Detailed Study for Chps 22-53

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University of Toronto Scarborough
Biological Sciences
Kamini Persaud

Chp. 22 The Mechanisms of Evolution Charles Darwin partook in a 5 year long voyage in the 19 century on the H.M.S. Beagle which traveled around the world o The Archipelago Galapagos Islands was where he gathered his most groundbreaking evidence of natural selection FINCHES o He postulated that some animals had come to the archipelago from mainland South America and then undergone different changes on each of the islands Darwins theory for evolutionary change: o Species are not immutable; they change over time o The process that produces these changes is natural selection Darwin, with the influence of economist Thomas Malthus, postulated that populations of all species have the potential for rapid increase in numbers Without high death rates, even the most slowly reproducing species would quickly reach enormous population sizes Darwin suggested that slight variations among individuals affect the chance that a given individual will survive and reproduce Natural selection (formally defined): the differential contribution of offspring to the next generation by various genetic types belonging to the same population o Individuals do not evolve, POPULATIONS do o Population: a group of individuals of a single species that live and interbreed in a particular geographic area at the same time Adaption refers both to the processes by which characteristics that appear to be useful to their bearersevolve and to the characteristics themselves For a population to evolve, its members must possess heritable genetic variation o Character: a particular o Genotype: the genetic feature of the individuals coding for that trait phenotype o Phenotype: physical o Trait: a specific form of a appearance character A population evolves when individuals with different genotypes survive or reproduce at different rates If there were NO evolution in a population: the Hardy-Weinberg equation shows that allele frequencies will remain the same from generation to generation (genotypic frequencies can be predicted from allele frequencies) unless some mechanism acts to change them o The models conditions are never met = population deviate from Hardy- Weinberg equilibrium o Therefore, there are mechanisms acting to change allele frequencies, and these mechanisms drive evolution Known evolutionary mechanisms include: o Mutation o Non-random mating o Gene flow o Natural selection o Genetic drift Mutations o Origin of genetic variation o Change in an organisms DNA and appear to be random o Both create and help maintain genetic variation within populations o Rates are low with only 1 mutation/locus in a million zygotes; but are sufficient to create considerable variation because each of a large number of genes may mutate o Minor deviation from H-W equilibrium Gene Flow o Migration of individuals and movements of gametes between populations o Common because few populations are completely isolated from others o Add new alleles to gene pool of population or may change the frequencies of alleles already present Genetic Drift o Random changes in allele frequencies from one generation to the next o Great affect on small populations where genetic variation can be reduced and harmful alleles may increase o May also affect large populations but it wont alleles influencing survival and reproduction o Population bottleneck: large populations are reduced to small ones and genetic variation is thus decreased o Founder effect: where few pioneering individuals colonize a new region and because of the colonized populations small size, genetic variation is reduced Non-random Mating o Mating patterns may alter genotype frequencies if individuals in a population choose other individuals of particular genotypes as mates o Self-fertilization (selfing) is common among many groups of organisms, especially plants; reduces the frequency of heterozygous individuals from H-W equil. And increases the frequency of homozygous individuals from H-W equil.; BUT does NOT change allele frequencies and thus does not result in adaptation o Sexual selection DOES change allele frequencies and often results in adaptation Adaption occurs when some individuals in a population contribute more offspring to the next generation than others, such that allele frequencies in the population change in a way that adapts individuals to the environment that influenced such reproductive success Natural selections acts on the PHENOTYPE rather than acting directly on the genotype Fitness o Fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype Stabilizing Selection o Average characteristics are favoured o Reduces variation in populations o Does not change the mean o Natural selection frequently acts in this way, countering increases in variation brought about by sexual recombination, mutation, or migration Directional Selection o Favouring individuals that vary in one direction from the mean of the population o If this selection operates over many generations, an evolutionary trend within the population is seen o May be reversed when the environment changes and different phenotypes are favoured o May be halted when an optimum phenotype is reached or when trade- offs oppose further change Disruptive Selection o Favouring individuals that vary in opposite directions from the mean of the population
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