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Chapter 19 Genetic Diversity in Populations
Chapter Outcomes: • Define a gene pool. • Describe the gene pool of a population at genetic equilibrium. • Summarize the five conditions upon which the Hardy-Weinberg principle is based. • Describe how the Hardy-Weinberg equation is used to determine whether a population is undergoing microevolution.
Chapter Outcomes • Calculate allele and genotype frequencies in a population. • Outline the conditions required to maintain genetic equilibrium. • Identify and compare the effects of mutations, gene flow, non-random mating and genetic drift on gene pool diversity. • Apply the Hardy-Weinberg principle to published data.
Chapter Outcomes • Distinguish between founder effect and the bottleneck effect on gene pools. • Explain how the process of natural selection is related to microevolution. • Explain the cause of heterozygote advantage and how it affects a gene pool. • Describe strategies used in captive breeding and population management. • Explain that genetic engineering can have intended and unintended effects on gene pools.
Genetic Diversity in Populations • Recall that a population is a group of organisms of the same species living in one area • Within a population, there are many genes • The sum of the genes (and their different alleles) is known as the gene pool • Gene pools are studied by population geneticists
Genotype, Phenotype & Allele Frequency • Genotype Frequency: • Phenotype Frequency: • Allele Frequency:
The Hardy-Weinberg Principle • the Hardy-Weinberg principle predicts that if other factors remain constant, the gene pool will maintain a constant composition over many generations • this is expressed by a mathematical equation:
The Hardy-Weinberg Equation p2 + 2pq + q2 = 1 Where: • p is the frequency of the A allele • q is the frequency of the a allele • if the values of p and q are known, we can calculate the frequency of the alleles AA, Aa, and aa (and vice-versa)
Limits to the Hardy-Weinberg Principle • Large populations • Random mating • No mutations • No migration • No natural selection against any of the phenotypes
Application of the Hardy-Weinberg Principle • In a population, we know that a dominant trait is present 82% of the time. Determine the percentage of individuals that make up each genotype.
The Hardy-Weinberg & Population Change • If a gene pool changes over time, one of the 5 conditions it is based on must also have changed • Therefore, the strength of this principle is to determine whether or not a population is evolving • The Hardy-Weinberg equation also allows us to determine what percentage of a population are “carriers” of a trait
Evolutionary Change • gene pools are unstable • factors that bring about evolutionary change are mutation, genetic drift, and migration (or gene flow)
The Founder Effect • New populations are often formed by only a few individuals (Founders) • The founders will only carry part of the original gene pool from the population • Therefore, the new gene pool will be limited
The 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, and therefore, the gene pool loses diversity • Gene pool change caused by a rapid decrease in population is known as the bottleneck effect
Examples of the Founder Effect • “Blue Fugates” • Philadelphia Amish
Examples of the Bottleneck Effect • Northern Elephant Seals • Cheetahs
Natural Selection • Natural selection is the only process that leads directly to evolutionary adaptation • Recall that natural selection occurs in the following order:
Advantage & Natural Selection • Sexual Selection: • Heterozygote Advantage & Lethal Alleles:
Human Activities & Genetic Diversity • Humans can affect genetic diversity of populations in many ways: • Habitat fragmentation • Unregulated hunting & habitat removal
Biotechnology & Gene Pools • We can use many techniques to modify organisms and the gene pools of populations • We now have added genes to species that come from completely different species, thereby adding a gene to a gene pool that most likely would never have been there otherwise
These genes could be passed on to others in a population • This could ultimately affect the process of natural selection • As well, because genes do not work alone (often they work together), the positive effect of the inserted genes may be negated because the gene affects other traits necessary for survival
Cloning to Save Species • Cloning can be one way to preserve gene pools • Creating clones of endangered species could reverse the threat of extinction • In 2000, a cloned Asian gaur (a rare ox-like mammal) was born in Iowa to a domestic cow that served as a surrogate mother