EVOLUTION: GENES AND POPULATIONS CH 23 http://evolution.berkeley.edu/evolibrary/ news/070401_lactose Organisms DO NOT evolve. Natural selection acts on individuals causing populations to evolve Microevolution: changes in allele frequencies over time. Caused by: Natural selection Genetic drift Gene flow Only natural selection causes adaptive evolution I. Genetic Variation and Evolution In order for evolution to occur, there must be genetic variation in a population A. Where does genetic variation come from? New genes and alleles come from mutation and gene duplication Gene recombination during meiosis: reshuffles alleles during sexual reproduction Mutation is most important in asexually reproducing organisms while gene recombinations are more important in sexually reproducing organisms B. Hardy Weinberg Equilibrium and Allele Frequencies 1. The Hardy Weinberg equilibrium Population: a group of actively interbreeding members of a species Gene pool: all of the alleles present in a population The Hardy-Weinberg principle states that allele frequencies in a population will remain unaltered from generation to generation IF the following criteria are met: Extremely large population No mutation No selection No gene flow (migration) Mating is random 2. Using Hardy Weinberg to calculate allele frequencies p2 + 2pq +q2 = 1 p + q =1 Ex: Albinism is a recessive disorder and occurs with a frequency of 1/20000 people What is the frequency of individuals who are: Dominant Recessive Heterozygous II. Why Do allele Frequencies Change? In natural populations, allele frequencies DO change over time (this is the basis for evolution) Hardy Weinberg equilibrium establishes baseline to measure change Allele frequencies can change due to mutation, gene flow, genetic drift, and natural selection A. Mutation: introduces new alleles B. Gene flow (migration): individuals and their alleles enter and leave the population gene flow limits variation and reduces speciation new genes can enter into the population, which may have a negative effect. This lowers survivability mating causes gene flow between central and eastern populations Immigration from mainland introduces alleles that decrease fitness o High immigration between mainland and central region o Low immigration between mainland and eastern region Gene flow can also increase fitness of population o Insecticide resistant genes entering a population can increase the fitness of that population C. Genetic drift: in small populations, allele frequencies change RANDOMLY from generation to generation https://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/ animation45.html Bottleneck effect: a drastic change in environment could significantly reduce population size. New allele frequency may not reflect individual population Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent. Think of the effect of excessive hunting on animal populations Founder effect: a few individuals from a population get separated from the rest Genetic drift is: More significant in small populations and RANDOMLY causes allele frequencies to change Can reduce variation in a population Can cause unfavorable alleles to become fixed D. Selection: environment or humans select for favorable alleles which are then passed on. Selection is the only mechanism of adaptive evolution It is the fitness of individuals that affects future generations Fitness is the ability to reproduce and pass on alleles Natural selection acts on organisms’ phenotype which thus favors certain genotypes Types of natural selection: o Selection to match environment o Selection to avoid predators: Camouflage Mimicry Poison o Selection for pesticide resistance o Sexual selection: natural selection for mating o Intrasexual selection: members of the same sex compete to mate o Intersexual selection: when females choose their mate The three modes of selection o Directional selection: favors individuals at one end of phenotypic range o Disruptive selection: favors individuals at both ends of phenotypic range o Stabilizing selection: acts against extreme phenotypes III. Maintaining Alleles in a population Natural selection tends to remove variation by favoring certain alleles Mechanisms to preserve variation: o Diploidy: Recessive allele is hidden in heterozygote. Explains why Tay Sachs is still around o Heterozygote advantage: in the case of malaria, being heterozygous for sickle cell anemia protects you from malaria and you don’t have sickle cell anemia o Frequency dependent selection: fitness of a phenotype decreases if it becomes too common in the population