Microevolution.docx

Microevolution Process of Evolution by Natural Selection Variation in heritable traits within a population + Differential survival and/or reproduction= change in traits over time Population A localized group of individuals capable of interbreeding and producing fertile offspring Heritable Traits Traits with a genetic basis Genotype Genetic composition of an individual Phenotype Observable characteristics of an individual Alleles Alternative versions of a gene that can produce a different phenotype effect Variation in Alleles Variation in genotype can give ride to variation in phenotype Natural selection is an example in phenotype Microevolution Change in allele frequencies in a population from one generation to the next TIME 1=Aa (A:P=0.8, a:P=0.4)-----------------Time 2=Aa (A:P=0.6, a:P=0.4) Remember Natural selection acts on PHENOTYPE but evolutionary change ultimately implies change in GENOTYPE in a population Why is Genetic Variation Important? Variation is the raw material for evolution Environment (and thus selection pressure) is unlikely to remain constant More genetic variation, less likely to change in the environment has detrimental consequences for all members of a population Gene pool All of the alleles for all in a population Polymorphism When there are two or more versions of a trait in a population >2 alleles and locus of interest An allele becomes fixed if Entire population is homozygous at that locus No variation, no opportunity to adaptive evolution by natural selection Fixed: No variation Creating Genetic Variation Mutation Change in structure of a gene or chromosome Only way to generate novel alleles Changing allele frequencies Sex Combining existing alleles in new ways Meiosis Crossing over Random segregation of chromosomes Fertilization Four Basic Processes Cause Changes in Allele Frequencies Between Generations 1. Mutation New allele has joined gene pool Mutation rates relatively low Mutation is most important for generating variation rather than driving changes in allele frequency Occurrence of new mutation directly changes allele frequencies Relatively low mutation rates in most organisms 2. Genetic Drift ?Stuff happens? Change in allele frequencies between generations due to random (chance) events Tends to reduce genetic variation through the lose of alleles Level of adaptation Increase Decrease Or not change Especially important in small populations 100 adult Tigers A a A a 25% AA 25% aa 50% Aa 2 Cub Scenario A 0.5 a 0.5 Aa Time 1------AA Time 2 100 Cub Scenario Aa Time 1----25%AA, 50% Aa, 25%aa Genetic drift is always present to some degree Some events can lead to large amounts of drift Founder effect Tiger Nation is fixed Few individuals separated from a source population these individuals become founders in likely to reflect population Island animals Always present to some degree Especially in small amounts Some events can lead to large amounts of drift Bottleneck effect Large population experiences catastrophic event that dramatically reduces population size. Many individuals and possibly many alleles are lost 3. Gene Flow Change in allele frequencies between generations due to input of individuals or gametes (ex. pollen) from other populations Tends to reduce genetic differences between populations Can result in neighboring populations combining into single population with a single gene pool When neighboring populations live in different environments, alleles transferred by gene flow may prevent a population from fully adapting to it environment Some times beneficial alleles are transferred very widely More likely than mutation to alter allele frequencies directly Natural selection may then cause the new allele to increase in frequency or decrease in frequency 4. Natural Selection Three way selection can alter frequencies Directional selection Occurs when conditions favor individuals exhibiting one extreme of a phenotype range, thereby shifting the frequency curve for the phenotypic character in one direction or the other Common when a population?s environment changes or when members of a population migrate to a new habitat Disruptive/Diversifying Selection Occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes Stabilizing Selection Acts against both extreme phenotypes and favors intermediate variants Reduces variation and tends to maintain the status quo for a particular phenotypic character Natural Selection is the only one of the four processes of evolution that consistently has positive effects on the degree of adaptation within the target population Conservation Focus Why are we concerned about small populations Population sizes of other organisms may be small Small populations experience greater effects of drift In small populations, drift can actually become more important than selection Mutational meltdown Case Study Impact of genetic drift on the Greater Prairie Chicken Prairie Chicken habitat destroyed in much of Illinois, leads to greatly reduced population size Use DNA from museum specimens to quantify genetic variation Use data from field studies to compare reproduction Sexual Selection Natural selection that is related to meeting success Survival isn?t enough-genes must get into the next generation Ability to locate mates Courtship behaviors that attempt to convince mates of an individuals suitability Ornaments that presumably play some function in increasing your attractiveness Darwin distinguished two types of sexual selection Intrasexual selection Acts on traits that affect success in competition with members of the same sex for mates Intersexual selection (mate choice) Acts on traits that affects success in being chosen for mating by the opposite sex Sexual dimorphism Different sizes, colors, etc. between sexes Can experience an opposition to standard survival-based natural selection Why is Evolution by Natural Selection Unlikely to Create Perfect Organisms Selection can act only on existing variation Needs genetic variation as raw material Selection is blind to the future Unlikely to fashion a current solution to a future problem Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection and the environment interact Evolution is ultimately a function of drift, mutation, gene flow, and selection Evolution takes time Evolutionary trap Evolutionary Trap Using an old cue in current environment, results in a maladaptive choice KEY CONCEPTS Why is genetic variation important? Variation is the raw material for evolution, and the environment is unlikely to ever stay constant What are the four basic processes that can cause changes in allele frequencies between generations? How do these processes differ? Mutation, genetic drift, gene flow, and natural selection Explain why natural selection is the only mechanism that consistently produces adaptive changes. Natural selection alters allele frequency by keeping the good traits and enhancing them in organisms Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection. Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotype range, thereby shifting the frequency curve for the phenotypic character in one direction or the other. Disruptive selection occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes. Stabilizing selection acts against both extreme phenotypes and favors intermediate variants. Intrasexual is sexual selection that occurs when members of the same species must compete for a mate. Intersexual selection is sexual selection based on traits given to organisms to attract mates. List four reasons why natural selection cannot produce perfect organisms. Natural selection is blind to the future Selection can only act on existing variation Adaptations are often compromises The environment changes Why is genetic drift important for conservation biology? Genetic drift is always present and effects small populations a lot. Genetic drift tends to reduce genetic variation through the lose of alleles. What is sexual selection? What are the two types of sexual selection? Sexual selection is natural selection that is related to mating success. AA Aa Aa aa