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Evolution. How Populations Evolve. Voyage of the Beagle. Jean Baptiste Lamarck. Charles Lyell. Artificial Selection. Artificial Selection. Natural Selection. Darwin saw natural selection as the basic mechanism of evolution

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How Populations Evolve

Voyage of the Beagle

Jean Baptiste Lamarck

Charles Lyell

Artificial Selection

Artificial Selection

Natural Selection

  • Darwin saw natural selection as the basic mechanism of evolution

    • As a result, the proportion of individuals with favorable characteristics increases

    • Allele frequencies (and therefore phenotypes within a population) gradually change over time

  • These five canine species evolved from a common ancestor through natural selection

African wilddog





Thousands tomillions of yearsof natural selection

Ancestral canine

Figure 13.4C

  • The evolution of insecticide resistance is an example of natural selection in action


Chromosome with geneconferring resistanceto insecticide

Additionalapplications of thesame insecticide willbe less effective, andthe frequency ofresistant insects inthe populationwill grow


Figure 13.5B

Fossils provide strong support for evolution

Ammonite casts; Fossilized leaf

  • Other evidence for evolution comes from

    • Comparative anatomy (e.g. homologous structures)


Comparative Embryology

Comparative Embryology

Molecular biology

Examples of natural selection – camouflage adaptations in different environments

Population Genetics and Modern Evolutionary Synthesis Theory

  • Population genetics

    • Studies how populations change genetically over time

  • The modern evolutionary synthesis theory

    • Connects Mendelian Inheritance, Darwin’s theory with population genetics

  • Gene pool

    • Is the total collection of genes in a population at any one time

  • Microevolution

    • Is a change in the relative frequencies of alleles in a given gene pool

Gene pool in a nonevolving population remains constant over generations.

Hardy-Weinberg Principle

  • States that allele and genotype frequency in a population remain constant (equilibrium) – from generation to generation unless an outside influence disturbs the balance.

Hardy-Weinberg Principle

  • Must satisfy five conditions

    • The population is very large

    • The population is isolated

    • Mutations do not alter the gene pool

    • Mating is random

    • All individuals are equal in reproductive success

Hardy-Weinberg Equation

  • p2 + 2pq + q2 = 1

  • p + q = 1


  • p is frequency of the dominant allele; q is the frequency of the recessive allele

  • p2 is the frequency of the homozygous dominant genotype

  • 2pq is the frequency of the heterozygous genotype

  • q2 is the frequency of the homozygous recessive genotype

  • The Hardy-Weinberg equation is useful in public health science

    • Public health scientists use the Hardy-Weinberg equation

      • To estimate frequencies of disease-causing alleles in the human population

Other contributors to evolution include

  • Genetic Drift – change in the relative frequency of an allele

    • Bottleneck effect

    • Founder effect

  • Gene Flow – movement of alleles from one population to another

    • Emigration/immigration

Bottleneck Effect

Founder Effect

  • Loss of genetic variability when a new population is established by a very small number of individuals from a larger population.

Founder Effect

Island of Krakotoa, 1883




  • Variation is extensive in most populations

    • Many populations exhibit polymorphism

      • Different forms of phenotypic characteristics

      • Functions to retain variety of morphs in a population living in a varied environment



  • Populations may also exhibit geographic variation

    • Variation of an inherited characteristic along a geographic continuum

Sexual recombination

Evolutionary Fitness?

Natural selection can alter variation in a population in three ways

  • Stabilizing selection

  • Directional selection

  • Disruptive selection

Modes of Selection

Figure 13.17B

Figure 13.17A


Sexual selection may produce sexual dimorphism

  • Sexual selection leads to the evolution of secondary sexual characteristics

    • Which may give individuals an advantage in mating

Processes and Mechanisms of Evolution

  • Adaptation

  • Genetic drift

  • Gene flow

  • Mutations

  • Natural selection

  • Speciation

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