CHANGE IN POPULATIONS AND COMMUNITIES

Important Terms Communities are made up of populations of different species of organisms that live and potentially interact with one another in a particular area at a particular time

Important Terms a Population is a group of individuals of the same species that live in a localized area A Species is a group of populations that have the potential to interbreed in nature

Populations Individuals within a species tend to cluster into groups or separate populations and tend to breed with other members of their population Because of this, members of populations tend to be more closely related to members of their own population

Populations A population’s gene pool is the total aggregate of genes in a population at any one time Even though members of a population are similar there is still lots of variation within that population Diploid species Alleles ‘P’ and ‘p’ Genotypes PP, Pp, pp Frequencies of alleles and genotypes in a population remain fairly constant over time

Populations Frequencies of alleles and genotypes in a population remain fairly constant over time Red haired Scotts Tounge rollers (dominant gene) 65% carry dominant gene Blood types A, B, AB, O I A, I B, I In N.A. 45% of Whites and 49% of blacks are Type O (recessive allele has greater frequency

Studying Populations Population sampling is used to study large populations A representative group of the population is randomly selected and the frequency of traits of this group are used as indicators for the entire population

Hardy-Weinberg Theorem Frequencies of alleles and genotypes in a populations gene pool remain constant from one generation to the next (Equilibrium) ***For non-evolving populations***

Hardy-Weinberg Equilibrium In order to maintain the Hardy-Weinberg Equilibrium the following 5 conditions must be met 1. Population sizes are large 2. Populations are isolated from other populations 3. No net mutations 4. Random mating 5. No Natural selection

Hardy-Weinberg Equilibrium The Hardy-Weinberg ratio is the ratio of genotype frequencies that evolve when mating is random and neither selection nor drift are operating For two alleles (A and a) with frequencies p and q, the Hardy-Weinberg frequencies are Genotype AA : Aa : aa Frequency p 2 : 2pq : q 2

Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states p 2 + 2pq + q 2 = 1

Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that allele and genotype frequencies do not change over time given the 5 conditions: 1. Population sizes are large 2. Populations are isolated from other populations 3. No net mutations 4. Random mating 5. No Natural selection What is the use of the H-W principle then?

Evolutionary Change Factors that change a population’s gene pool contradict the 5 conditions of the H-W Principle and lead to evolution Mutation Genetic Drift Migration

Mutation Very rare events (1/half million genes per generation) Usually occurs during meiosis and present in either egg or sperm The mutation will be present for life of the individual and will possibly be passed on to future generations

Genetic Drift Disruption of genetic equilibrium due to small population sizes and chance events Larger population sizes have less sampling error (probabilities) Small population – gene pool may not be accurately represented in following generations

Causes of Genetic Drift Bottleneck effect Drastic reduction in population size due to a catastrophic event (fire, flood, earthquake, disease) Small surviving population is unlikely to be representative of original population

Causes of Genetic Drift Founder effect (Darwin’s Finches) A few individuals from an original population move to an isolated location (island) Most radical example would be a pregnant female moving to an island with no other members of her species

Migration (gene flow) Movement of members of a species in or out of a population Immigration Emigration