Chapter 23 Microevolution Mechanisms Microevolution the change in the frequency of alleles of a population over time I 1 Unit of evolution the population a Group of individuals of the same species that live in a given area b Ability to mate often somewhat separated from others c Tailings of an abandoned mine rich in heavy metals toxic conditions to most plants i Many seeds are blown to this toxic site ii Only individuals that have mutation to tolerate the metals will grow iii Individual plants cannot evolve resistance 2 Modern synthesis how populations evolve a Combination of Darwin and Mendel s work as well as paleontology taxonomy biogeography and population genetics i Populations as the units of evolution ii Natural selection as the most important mechanism of evolution iii Gradualism small changes over long periods of time can result in large changes 3 Gene pool a population a Total genes in a population at any one time all alleles at all gene loci in all individuals of i Diploid species each individual has two of every gene heterozygous or ii Total number of alleles in the population equals total number of individuals homozygous times 2 b Frequency of alleles 0 1 i Allele is fixed Frequency 1 no variation all individuals are homozygous for the same allele ii More than one possible allele 1 Frequency of each allele 1 but must equal 1 in total iii Example gene that codes for earlobe attachment 1 2 possible alleles 2 F free lobes f attached lobes 3 Population of 500 individuals 1000 total alleles c Calculating the frequency of alleles i 455 individuals have Free earlobes phenotype 1 245 with FF 210 with Ff 2 45 individuals have attached earlobes ff ii Frequencies 1 of allele F homozygotes times 2 of heterozygotes 2 245 times 2 210 3 700 4 Frequency is 700 1000 d To calculate the frequency of allele f you can just subtract 1 frequency of allele F 4 Hardy Weinberg Theorem a Describes the gene pool of a non evolving population b Ratio of genotypes in a population s gene pool should stay stable generation to c generation unless influenced by a mechanism of evolution Shuffling of alleles in meiosis should not affect the frequency of alleles in a population that mates randomly i Each gamete has 1 copy of the allele for ear lobe attachment ii Gamete chosen at random would have a 70 chance of having the F allele and 30 chance of having the f allele iii Chance of getting two gametes with F FF would be 7 times 7 49 a Can find the of the human population that carries the recessive allele for a particular d If random mating i Chance of getting Ff 21 21 5 Applying the Hardy Weinberg Equation inherited disease PKU in this case b Equation P 2 2pq q 2 1 c 1 in 10000 q 2 0001 take square root p 99 d Frequency of dominant normal allele i P 99 q 01 ii Frequency of carriers heterozygotes 1 2pq 0198 2 Close to 2 of the population 6 Assumptions of the Hardy Weinberg Theorem a Very large population size no genetic drift b No migrations no gene flow c No net mutations d Random mating no sexual selection e No natural selection reproductive success is not affect by genotype i NO EVOLUTION 7 Mechanisms of Evolution a Causes a change in the frequency of alleles in a population over time If frequency changed from 70 to 71 i ii Any change from Hardy Weinberg frequencies shows that evolution has occurred b What can cause this change i Genetic drift gene flow mutation and natural selection 8 Mutation change in the DNA a Creates new alleles i Error during DNA replication ii Error during Cell division b Mutation at one locus does not have a great effect on allelic frequencies of a large c Mutation can be important if new allele increases in frequency due to natural selection population or genetic drift i Very important in the long run as it is the original source of genetic variation 9 Genetic Drift change in the frequency of alleles in a population over time due to chance events sampling errors that occur when populations are small in size a Example tossing a coin 10 times vs 1000 times 10 How genetic drift works a In any population only some individuals will contribute their genes to the next generation mate b The genetic makeup of the next generation is dependent of those individuals that reproduced in the prior generation c Change in the frequency of alleles is NOT due to the adaptive quality of these alleles not related to fitness Change is DUE TO CHANCE d Genetic drift typically decreases the variation in a population i Small wildflower population ten plants ii Alleles could be eliminated by genetic drift 11 Population bottleneck a Severe population reduction can create severe genetic drift genetic bottleneck b Some alleles from population may be lost entirely Northern elephant seals cheetahs in wild face grim future due to loss of genetic variability c Human example i Complete achromatopsia total color blindness 1 Differs from normal color blindness cannot see colors at all lack the sharp vision most people use to read eyes are overwhelmed by sunlight 2 The island of Pingelap is part of the Federated States of Micronesia 3 000 Pingelapese have the condition 3 Traced back to one man who was one of many survivors of a typhoon that struck the island 12 Founder Effect limited genetic variability if population founded by a small number of individuals a Few individuals colonize a new island gene pool of the new population on the island is based on the alleles found in the few founders 13 Gene Flow change in the frequency of alleles in a population over time due to migration followed by mating move to a pre existing population very common in nature a Gene flow generally increases the genetic diversity of a population b Gene flow can counteract genetic drift c It can reduce differences between populations limit local adaptations 14 Natural Selection a Change in the frequency of alleles in a population over time due to differential reproductive success of individuals with heritable traits b Only mechanism of evolution that adapts populations to their environment c Alleles conferring adaptive traits increase in frequency in future generations 15 Darwinian fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals Lifespan means nothing if an individual fails to reproduce successfully a b Directional forward disruptive dent in middle stabilizing mountain 16 Disruptive selection a Two distinct bill types are present in black bellied