Chapter 17: Evolution of Populations

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Chapter 17: Evolution of Populations. Evolution of Populations. When Darwin developed his theory of evolution, he did not understand: how heredity worked. This left him unable to explain two things: a. source of variation - PowerPoint PPT Presentation


  • Chapter 17: Evolution of Populations

  • When Darwin developed his theory of evolution, he did not understand:how heredity worked.This left him unable to explain two things:a. source of variationb. how inheritable traits pass from one generation to the nextEvolution of Populations

  • In the 1940s, Mendels work on genetics was rediscovered and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular population. This evolution of populations is called microevolution.

    Evolution of Populations

  • Vocabulary: Population: group of individuals of the same species living in the same area that breed with each other.

  • gene poolcombined genetic info. for all membersof a population

  • Allele: one form of a gene

  • 2. relative frequency of an allele: # times an allele occurs in the gene pool compared to other alleles (percent)

    ExampleRelative Frequency:70% Allele B30% Allele b

  • 3. Sources of Variation: a. mutations: any change in DNA sequence

    Can occur because of: mistakes in replication environmental chemicals

    May or may not affect an organisms phenotype

  • 3. Sources of Variation

    b. Gene Shuffling: recombination of genes that occurs during production of gametesCause most inheritable differences between relativesOccurs during meiosisAs a result, sexual reproduction is a major source of variation in organisms.Despite gene shuffling, the frequency of alleles does not change in a population. Explain why this is true.Similar to a deck of cards no matter how many times you shuffle, same cards (alleles) are always there.

  • 4. Gene Traits:

    A) Single gene trait: controlled by single gene with two allelesExamples: widows peak, hitchhikers thumb, tongue rolling

  • (4. Gene Traits:)B) Polygenic trait: controlled by 2 or more genes, each with 2 or more allelesExamples: height, hair color, skin color, eye colorMost human traits are polygenic.

  • Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain.

    type: single genewhy? Only two phenotypes possibleExample: tongue roller or non-tongue rollertype: polygenicwhy? Multiple (many) phenotypes possibleExample: height range 4feet to 9 feet all

  • 5. Natural selection acts on phenotypes, not genotypes.

    Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white? Brown, more hidden.

  • 5. If brown is dominant can a predator tell the difference between:

    Mouse with highest fitness will have the most alleles passed on to the next generation.White mouse will have low fitnessBB Bb?

  • 5. Which mouse will have the lowest fitness? White, bb (recessive)

    Will the fitness of BB and Bb differ? Why?No, Both BB and Bb have the same fitness advantage of being brownBB Bb?

  • Three ways in which natural selection affects polygenic traits.

    Natural SelectionDirectional SelectionStabilizing SelectionDisruptive Selection

  • Directional Selection: individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait

    Individuals with highest fitness: those at one end of the curveExample: Galapagos finches beak size

  • Directional SelectionDirectional Selection

  • Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curveIndividuals with highest fitness: near the center of the curve (average phenotype)

    Example: human birth weight

  • Disruptive Selection: individuals at both ends of the curve survive better than the middle of the curve.

    Individuals with highest fitness: both ends of curve

    Example: birds where seeds are either large or small

  • Stabilizing Selection

  • Disruptive Selection

  • However:

    No examples ever observed in animalsA couple examples that may demonstrate speciation exist in plants and some insects.

  • Genetic Drift random change in allele frequency that occurs in small populations

  • Two phenomena that result in small populations and cause genetic driftFounder EffectBottleneck Effect

    Genetic Drift

  • The results of genetic crosses can usually be predicted using the laws of probability.

    In small populations, however, these predictions are not always accurate.

    Genetic Drift

  • Founder effect Allele frequencies change due to migration of a small subgroup of a population

  • Founder EffectTwo groups from a large, diverse population could produce new populations that differ from the original group.

  • 2. Bottleneck effectMajor change in allele frequencies when population decreases dramatically due to catastropheExample: northern elephant sealsdecreased to 20 individuals in 1800s, now 30,000no genetic variation in 24 genes

  • Bottleneck Effect: Northern Elephant Seal PopulationHunted to near extintionPopulation decreased to 20 individuals in 1800s, those 20 repopulated so todays population is ~30,000No genetic variation in 24 genes

  • Bottleneck EffectOriginal population

  • Bottleneck EffectOriginal populationCatastrophe

  • Bottleneck EffectOriginal populationCatastropheSurviving population

  • Evolution Versus Genetic EquilibriumWhat conditions are required to maintain genetic equilibrium?According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium:

    The population must be very largethere can be no mutations there must be random matingthere can be no movement into or out of the population(5) no natural selection can occur

  • Genetic equilibrium = no evolutionA population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies dont change, the population will not evolve.

    The Hardy-Weinberg principle describes theconditions under which evolution does not occur.

    The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.

  • Hardy-Weinberg principle 1. Large Population

    Genetic drift can cause changes in allele frequencies in small populations.Genetic drift has less effect on large populations. Large population size helps maintain genetic equilibrium2. No MutationsIf mutations occur, new alleles may be introduced into the gene pool, and allele frequencies will change.

  • Hardy-Weinberg principle 3. Random MatingAll members of the population must have an equal opportunity to produce offspring. Individuals must mate with other members of the population at random. In natural populations, however, mating is not random. Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. Such non-random mating means that alleles for those traits are under selection pressure.

  • Hardy-Weinberg principle 4. No Movement Into or Out of the PopulationIndividuals who join a population may introduce new alleles into the gene pool. (Immigration)Individuals who leave may remove alleles from the gene pool. (emigration)Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population.

  • Hardy-Weinberg principle 5. No Natural SelectionAll genotypes in the population must have equal probabilities of surviving and reproducing. No phenotype can have a selective advantage over another.

  • Sexual Reproduction and Allele FrequencyMeiosis and fertilization do not change the relative frequency of alleles in a population.

    The shuffling of genes during sexual reproduction produces many different gene combinations but does not alter the relative frequencies of alleles in a population.

  • The Process of SpeciationThe formation of new biological species, usually by the division of a single species into two or more genetically distinct one.

  • Three Isolating Mechanisms: Isolate species forming subspecies and perhaps causing speciation. Geographic IsolationBehavioral IsolationTemporal Isolation

  • Example: Eastern and Western Meadowlark Male birds sing a matting song that females like, East and West have different songs. Females only respond to their subspecies song.

  • 1. Geographic IsolationTwo populations separated by a geographic barrier; river, lake, canyon, mountain range.

  • Example: 10,000 years ago the Colorado River separated two squirrel populations.

    Kaibab Squirrel Abert Squirrel

  • This resulted in a subspecies, but did not result in speciation because the two can still mate if brought together. Kaibab Squirrel Abert Squirrel

  • Example: Eastern and Western Meadowlark Eastern and Western Meadowlark populations overlap in the middle of the US

  • 2. Behavioral IsolationTwo populations are capable of interbreeding but do not interbreed because they have different courtship rituals or other lifestyle habits that differ.

  • 3. Temporal IsolationPopulations reproduce at different timesJanuary 1 2 3 4 5 67 8 9 10 11 12 13

  • Example: Northern Leopard Frog & North American Bullfrog Mates in: Mates in: April July

  • Conclusion:Geographic, Behavioral andTemporal Isolation are allbelieved to lead tospec