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BIOLOGY 1A03 (699)


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McMaster University
Grant B Mc Clelland

Chapter 22 The Mechanisms of Evolution221 What Facts Form the Base of Our Understanding of EvolutionDarwin developed the major features of an explanatory theory for evolutionary change based on two major propositionsoSpecies are not immutable they change over timeoThe process that produces these changes is natural selectionDarwin also observed that although offspring ten to resemble their parents the offspring of most organisms are not identical to one another or to their parentsHe suggests that slight variations among individuals affect the chances that a given individual will survive and reproducenatural selection differential contribution of offspring to the next generation by various genetic types belonging to the same populationIndividuals do no evolve populations doA 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 meaningsRefers both to the processes by which characteristic that appear to be useful to their bearers evolve and to the characteristics themselvesWith respect to characteristics an adaptation is a phenotypic characteristic that has help an organism adjust to conditions in its environmentPopulation genetics provides an underpinning for Darwins TheoryWe cannot directly observe the genetic composition of organisms what we do see in nature are phenotypes the physical expression of organism genesThe features of a genotype are its characters eg eye colourThe specific form of a character eg brown eyes is a traitA heritable trait is a characteristic of an organism that is at least partly determined by its genesA population evolves when its individuals with different genotypes survive or reproduce at different ratesPopulation genetics has three main goalsoTo explain the origin and maintenance of genetic variationoTo explain the patterns and organization of genetic variationoTo understand the mechanism that cause changes in allele frequencies in populationsDifferent forms of a gene alleles 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 belongsThe sum of all copies of all alleles at all loci found in a population constitutes its gene poolThe gene pool contains the genetic variations that produce the phenotypic traits on which natural selection actsMost populations are genetically variableNearly all populations have genetic variation for many charactersEvolutionary change can be measure by allele and genotype frequenciesAllele frequencies are usually estimated in locally interbreeding groups Mendelian populations within a geographic population of a species
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