Chapter 21: Genetic Diversity in Populations PDF

Chapter 21 Genetic Diversity in Populations Genetic Diversity in Populations Population: a group of organisms of the same species living in one area Within a population, there are many genes Gene pool: the sum of the genes and their different alleles They are studied by population geneticists...

Chapter 21 Genetic Diversity in Populations Genetic Diversity in Populations Population: a group of organisms of the same species living in one area Within a population, there are many genes Gene pool: the sum of the genes and their different alleles They are studied by population geneticists in order to study the health of a population (is it stable) and microevolution. 1 The Hardy-Weinberg Principle It is a mathematical method of studying the gene pool of a population It predicts that if other factors remain constant, the gene pool will maintain a constant composition over many generations (equilibrium = stable) Any changes in the gene frequencies in the population over time can be detected. In order for equilibrium to remain in effect (no evolution is occurring) then the following five conditions must be met: No mutations must occur so that new alleles do not enter the population. No gene flow can occur (i.e. no migration of individuals into, or out of, the population). Random mating must occur (i.e. individuals must pair by chance) 2  The population must be large so that no genetic drift (random chance) can cause the allele frequencies to change. No natural selection can occur so that certain alleles are not selected for, or against. When some of these conditions are not met then microevolution occurs - evolution on a small scale = within a single population Genotype Frequency: the number of organisms in a population having a particular genotype Phenotype Frequency: the number of organisms in a population having a particular phenotype Allele Frequency: The proportion that a particular allele exists in the entire population 3  The Hardy-Weinberg Equations p2 + 2pq + q2 = 1, p+q=1 Variables are the frequency of: p = allele #1 (often the dominant one) q = allele #2 (often the recessive one) p2 = homozygous allele #1 q2 = homozygous allele #2 2pq = heterozygotes Boxed area = found on data sheet Note – pay attention to what type of frequency you are looking for: p and q = p2, q2 , 2pq = p2 + 2pq = phenotypic of trait p if it is dominant q2 = phenotypic of trait q if it is recessive Note: p2, q2 , 2pq = genotype and phenotypic if dealing with incomplete and co dominance 4 Another way to think about the formula: Sample Calculations 1) p. 722 8a and 8b – from text 5  8b solution Examples In a population of pigs 80% show the dominant trait of a pink coat colour, with a black colour being recessive. Calculate the allele and genotypic frequencies. 6  Solution Other types of problems - Snail 7  Evolutionary Change gene pools can be unstable factors that bring about evolutionary change are mutation, genetic drift, sexual selection, migration (or gene flow), and natural selection. Mutation Randomly occurring events in the gametes Create genetic diversity Can be neutral (most), beneficial or harmful. Lack of genetic diversity can result in extinction – bananas – yet again! 8  Gene Flow Exchanging of alleles between two populations due to migration. Having gene flow increases genetic variability. Wild Banana Cavindish Banana 9  Non-Random Mating Much of our natural world, including ourselves, involves non – random mating. Mates are chosen through female choice and male to male competition so that traits are being selected for. Can lead to sexual dimorphism (distinctive male and female traits) See text: p. 728 Genetic Drift Occurs in all populations but is far more noticeable in small populations. It is the change in gene pool of a population due to chance. Two types: Founder Effect – 10 Bottleneck Effect: Starvation, disease, human activities, or natural disasters can quickly reduce a large population The survivors only have a subset of the alleles present before the disaster; the gene pool loses diversity Natural Selection Mutations provide the raw material for this mechanism. Sickle cell anemia – heterozygote advantage keeps the allele for the anemia common in populations where malaria is present. The allele should be at a low rate but due to it’s role in malaria protection it remains at a high frequency where malaria is present. See text. 727 11