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Ch. 16 Evolution of Populations Copyright Pearson Prentice Hall • Microevolution = gradual change in a population overtime • Speciation = the origin of new species, is at the focal point of evolution • Macroevolution = evolutionary change on a grand (LARGE) scale - Replacement of one species with another - Increases biodiversity Figure 14.1 What is a species? Carolus Linnaeus - Swedish physician and botanist, used physical characteristics to distinguish species and developed the binomial system of naming organisms Scientific name = the genus and species names; Should be italicized or underlined Homo sapiens »Homo = genus »sapien = species Linnaeus’ system established the basis for taxonomy, the branch of biology concerned with naming and classifying the diverse forms of life Similarities between some species and variation within a species can make defining species difficult. Different species! SAME species! Species: A population or group of populations whose members can interbreed and produce fertile offspring NOT the SAME SPECIES! LIGER Scientific Name: Panthera leo × Panthera tigris Copyright Pearson Prentice Hall Copyright Pearson Prentice Hall Variation and Gene Pools A population is a group of individuals of the same species that interbreed. A gene pool consists of all genes, including all the different alleles, that are present in a population. Copyright Pearson Prentice Hall Variation and Gene Pools Gene Pool for Fur Color in Mice Sample Population Frequency of Alleles allele for brown fur Copyright Pearson Prentice Hall allele for black fur 16-216-2 Evolution Evolution as Genetic as Genetic Change Change • Evolution is any change over time in the relative frequencies of alleles in a population. • Populations, not individual organisms, can evolve over time. Copyright Pearson Prentice Hall Possible effects to a population of mice due to natural selection Frequency of individuals Original population Original population Figure 13.16 Evolved population Stabilizing selection Phenotypes (fur color) Directional selection Disruptive selection What type of selection is this??? Copyright Pearson Prentice Hall What is genetic drift? • A random change in allele frequency is called genetic drift. Bottleneck Effect Founder Effect Copyright Pearson Prentice Hall Genetic Drift Founder Effect- genetic drift when a small group of individuals colonizes a new habitat and allele frequencies shift The new population will be genetically different from the parent population. Copyright Pearson Prentice Hall Genetic Drift Bottleneck effect – a form of genetic drift causing a shift in allele frequencies due to a drastic event (earthquake, volcano, etc.) Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium • The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. • When allele frequencies remain constant it is called genetic equilibrium. Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium Five conditions are required to maintain genetic equilibrium from generation to generation: 1. Random mating (no sexual selection) 2. Very large population 3. No movement into or out of the population 4. No mutations 5. No natural selection Copyright Pearson Prentice Hall The Process of Speciation Copyright Pearson Prentice Hall 16-3 The Process of Speciation Natural selection and chance events can change the relative frequencies of alleles in a population Speciation = formation of NEW species! Figure 14.1 Copyright Pearson Prentice Hall Isolating Mechanisms What factors are involved in the formation of new species? The gene pools of two populations must become separated for them to become new species = isolating mechanisms • When populations become reproductively isolated from each other, they cannot interbreed and produce fertile offspring. Copyright Pearson Prentice Hall Isolating Mechanisms Reproductive isolation can develop in a variety of ways, including: • behavioral isolation • geographic isolation • temporal isolation Copyright Pearson Prentice Hall Isolating Mechanisms Behavioral Isolation - populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. Copyright Pearson Prentice Hall MECHANISMS OF SPECIATION (the “HOW”) Geographic isolation – separation by geographic barriers A. harrisi A. leucurus Isolating Mechanisms Temporal isolation - when two or more species reproduce at different times. Copyright Pearson Prentice Hall How did speciation in the Galápagos finches occur? By the : • founding of a new population • geographic isolation • changes in new population's gene pool • reproductive isolation • ecological competition Copyright Pearson Prentice Hall Speciation in Darwin's Finches Founders Arrive A few finches— species A—travel from South America to one of the Galápagos Islands. There, they survive and reproduce. Copyright Pearson Prentice Hall Speciation in Darwin's Finches Geographic Isolation Some birds from species A cross to a second island. The two populations no longer share a gene pool. Copyright Pearson Prentice Hall Speciation in Darwin's Finches Changes in the Gene Pool Seed sizes on the second island favor birds with large beaks. The population on the second island evolves into population B, with larger beaks. Copyright Pearson Prentice Hall Speciation in Darwin's Finches Reproductive Isolation • If population B birds cross back to the first island, they will not mate with birds from population A. • Populations A and B are separate species. Copyright Pearson Prentice Hall Speciation in Darwin's Finches Ecological Competition • As species A and B compete for available seeds on the first island, they continue to evolve in a way that increases the differences between them. • A new species—C—may evolve. Copyright Pearson Prentice Hall