Microevolution

Introduction

  • Speciation: the origin of new species; at the focal point of evolutionary theory 
  • Evolutionary theory must explain how new species originate and how populations evolve
  • Microevolution: consists of changes in a single gene in a population over time; a change in allele frequencies in a population over generations
  • Three mechanisms cause allele frequency change   * natural selection   * random genetic drift   * gene flow through migration and nonrandom mating

Populations and Hardy Weinberg Equilibrium

  • A population is in Hardy-Weinberg equilibrium if the genotype frequencies are the same in each generation. 

  • Conditions that must be met include   * there is no migration (“gene flow”) in or out of the population   * natural selection is not occurring   * mutation is not occurring (specifically in our germ cells)   * each member of the population is equally likely to breed   * the population is infinitely large

  • As long as a population satisfies biological conditions the allele frequencies (p and q -> dominant and recessive allele, respectively) are the same in each generation.

  • p^2 + 2pq + q^2 (Punnett square)

Agents That Change Allele Frequency In A Population

  • Natural Selection   * Some are more successful than others in surviving and reproducing owing to traits that give them a better fit to their environment.     * Relative fitness: the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals       * Includes intrasexual and intersexual (mate choice) sexual selection   * Causes adaptive evolution     * Leads to greater relative fitness   * Outcomes of natural selection     * directional selection     * disruptive selection     * stabilizing selection     * balancing selection

  • Genetic drift   * Founder effect: small number of individuals become isolated from a larger population     * Establishes different gene pool from population due to limited variation   * Bottleneck: when disaster strikes and chance alone leads to certain alleles being more or less present in survivors    * Migration and nonrandom mating: one member of the population is not equally likely to mate with any other member.     * can reduce genetic variation in populations, potentially reducing its ability to evolve in response to new selective pressures.     * can have drastic results in smaller populations.       *  Endangered species.     * can contribute to speciation.        * a small isolated population may diverge from the larger population

  • Gene flow   * Gene flow: the movement of alleles among populations     * Immigration     * Emigration

    * Alleles can be transferred through the  movement of fertile individuals or gametes       * Gene flow tends to reduce differences between populations over time     * Gene flow is more likely than mutation to alter allele frequencies directly

  • Mutation   * A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations.   * It must occur in the germ line
  • Sex    * Introduces new gene combinations into a population.     * It’s the type of genetic shuffling that is a source of genetic variation. 

Balancing Selection

  • Balancing selection: maintains genetic diversity
  • Balanced polymorphism: two or more alleles are kept in balance, and therefore are maintained in a population over \n the course of many generations
  • Two common ways   * For a single gene, heterozygote favored     * Heterozygote advantage – HS allele   * Negative frequency-dependent selection     * Rare individuals have a higher fitness

Causes of Adaptive Evolution

  • Natural selection is the only mechanism that consistently causes adaptive evolution
  • Evolution by natural selection involves both chance and “sorting”   * new genetic variations arise by chance   * beneficial alleles are “sorted” and favored by natural selection
  • Only natural selection consistently increases the frequencies of alleles that provide reproductive advantage
  • Natural selection brings out adaptive evolution by acting on an organism’s phenotype

Genetic Drift

  • Genetic drift: unpredictable changes in allele frequency in a small population
  • Genetic drift is significant (and faster) in small populations

     1. Causes allele frequencies to change at random

           1. Unrelated to fitness   2. Genetic drift can lead to a loss or fixation of an allele (Frequency = 0 or 100%)   3. Genetic drift can cause harmful alleles to become fixed

Nonrandom Mating

  • Forms of nonrandom mating   * Assortative: individuals with similar phenotypes are more likely to mate     * Increases the proportion of homozygotes   * Disassortative: dissimilar phenotypes mate preferentially     * Favors heterozygosity   * Inbreeding: does not favor any particular allele but does increase the likelihood the individual will be homozygous     * May have negative consequences with regard to recessive alleles

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