Chapter 24 Notes Evolutionary Processes Population?group of individuals from the same species that live in the same area and regularly interbreed Four mechanisms that shift allele frequencies in populations Natural selection?frequency of certain alleles?the ones that contribute to improved reproductive success Genetic drift?causes allele frequencies to change randomly Gene flow occurs when individuals immigrate or emigrate a populations Mutation?modifies allele frequencies by continually introducing new alleles Analyzing Change in Allele Frequencies: the Hardy-Weinberg Principle Hardy and Weinberg wanted to know what happened in an entire population when all individuals bred Gene pool?single collective bin of gametes of a population In two allele population, three genotypes are possible Frequency of A1A1: p (squared) Frequency of A?A?: 2pq Frequency of A?A?: q (squared) Hardy Weinberg principle?page 518!! Assumptions made by model-5 conditions must be met for equation to work No natural selection at the gene in question. No genetic drift, or random allele frequency changes No gene flow No mutation of genes Random mating with respect to the gene in question Null hypothesis?specifies what a researcher should observe when the hypothesis being tested is wrong H-W principle tells what frequencies will be when natural selection is not affecting the gene Scientists can determine whi MN Blood Types in Humans Gene codes for a surface protein of red blood cells After experiment, the frequencies observed were extremely accurate HLA Genes in Humans Important in human immune system?code for proteins that help immune system cells recognize and destroy invading bacteria and viruses HLA-A and HLA-B Codominant?both are expressed and create the phenotype Experiment showed that Mating may not be random with respect to the HLA genotype Heterozygous individuals may have higher fitness Natural Selection and Sexual Selection Heterozygote advantage?when heterozygous individuals have higher fitness than homozygous individuals do, natural selection maintains genetic variation in populations Genetic variation?refers to the number and relative frequency of alleles that are present in a particular population Lack of genetic variation is usually a bad thing Directional Selection?allele frequencies change in one direction Works well when trait in question has a bell-shaped population curve Stabilizing Selection Reduces the amount of variation in a trait Caused when directional selection on a trait is counterbalanced by a different factor that causes selection in the opposite direction Disruptive Selection Eliminates phenotypes near the average value and favors extreme phenotypes Example?black-bellied seedcrackers Disruptive selection can lead to speciation Speciation?the formation of new species Sexual Selection Mechanism of evolutionary change Individuals within a population differ in their ability to attract mates Normally found to act more on males than females?called fundamental asymmetry of sex Because eggs are large and energetically expensive, females produce small amounts of them over their lifetime Males can produce a nearly limitless amount of sperm over their lifetime due to its ability to be easily produced Sexual selection via Female Choice Great deal of time and energy Because of this, female organisms have a tendency to mate with males that are well fed and in good health Male v. Male Competition Territory?area actively defended; provides exclusive use by the owner Males that establish territories mate with the females within their territory SEXUAL SELECTION IS MUCH MORE INTENSE IN MALES THAN IN FEMALES! Sexual dimorphism?any trait that differs between males and females Genetic Drift Any change in allele frequencies that are due to chance in a population. Sampling error?allele frequencies changing due to blind luck during genetic drift Important points Genetic drift is random with respect to fitness- allele frequencies are not adaptive Genetic drift is most pronounced in small populations Over time, genetic drift can lead to the random loss or fixation of alleles Experimental Studies Hair on fruit fly legs Genetic Drift in Natural Populations Sampling error is high in small populations Genetic drift is caused by sampling error?any process that involves sampling can cause genetic drift! Founder effects and drift Founder event?when a group of individuals emigrate to a new geographic area and found a new population Change in allele frequencies when a founder event occurs Population bottleneck If a large population experiences a sudden reduction in size, bottleneck occurs. Most normally caused by floods, tires, storms or other disasters Genetic bottleneck?follows population bottleneck?sudden reduction in the number of alleles in a population Can cause a change in frequencies Achromatopsia on Pingelap Atoll?proves that bottleneck population caused change in frequencies, and frequencies may not always be geared in advancing natural selection Gene flow Movement of alleles from one population to another Occurs when individuals leave one population, join another, and breed Movement of alleles between populations always tends to reduce genetic differences between them. Mutation Mutation?evolutionary mechanisms lead to a loss or gain of genetic diversity Increases diversity in populations Mutations are a random process Most mutations result in deleterious alleles?alleles that lower fitness Mutation as an Evolutionary Mechanism Mutation is not a significant mechanism of evolutionary change by itself If one mutation occurred in a large population, it would take a significant amount of time for that new frequency to grow in percentage Role of mutation in Evolutionary change E. Coli experiment by Richard Lenski and colleagues Cells had beneficial mutation which grew rapidly and came to dominate the population Mutation is the ultimate source of variation Inbreeding Mating between relatives Violates the assumptions of the Hardy-Weinberg principle Self-fertilization is the most extreme form of inbreeding Increases frequency of homozygotes and decreases frequency of heterozygotes?leads to inbreeding depression?population becomes less fit?results from two processes Many recessive alleles represent loss of function mutations Many genes are under intense selection for heterozygote advantage As a result, offspring are expected to have lower fitness Inbreeding affects all genes; sexual selection affects only genes involved in competition for mates
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