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darwinian theory

The Structure of Evolutionary Theory

The Structure of Evolutionary Theory (2002) is a technical book on macroevolutionary theory by the Harvard paleontologist Stephen Jay Gould. The volume is divided into two parts. The first is a historical study and exegesis of classical evolutionary thought, drawing extensively upon primary documents. The second is a constructive critique of contemporary Darwinian theory, and presents a case for a hierarchical interpretation of biological evolution based largely on the author's theory of punctuated equilibrium.

According to Gould, classical Darwinism encompasses three essential core commitments. These are: agency, efficacy, and scope. Agency is the unit upon which natural selection acts. For Darwin, this fundamental unit was the organism. Efficacy encompasses the power of natural selection—over all other forces—in shaping evolution at ecological scales. (Auxiliary forces include sexual selection, as well as historical, structural, and developmental constraints.) Scope is the degree to which natural selection can be extrapolated to explain biological diversity at the macroevolutionary level, including the evolution of higher taxonomic groups.

Gould described these three propositions as the "tripod" of Darwinian central logic, each being so essential to the structure that if any branch were cut, it would either kill, revise, or superficially refurbish the whole structure (depending on the severity of the cut). According to Gould "substantial changes, introduced during the last half of the 20th century, have built a structure so expanded beyond the original Darwinian core, and so enlarged by new principles of macroevolutionary explanation, that the full exposition, while remaining within the domain of Darwinian logic, must be construed as basically different from the canonical theory of natural selection, rather than simply extended."

In the arena of agency, Gould explores the concept of "hierarchy" in the action of evolution (the idea that evolution may act on more than one unit simultaneously, as opposed to only acting upon individual organisms). In the arena of efficacy he explores the forces beside natural selection that have been considered in evolutionary theory. In the arena of scope he considers the relevance of natural selection to the larger scale patterns of life.

Outline

Chapter 1 introduces and outlines the central issues that Gould wishes to discuss. Part I of the book focuses on the early history of evolutionary thought (pre-1959), with Chapter 2 covering the structure of The Origin of Species, Chapter 3 focusing on issues surrounding agency, Chapters 4 & 5 covering efficacy, and Chapters 6 & 7 covering Scope. Part II (comprising the bulk of the text) focuses on the modern discussion and debate (post-1959). Chapters 8 & 9 cover agency, 10 & 11 cover efficacy, and 12 covers scope.

Sections of the book dealing with punctuated equilibrium (chiefly Chapter 9) have been posthumously reprinted as a separate volume by Belknap Harvard.

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