Chapter 2 Notes: The Development of Evolutionary Theory
Natural Selection
Natural selection, first explained by Charles Darwin, refers to genetic change or changes in the frequencies of certain traits in populations due to differential reproductive success between individuals. It is the most critical mechanism of evolutionary change. Many of these same conclusions were independently reached by Alfred Russel Wallace. The theory of natural selection proposes that in each generation, more offspring are born than can survive, leading to constant competition for resources. Individuals with favorable variations or traits have an advantage—greater fitness—increasing their likelihood of survival and reproduction. These traits are inherited and passed on, leading to an accumulation of successful variations over time, which can result in the formation of new species. Geographical isolation further contributes to new species formation as individuals adapt to different environments and selective pressures.
Evolution Is a Theory
In science, a theory is something that has been tested and shows robust evidentiary support. The theory of evolution has been extensively tested, subjected to verification through accumulated evidence, and has not been disproved. It is supported by a mounting body of genetic evidence, has stood the test of time, and continues to grow. It explains how life forms have changed over millions of years.
A Brief History of Evolutionary Thought
Evolutionary principles developed in western Europe, enabled by scientific thinking from the century, though Western science also drew ideas from Arab, Indian, and Chinese cultures where notions of biological evolution had already emerged. By the century, evolution was not a new concept, but natural selection was a novel theory. The Scientific Revolution, starting in the century, challenged traditional beliefs like Aristotle's earth-centered universe, exposing new biological diversity and prompting new ideas. Precursors like John Ray developed the concept of species, and Carolus Linnaeus created binomial nomenclature and a classification system (genus, species, class, order), although he still believed in the fixity of species. George-Louis Leclerc, Comte de Buffon, recognized the dynamic relationship between the environment and living forms, suggesting alterations in climate could be agents of change. Erasmus Darwin, Charles Darwin's grandfather, hypothesized that life originated in the seas and all species descended from a common ancestor. These ideas laid the groundwork for a comprehensive theory of evolution.
Key Scientists and Their Contributions
Jean-Baptiste Lamarck: Developed the theory of inheritance of acquired characteristics (or "use-disuse theory"). He suggested a dynamic relationship between species and the environment, where characteristics acquired during an organism's life (e.g., giraffe neck elongation from stretching) could be passed to offspring. While ultimately disproven in its mechanism, it was an important step in recognizing environmental influence on biological change.
Charles Darwin: A naturalist on HMS Beagle, his ideas were shaped by observations of biological variation, especially in places like the Galápagos Islands. Influenced by Thomas Malthus's ideas on population growth outpacing resources and Charles Lyell's concept of uniformitarianism, Darwin recognized that nature acted as the selective agent on biological variation. He noted that sexual reproduction increased variation. His observations led him to meticulously formulate the theory of natural selection, explaining that individuals with advantageous, inherited traits are more likely to survive and reproduce, passing those traits to the next generation, leading to evolutionary change.
Alfred Russel Wallace: Independently developed a very similar theory of natural selection. In , he sent Darwin an article outlining his ideas. This prompted Darwin, who had been working on his theory for years, to quickly publish his seminal work, On the Origin of Species, solidifying the theory of evolution by natural selection. Wallace's independent discovery underscored the validity of the concept and remains a cornerstone of evolutionary biology.