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Population Genetics How does a population evolve? Outline 1. Key Concepts 2. Individuals Don’t evolve, Populations Do 3. The Hardy-Weinberg Theorem 4. The Microevolution and Natural Selection 5. Genetic drift 6. Gene flow Key Concepts: Individuals of a population have the same number and kinds of genes In a population, a gene may exist in different forms Microevolution means that changes have occurred in a population’s allele frequencies Allele frequencies can change through mutations, gene flow, genetic drift, nonrandom mating, and natural selection 1 Individuals Don’ Don’t Evolve, Populations Do Population - Is a localized group of individuals that are capable of interbreeding and producing fertile offspring Polymorphism - traits come in two or more distinct forms Gene Pool - pool of genetic resources that, in theory, is shared by all members of the population Polymorphism A Gene Pool 2 The HardyHardy-Weinberg Theorem The Hardy-Weinberg Theorem: The frequences of alleles in the gene pool will remain constant unless acted by other agents This is a theoretical state in which a population is not evolving Only if: – There is no mutation – The population is very large and – Isolated from other populations – There is no selection – Random mating The Hardy-Weinberg theorem Punnett Square p A q a p A AA(p2) Aa(pq) q a Aa(pq) aa(q2) 3 Frequencies in Gametes F1 genotypes: Gametes: 0.49 AA A 0.42 Aa A A 0.09 aa a a a 0.49 + 0.21 0.21 + 0.09 0.7A 0.3a Five Causes of Microevolution Mutations Gene flow - Emigration and immigration of individuals (Flow of alleles) Genetic Drift – Changes in the gene pool of a small population due to chance Nonrandom mating Natural selection Natural Selection Steps in the process of evolution by natural selection – 1. There is genetically-based variation in a population (from random mutation) – 2. Some individuals are more fit a certain environment (with certain genetic-based traits have greater reproductive success than others – 3. Individuals with phenotypes that are better adapted to the environment pass more copies of their alleles into next generation – 4. As a result, there is a change in allele frequency overtime ( = microevolution) 4 Natural Selection Natural selection causes changes in gene frequencies of a population – 1. Natural selection does not cause genetic changes in individuals – 2. Natural selection befalls individuals, but evolution occurs in populations – 3. Only natural selection generally leads to an accumulation of favorable adaptations in a population (Darwinian Changes) Natural selection is an outcome of differences in survival and reproduction among individuals Genetic drift Genetic drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation CW CW C RC R C RC R Only 5 of 10 plants leave offspring C RC W CW CW C RC R C RC R CW CW C RC R C RC W C RC W C RC R CW CW C RC W C RC R C RC R C RC W C RC W Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3 Only 2 of 10 plants leave offspring C RC R C RC R C RC R C RC R C RC R C RC R C RC R C RC R C RC R C RC W C RC W Generation 2 p = 0.5 q = 0.5 C RC R Generation 3 p = 1.0 q = 0.0 Figure 23.7 The Bottleneck Effect In the bottleneck effect – A sudden change in the environment may drastically reduce the size of a population – The gene pool may no longer be reflective of the original population’s gene pool (a) 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. Figure 23.8 A Original population Bottlenecking event Surviving population 5 Bottlenecking is an important concept in conservation biology of endangered species. – Populations that have suffered bottleneck incidents have lost at least some alleles from the gene pool. – This reduces individual variation and adaptability. – For example, the genetic variation in the three small surviving wild populations of cheetahs is very low when compared to other mammals. bottleneck effect Understanding the bottleneck effect – Can increase understanding of how human activity affects other species (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction. Figure 23.8 B Gene Flow Gene flow – Causes a population to gain or lose alleles – Results from the movement of fertile individuals or gametes – Tends to reduce differences between populations over time 6