One common misconception about evolution is that organisms evolve, in a Darwinian sense, during their lifetimes. Natural selection does act on individuals. Each individual's combination of inherited traits affects its survival and its reproductive success relative to other individuals in the population. However, the evolutionary impact of natural selection is only apparent in the changes in a population of organisms over time. It is the population, not the individual, that evolves. Consider the example of bent grass (Agrostis tenuis) growing on the tailings of an abandoned mine. These tailings are rich in toxic heavy metals. While many bent grass seeds land on the mine tailings each year, the only plants that germinate, grow, and reproduce are those that possess genes enabling them to tolerate metallic soils. These plants tend to produce metal-tolerant offspring. Individual plants do not evolve to become more metal-tolerant during their lifetimes. Population genetics provides a foundation for studying evolution Darwin proposed a mechanism for change in species over time. What was missing from 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. The widely accepted hypothesis of the time—that the traits of parents are blended in their offspring—would eliminate the differences in individuals over time. Just a few years after Darwin published On the Origin of Species, Gregor Mendel proposed a model of inheritance that supported Darwin's theory. Mendel's particulate hypothesis of inheritance stated that parents pass on discrete heritable units (genes) that retain their identities in offspring. Although Gregor Mendel and Charles Darwin were contemporaries, Darwin never saw Mendel's paper, and its implications were not understood by the few scientists who did read it at the time. Mendel's contribution to evolutionary theory was not appreciated until half a century later. The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance. When Mendel's research was rediscovered in the early 20th century, many geneticists believed that his 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. It was not obvious that there was a genetic basis to quantitative characters. Within a few decades, geneticists determined that quantitative characters are influenced by multiple genetic loci and that the alleles at each locus follow Mendelian laws of inheritance. These discoveries helped reconcile Darwin's and Mendel's ideas and led to the birth of population genetics, the study of how populations change genetically over time. 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. The first architects of the modern synthesis included statistician R. A. Fisher, who demonstrated the rules by which Mendelian characters are inherited, and biologist J. B. S. Haldane, who explored the rules of natural selection. Later contributors included geneticists Theodosius Dobzhansky and Sewall Wright, biogeographer and taxonomist Ernst Mayr, paleontologist George Gaylord Simpson, and botanist G. Ledyard Stebbins. The modern synthesis emphasizes: The importance of populations as the units of evolution. The central role of natural selection as the most important mechanism of adaptive evolution. The idea of gradualism to explain how large changes can evolve as an accumulation of small changes over long periods of time. While many evolutionary biologists are now challenging some of the assumptions of the modern synthesis, it has shaped our ideas about how populations evolve. A population's gene pool is defined by its allele frequencies. A population is a localized group of individuals that belong to the same species. One definition of a species is a group of natural populations whose individuals have the potential to interbreed and produce fertile offspring. Populations of a species may be isolated from each other and rarely exchange genetic material. Members of a population are far more likely to breed with members of the same population than with members of other populations. Individuals near the population's center are, on average, more closely related to one another than to members of other populations. The total aggregate of genes in a population at any one time is called the population's gene pool. It consists of all alleles at all gene loci in all individuals of a population.

• published: 28 Jan 2014
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