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Grant B Mc Clelland

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Chapter 22 The Mechanisms of Evolution221 What Facts Form the Base of Our Understanding of Evolution Darwin went on a 5yearlong 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 propositionsSpecies are not immutable they change over timeThe process that produces these changes is natural selectionDarwin 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 reproducenatural selection differential contribution of offspring to the next generation by various genetic types belonging to the same populationIt 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 timeAdaptation has two meaningsAdaptation 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 environmentPopulation genetics provides an underpinning for Darwins theory For a population to evolve its members must possess heritable genetic variation The physical expressions of an organisms genes are what one sees The features of a phenotype are its characters eg eye colour The specific form of a character eg 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 Mendels publications paved the way for the development of the field of population genetics It has three main goals1To explain the origin and maintenance of genetic variation2To explain the patterns and organization of genetic variation3To understand the mechanisms that cause changes in allele frequencies in populationsDifferent forms of a gene are called alleles and may exist at a particular locusAt 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 which produces the phenotypic traits on which gene pool natural selection actsMost populations are genetically variableNearly 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 phenotypes Different phenotypes can be produced by a given genotype depending on the environment encountered during developmentEvolutionary change can be measured by allele and genotype frequenciesAllele frequencies are usually estimated in locally interbreeding groups within a geographic populationMendelian populationAn alleles frequency is calculated using the following formulapopulation in the alleletheof copies ofnumber Refer to ppopulation in the alleles of sumPage 491492A locally interbreeding group within a geographic population is called a Mendelian population
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