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Chapter 22

Chapter 22 Study Guide

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Biological Sciences

Chapter 22 The Mechanism of Evolution Snake eats poisonous newt – and lives! -some amphibians have evolved chemical defence against predation, becoming poisonous -tetrodotoxin (TTX) is a neurotoxin that paralyzes nerves and muscles by blocking sodium channels -some snakes have TTX-resistant sodium channels in nerves and muscles -TTX-resistant snakes move slowly than TTX non-resistant snake after eating a newt -more vulnerable to their own predators than are TTX-sensitive snakes -production and resistance to defensive chemicals are evolutionary adaptations -improvement of performance in one area is reduced performance in some other area, known as trade-off 22.1 What Facts Form the Base of Our Understanding of Evolution? -Darwin found that species were more similar in regions close together than regions that have the same climate -species on the island were not found in anywhere else, but similar to the surrounding main lands -proposed explanatory theory for evolutionary change 1. species are not immutable, they change over time 2. process that produces these changes is natural selection -populations of all species have the potential for rapid increase in numbers, but death rate is also high in nature -natural selection is differential contribution of offspring to the next generation by various genetic types belonging to the same population -artificial selection is breeding between individuals with desirable traits by breeders -individuals do not evolve, populations do -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 of the development of useful characteristics and the characteristics themselves -process is the evolutionary mechanisms that produce the characteristics -characteristic is an adaptation that is a phenotypic characteristic that has helped an organism adjust to conditions in its environment -adaptation to a particular environment is when organism reproduces and survive less well in that environment Population genetics provides an underpinning for Darwin’s theory -heritable trait is a characteristic that is at least partly determined by its genes -population evolves when individuals with different genotypes survive or reproduce at different rates -Mendel developed population genetics -has 3 main goals: www.notesolution.com 1. to explain origin and maintenance of genetic variation 2. to explain patterns and organization of genetic variation 3. to understand mechanism that cause changes in allele frequencies in population -alleles are different forms of gene, exist at a particular locus -gene pool is the sum of all copies of all alleles at all loci found in a population Most populations are genetically variable -genetic variation leads to offspring with different phenotype -dominant traits are expressed more that recessive trait Evolutionary change can be measure by allele and genotype frequencies -Mendelian population are locally interbreeding groups that are used to estimate allele frequencies -every allele at every locus in individual is counted to measure allele frequency -frequency is proportion of the gene at the locus -sum of all allele frequencies at a locus is 1 -measure of allele frequency range from 0 to 1 -frequency = number of copies of allele in population/ sum of alleles in population -polymorphic at a locus is when there is more than one allele -Let NAA be the number of individuals that are homozygous for A allele (AA) -Let NAa be the number that are heterozygous (Aa) -Let Naa be the number that are homozygous for the a allele (aa) -NAA + NAa + Naa = N, the total number of individuals in the population -2N is the number of copies of both alleles in the population because each individual is diploid -p represents frequency of A, p = (2NAA + NAa)/ 2N -q represents frequency of a, q = (2Naa + NAa)/ 2N -monomorphic at the locus is when there is only one allele at the locus -allele frequency is 1 and the allele is said to be fixed -genetic structure is the combination of allele frequency and genotype frequency -allele frequencies measure the amount of genetic variation in a population -genotype frequency shows show a population’s genetic variation is distributed among its members The genetic structure of a population does not change over time if certain conditions exist -dominant alleles do not replace recessive alleles -if an allele is not advantageous, its frequency remains constant from generation to generation (Hardy-Weinberg equilibrium) -equation explains allele frequencies do not change across generation and genotype frequencies can be predicted allele frequencies -equilibrium applies to sexually reproducing organisms -some condition must be met: 1. Mating is random 2. Population size is infinite -small population may exhibit genetic drift (random chance) fluctuations in allele frequencies www.notesolution.com 3. no gene flow, no migration into or out of the population 4. no mutation, no change to allele and no new alleles are added to gene pool 5. natural selection does not affect survival of particular genotypes, no differential survival of individuals with different genotypes -Hardy-Weinberg equation: p2 + 2pq + q2 = 1 Deviations from hardy-Weinberg equilibrium show that evolution is occurring -equation is useful to predict the approximate genotype frequencies of population from allel frequencies -describe conditions that would result if there were no evolution What Are the Mechanism of Evolutionary Change -Hardy-Weinberg equilibrium is a null hypothesis -evolutionary mechanisms include mutation, gene flow Mutations generate genetic variation -mutation is change in DNA -most mutations are harmful and neutral -restore to populations alleles that other evolutionary processes have removed -help maintain genetic variation -mutation rate is low for most loci Gene flow may change allele frequencies -gene flow is migration and movement of gametes between populations -new alleles are added to gene pool -change allele frequencies of the population Genetic drift may cause large changes is small populations -genetic drift is random changes in allele frequency that produce large changes in allele frequencies from one generation to the next -harmful alleles increase in frequency and rare advantageous alleles are lost -population suffers large losses of members -population bottleneck is when only a small number of individuals survive -it is a genetic drift that leads to reduction in genetic variation -allele frequency of dominant/ recessive allele has a higher frequency and the other has a lower frequency than previous generation -population loses genetic diversity -founder effect is the change in genetic variation when pioneering individuals colonize a new region -small colonizing population does not have all the alleles from its source population -resulting change in genetic variation is equivalent to that of population bottle neck -population in the new area has less chromosomal inversion -lower allele diversity at certain enzyme-producing genes than the original population www.notesolution.com Nonrandom mating changes genotype frequencies -mating pattern may alter genotype frequencies -nonrandom mating is when individuals in a population choose other individuals of particular genotypes as mates -self-fertilization is a non-random mating, common form in plants -reduce frequencies of heterozygous individual -alleles frequencies are not changed so does not result in adaptation -sexual selection is a non-random mating that changes allele frequencies and results in adaptation What Evolutionary mechanisms Result in Adaptation? -adaptation occurs when some individuals in a population contribute more offspring to the next generation -allele frequencies change in a way that adapts individuals tot eh environment that influence such reproductive success -natura
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