Origin’s Chapter V: How “Random” Is Evolutionary Change?

Abstract

Darwin’s fifth chapter, “The Laws of Variation,” may stand in the shadow of the first four that climax with his presentation of “Natural Selection,” but its importance should not be underestimated. It deals with philosophical and methodological issues in the study of variation that would be hotly debated for decades after the publication of the book, many of which are still unsettled today. As the chapter title suggests, Darwin felt that a proper scientific study of variation had to discover the laws of nature that governed it. He could not simply let it be random. He looked for laws in the patterns of co-occurrence of changes in different parts of the body or the same part in different species and also in the apparent consistencies in the effects of environment and habit. There is no one main line of argument in this chapter, but rather an exploration of multiple possible conceptions, patterns, and laws of variation—all ways in which variation might not be entirely random. For example: To what extent was variation responsive to environmental changes or to the organism’s needs and habits? And if Darwin admitted such responses, then how was his theory any different from Lamarck’s? Do all parts of the organism vary freely and independently or are there hidden connections, correlations, or trade-offs between varying parts of the body? Why do related species seem often to vary in similar ways? Many of these questions are still with us, in modern evolutionary developmental biology (“evo-devo”) and studies of epigenetics.