social Darwinism." While Darwin himself did not promote the idea that certain "races" were more "advanced" than others, some would apply the idea of natural selection to subjugate and degrade human populations throughout the 19th and 20th centuries.

Finally, one of the main contributions of this chapter is Darwin's cladogram (branch diagram used in science). The cladogram is based on an unnamed or hypothetical genus, but Darwin goes on to connect it to a metaphorical tree of life. While some have argued over the inaccuracy of the tree of life metaphor, there is no denying the impact it has had in the popular belief in evolution and natural selection. Throughout the book, Darwin refers to this figure to visually represent the concepts involved with the theory of natural selection.

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