On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 1 : Variation Under Domestication | Summary



Darwin states a large degree of the variability in species is due to humankind's "domestic production." He argues the cultivation of crops and animals for food inherently causes variation. He claims there are two ways variation occurs: directly on the organism or through reproduction.

Darwin argues this indirect—or "indefinite"—variation accounts for why offspring will have traits neither parent possesses. He states over many generations, these individual variations can result in "monstrosities"—variation between species of the same genera, and especially between the domestic and non-domestic varieties. These indirect variations can be seen most vividly in domestic plants and animals.

Despite his emphasis on reproductive variation, Darwin returns to the topic of direct variation and discusses how "changed habits produce an inherited effect" in a species. He provides a variety of examples, such as the differences in bone weight of the domestic and wild duck. He discusses the idea of "correlated variation," or the idea changes in certain traits automatically result in changes in others. He argues correlated variation is the reason human domestication can often result in the unexpected modification of certain traits. Darwin then describes the variation in domesticated pigeons to illustrate his ideas.

Darwin argues the main benefit of domesticated races is the appearance of features beneficial to humankind. He distinguishes between two forms of selection: methodological and unconscious. Methodological selection refers to the predetermined effort to select certain traits within an animal species with the goal of creating a new breed for a specific purpose. In contrast, unconscious selection refers to the unintended selection of certain traits in a species. The conclusion returns to Darwin's original thesis statement, arguing the main cause of domestication was due to "the accumulative action of Selection, whether applied methodically and quickly, or unconsciously and slowly."


Darwin begins his discussion of natural selection by addressing the popular idea a change in species can occur on the organismal level. The oft-used example is the long neck of the giraffe is the result of individual organisms stretching to obtain leaves from increasingly taller trees. However, it is impossible for individual changes that occur over an organism's lifetime to be transferred to their offspring. For instance, an organism that breaks their leg will not have an offspring with a similarly broken leg.

According to Darwin, the only form of natural selection is the second way variation can occur: through sexual reproduction. In order to support his theory, he uses the remainder of the chapter to illustrate various examples of how traits not present in either parent can appear and disappear in their offspring.

Darwin discusses the ability of domesticated animals to revert to their pre-domesticated state. He claims different domestic races or breeds that revert are part of the same species. This is the primary means for defining species, as dictated by Carl Linnaeus, who developed the categorization system of families, genera, and species still in use today. More recently, with the advent of genetic research, scientists have shifted to categorizing species based not on their physical characteristics but on their genetic similarities.

Despite the change in species categorization due to technological and theoretical advances in gene studies, Darwin's work represents the first systematic attempt to identify variations in species. In his final paragraph, he discusses two forms of animal and plant domestication: purposeful and directional, and unintended. Today, it is believed the initial domestication of plants and animals by early human populations was due to a combination of both types of domestication.

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