Evolutionary Theory

The theory of evolution or “the Modern Synthesis” can be applied to any system that changes, with the theory part of evolution used to explain how these changes occur. It is the unifying theory for disciplines as diverse as genetics, archaeology, primatology, biology, paleontology, systematics, and ecology. It is flourishing in psychology, economics, and anthropology and is slowly making inroads into sociology.

Historically, the word evolution was derived from the Latin verb evolvere, or an “unfolding process.” In the eighteenth and early nineteenth century, this usage was combined with a theory of progress to promote a new scientific theory that higher forms had slowly developed out of lower forms. Evolution was also the centerpiece for the first social science paradigm, which viewed human societies as evolving from simple to complex forms. Three of sociology's founding fathers, Auguste Comte, Herbert Spencer, and Émile Durkheim, imported this original definition of evolution into their theories of society. Unfortunately, early evolutionary ideas became intermingled with racism and Social Darwinism as aboriginal peoples were characterized as biologically inferior to members of Western populations.

By the mid-nineteenth century, natural scientists realized that evolution involved much more than an unfolding sequence from simple to complex forms. Charles Darwin in On the Origin of Species by Natural Selection (1859) provided the first meaningful understanding of evolution, by proposing that the environment itself is the agent for evolution by “selecting” for survival those members of a population with useful or adaptive traits. Darwin named this process natural selection, because it shapes the traits of a species for a local environment. Traits with adaptive value enhance fitness, or an organism's ability to leave behind offspring.

Darwin's theory of natural selection accounted for the “survival of the fit” but not the “arrival of the fit,” or how variations were transmitted to the next generation. In 1866, Gregor Mendel discovered that what are now called “genes” preserve and transmit heritable traits by self-replication. By the 1940s, Darwinian selection and Mendelian genetics were merged into the integrative theory of evolution, or the Modern Synthesis, which rests on the idea of adaptation and change through natural selection. Today, the Modern Synthesis recognizes four agents of evolution: (1) natural selection, (2) mutation, (3) gene flow, and (4) genetic drift. Natural selection is the primary agent, favoring individuals better able to survive in a local environment. Yet evolution, or what Darwin called “a descent with modification,” is a population concept, because each new generation is the genetic product of the last breeding population, or deme. To capture this process at the population level, biologists adopted the term gene pool, or the [Page 258]pooling together of genes from breeding members of a local population. Selection, however, can only fine-tune the existing variation in a population. New variation must come from other sources, which is why evolution involves mutation, which increases variation by adding new genetic material to a gene pool; gene flow, which increases variation through the exchange of genes between demes or local gene pools; and genetic drift, which increases the likelihood of random fluctuations in gene pool frequencies.

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