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BLG 230

Chapter 22: The Mechanisms of Evolution 22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin went on a 5-year-long voyage and made a lot of observations. When he returned to England, in 1836, he tried to understand and analyze these observations. He developed the major features of an explanatory theory for evolutionary change based on two major propositions: • Species are not immutable; they change over time. • The process that produces these changes is natural selection. Darwin observed that although offspring tend to resemble their parents, the offspring of most organisms are not identical to one another or to their parents. He suggests slight variations among individuals affect the chance that a given individuals will survive and reproduce  natural selection: differential contribution of offspring to the next generation by various genetic types belonging to the same population. It is important to remember that individuals do not evolve; populations do. A population is a group of individuals of a single species that live and interbreed in a particular geographic area at the same time. • Adaptation has two meanings Adaptation refers both to the processes by which characteristics that appear to be useful to their bearers evolve and to the characteristics themselves. In other words, an adaptation is a phenotypic characteristic that has helped an organism adjust to conditions in its environment. An organism is adapted to a particular environment when they can demonstrate that a slightly different organism reproduces and survives less well in that environment. • Population genetics provides an underpinning for Darwin’s theory For a population to evolve, its members must possess heritable genetic variation. The physical expressions of an organism’s genes are what one sees. The features of a phenotype are its characters (e.g. eye colour). The specific form of a character (e.g. brown eyes) is a trait. A heritable trait is a characteristic of an organism that is at least partly determined by its genes. The genetic constitution that governs a character is called a genotype. A population evolves when individuals with different genotypes survive or reproduce at different rates. Gregor Mendel’s publications paved the way for the development of the field of population genetics. It has three main goals: 1. To explain the origin and maintenance of genetic variation 2. To explain the patterns and organization of genetic variation 3. To understand the mechanisms that cause changes in allele frequencies in populations Different forms of a gene are called alleles and may exist at a particular locus. At any particular locus, a single individual has only some of the alleles found in the population to which it belongs. The sum of all copies of all alleles at all loci found in a population makes up the gene pool; which produces the phenotypic traits on which natural selection acts. • Most populations are genetically variable Nearly all populations have genetic variation for many characters. The study of the genetic basis of natural selection is difficult because genotypes alone do not uniquely determine all phenotype
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