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Transcript
Chapter 23~ The Evolution of Populations Microevolution – change in allele frequency of a population over generations (evolution on the smallest scale) Mutations – the only source of new genes and new alleles Point Mutations – change in one base of a gene AGCCTA AGCTTA Chromosomal Mutations – delete, disrupt, duplicate, rearrange many loci at once Sexual Recombination offers the most common genetic variation 1) crossing over during Prophase I of Meiosis 2) independent assortment of chromosomes (chromosomes pairing up in different combinations) 3) fertilization (so many sperm to choose from!) Population Genetics – study of how populations change genetically over time Population – groups of individuals of the same species living in the same area at the same time Gene Pool – all the alleles of all the individuals of a population ***if all members of a population are homozygous for the same allele, they are termed: “FIXED” Hardy-Weinberg Theorem – used to describe a population that is NOT evolving The population will remain constant over time, unless “forces” change it Populations remaining the same is not likely, therefore allele frequencies change…leading populations to evolve In order for Hardy-Weinberg to display no evolution occurring three conditions must be met 1. 2. 3. 4. 5. No mutations Random mating No natural selection Large population No gene flow ***If there are any of these things occurring, then HW equilibrium cannot be met p^2 + 2pq + q^2 = 1 p+q=1 “p” is dominant (so p^2 is homozygous dominant) “q” is recessive (so q^2 is homozygous recessive) That make “pq” heterozygous [this equation is used to predict the frequencies (percentage) of the distribution of alleles] 1. 2. 3. 4. 5. 6. Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait? “q^2” frequency is 25% (or 0.25), which means “q” must be 0.5 p + q = 1 (this is reduced from p^2 + 2pq + q^2 = 1), so “p” must also be 0.5 p^2 + 2pq + q^2 = 1 From the reading 25% were recessive (125 individuals) We deduced 25% are Homo. Dom. (125 individuals) That means 50% are heterozygous (250 individuals) ***BUT then again this is a perfect world, in reality it might not be so nice with the numbers Population: a localized group of individuals belonging to the same species Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring Gene pool: the total aggregate of genes in a population at any one time Population genetics: the study of genetic changes in populations Modern synthesis/neo-Darwinism “Individuals are selected, but populations evolve.” Serves as a model for the genetic structure of a nonevolving population (equilibrium) 5 conditions: 1- Very large population size; 2- No migration; 3- No net mutations; 4- Random mating; 5- No natural selection p=frequency of one allele (A); q=frequency of the other allele (a); p+q=1.0 (p=1-q & q=1-p) P2=frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q2=frequency of aa genotype; p2 + 2pq + q2 = 1.0 A change in the gene pool of a population over a succession of generations 1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability) The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population 2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) 3- Mutations: a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection) 4- Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes) 5- Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population Geographical variation: differences in genetic structure between populations (cline) Prevention of natural selection’s reduction of variation Diploidy 2nd set of chromosomes hides variation in the heterozygote Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host) Fitness: contribution an individual makes to the gene pool of the next generation 3 types: A. Directional B. Diversifying C. Stabilizing Sexual dimorphism: secondary sex characteristic distinction Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism