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Chapter 23 - CHAPTER 23 THE EVOLUTION OF POPULATIONS...

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CHAPTER 23 - THE EVOLUTION OF POPULATIONS Introduction One obstacle to understanding evolution is the common misconception that organisms evolve, in a Darwinian sense, in their lifetimes. Natural selection does act on individuals by impacting their chances of survival and their reproductive success. However, the evolutionary impact of natural selection is only apparent in tracking how a population of organisms changes over time. It is the population, not its individual, that evolves. Evolution on the scale of populations, called microevolution, is defined as a change in the allele frequencies in a population. For example, the bent grass ( Argrostis tenuis ) in this photo is growing on the tailings of an abandoned mine, rich in toxic heavy metals. While many seeds land on the mine tailings each year, the only plants that germinate, grow, and reproduce are those that had already inherited genes enabling them to tolerate metallic soils. Individual plants do not evolve to become more metal-tolerant during their lifetimes. A. Population Genetics The Origin of Species convinced most biologists that species are the products of evolution, but acceptance of natural selection as the main mechanism of evolution was more difficult. What was missing in Darwin’s explanation was an understanding of inheritance that could explain how chance variations arise in a population while also accounting for the precise transmission of these variations from parents to offspring. Although Gregor Mendel and Charles Darwin were contemporaries, Mendel’s discoveries were unappreciated at the time, even though his principles of heredity would have given credibility to natural selection. 1. The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance When Mendel’s research was rediscovered in the early twentieth century, many geneticists believed that the laws of inheritance conflicted with Darwin’s theory of natural selection. Darwin emphasized quantitative characters, those that vary along a continuum. These characters are influenced by multiple loci. Mendel and later geneticists investigated discrete “either-or” traits. An important turning point for evolutionary theory was the birth of population genetics , which emphasizes the extensive genetic variation within populations and recognizes the importance of quantitative characters. Advances in population genetics in the 1930s allowed the perspectives of Mendelism and Darwinism to be reconciled. This provided a genetic basis for variation and natural selection. A comprehensive theory of evolution, the modern synthesis , took form in the early 1940s. It integrated discoveries and ideas from paleontology, taxonomy, biogeography, and population genetics.
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