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Download Lecture Ch 23 The evolution of populations
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CH 23: The Evolution of Populations I. Population Genetics A. Relates genetics to evolution in various populations populations Modern Synthesis- importance of populations as units of evolution with natural selection as the main mechanism. Emphasizes gradualism to explain how large changes occurred by smaller changes over time but that idea is continually being challenged. B. Populations 1. Population- localized group of individuals belonging to the same species 2. species-individuals belong to the same species when they have the potential to interbreed and produce fertile offspring in nature 3. gene pool-total aggregate of genes in a population at a given time/this is what natural selection acts on at that time A. 4. microevolution – a generation to generation change in a population’s frequencies al alleles or genotypes-a change in a population’s genetic structure II. Hardy-Weinberg Theorem (nonevolving) states that the frequencies of alleles and genotypes in a population’s gene pool remain constant over generations unless acted on by agents other than sexual recombination (meiosis/random fertilization) A. Conditions needed to maintain equilibrium a. the population size must be relatively large-if not, genetic drift will occur b. the population must be isolated from other populations-if not gene flow will begin c. there must be no mutations of genes d. mating must be random-if not, certain traits will be passes vs others e. natural selection must not be present 2. Vocabulary and Equations 1. p = frequency of dominant alleles q = frequency of recessive alleles p2 + 2pq + q2 = 1 2 2. p (AA) both alleles are homozygous dominant. (Remember you get two alleles- one from mom and one from dad) 3. 2pq (Aa or aA)– one allele is dominant and one is recessive. (Multiply by 2 because you could get p from mom and q from dad or p from dad and q from mom so two variations) 4. q2 (aa) both alleles are homozygous recessive 5. PRACTICE YOUR H-W PROBLEMS II. Microevolution A. Causes of microevolution 1. genetic drift- changes in the gene pool of a small population due to chance if you flip a coin 1000 times and 700=heads, 300=tails, suspicious if flip a coin 10 times and 7=heads, 3=tails, think chance a small number of samples (trials) allows chance to play a more important role Therefore, small populations offset H-W equilibrium ( large populations are best to show if evolution is occurring based on the H-W equilibirum) 2. bottleneck effect- can cause genetic drift by some disaster (natural or human-caused) unselectively killing individuals The surviving population is not representative of the original population, therefore it’s like genetic drift. 3. founder effect- can cause genetic drift by a few individuals colonizing a new isolated island, bringing in small amounts of genetic variation 4. gene flow- genetic exchange due to the migration of fertile individuals or gametes between populations. Tends to reduce differences between populations due to genetic drift or natural selection and can make them one population again with a common genetic make-up 5. Mutations- mutations of genes may affect p and q frequencies Although mutations are a rare event, if the mutant form is being passed on more frequently, genetic frequencies will move away from the H-W equilibrium. Mutations usually harm an organism, but can be beneficial when the environment is changing. Mutations’ effects can be seen in faster reproducing species (bacteria, insects) 6. Nonrandom mating a. Inbreeding-mating between closely-related partners b. assortative mating-individuals select partners that are like themselves and will cause genetic frequency to move away from the H-W equilibirium 7. Natural selection- H-W equilibrium requires that all individuals within a population have an equal chance of passing on genes to offspring, but natural selection doesn’t make this so III. Genetic variation-raw material for natural selection to work on which then causes evolution A. polymorphism-when a population has two or more distinct morphs (discrete traits like red and white flower petals) at equally high frequencies to be noticeable Ex. spotted and striped snakes B. geographical variation-variation between populations due to differences in the environment a. cline-a graded change of some trait along a geographical axis ex. the average body size of several North American bird and mammal species increase gradually with increasing latitude C. Mutation and sexual recombination- Sexual recombination is more important than mutation on generation to generation variation which then makes evolution occur. Recombination occurs during meiosis. D. Diploidy and balanced polymorphism makes sure variation lives on 1. diploid-each individual have two types of alleles for a gene (one from each parent) 2. ensure that even recessive alleles are maintained within the gene pool E. Balanced polymorphism-ability of natural selection to maintain genetic variability a. heterozygote advantage-if heterozygotes are most successful, two or more alleles will remain in the gene pool Ex. sickle-cell anemia, if homozygous recessive, organisms die due to lack of O2, if homozygous dominant, they die if bit by mosquitoes carrying malaria, if heterozygous, organisms live since they have only a few abnormal red blood cells and have immunity to malaria IV. Natural Selection as a Mechanism A. Evolutionary Fitness- the relative contribution an individual makes to the gene pool of the next generation based on what genotypes are. The relative fitness of an individual is based not only on one allele, but how that alleles works with all the alleles in the individual Ex an allele for high tree trunk growth rate doesn’t help if the alleles for root support are lacking B. Modes of natural selection - natural selection can affect phenotype frequency in 3 ways 1. stabilizing selection-acts against extreme phenotypes Ex. human baby weights 2. directional selection-shifts the frequency curve toward a certain phenotype due to environment at that time 3. disruptive selection-favors extreme phenotypes C. sexual dimorphism-Ex male peacocks very beautiful while females are less colorfulmales and females look very different. Sexual selection leads to sexual dimorphism where females choose the more ornamental males or the ones with the best nests D. Natural Selection doesn’t produce “perfect” creatures 1. organisms are locked into historical constraints- adaptations are usually compromiseswhile some adaptations may be beneficial, they may take away from other adaptations that are also. Ex. seals would move better on rocks with legs, but less efficient in water due to lack of flippers 2. not all evolution is adaptive-genetic drift will cause evolution due to chance not adaptation 3. selection can only edit variations that exist now, not what might happen in the future.