1. Chapter 13 Lecture Outline See PowerPoint Image Slides
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2. The Scientific Concept of Evolution
Evolution is the change in the frequency of genetically determined characteristics within a population over time.
Microevolution involves changes in allele frequencies between populations of the same species.
Macroevolution involves major genetic changes that occur over long periods of time that generate new species.
The mechanisms of micro- and macroevolution are the same.
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3. Microevolution vs. Macroevolution
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4. Early Thinking About Evolution
Mid 1700s
Buffon observed fossils were so different from modern animals, implying that animals have changed over time (evolution).
Early 1800s
Lamarck suggested a process by which the changes could occur.
He proposed that acquired characteristics could be passed on to offspring.
Example: Giraffes have long necks because their ancestors stretched to reach leaves in trees.
Lamarck was wrong, but his ideas stimulated thought about evolution.
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5. The Theory of Natural Selection
1858
Darwin and Wallace suggested the theory of natural selection as a mechanism for evolution.
Darwin wrote Origin of Species by Means of Natural Selection.
The theory of natural selection
The idea that some individuals have genetic combinations that allow them to survive, reproduce, and pass their genes on to the next generation
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6. Assumptions of the Theory of Natural Selection
All organisms produce more offspring than can survive.
No two organisms are exactly alike.
Among organisms, there is a constant struggle for survival.
Individuals that possess favorable characteristics for their environment have a higher rate of survival and produce more offspring.
Favorable characteristics become more common in the species.
Unfavorable characteristics are lost over time.
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7. Natural Selection vs. Acquired Characteristics
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8. Modern Interpretations of Natural Selection
Darwin and Wallace’s theories were not immediately accepted because meiosis, genes, and inheritance were poorly understood.
Mendel’s discovery provided an explanation for how characteristics could be transmitted from one generation to the next.
Knowledge of mutation, gene flow, and reproductive isolation supported Darwin/Wallace’s theory.
Now we can integrate Darwin and Wallace’s hypothesis with what we know about meiosis, genes, and inheritance.
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9. Darwin and Wallace’s Basic Assumptions Can Be Updated
An organism’s ability to overproduce results in surplus organisms.
Due to mutation, new traits enter the gene pool and due to meiosis new combinations of alleles can be generated.
Resources such as food, soil nutrients, water, etc., are in short supply, so some individuals will not survive.
Disease, predators, etc., will also affect survival.
These are called selecting agents.
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10. Darwin and Wallace’s Basic Assumptions Can Be Updated
Selecting agents favor individuals with the best combinations of alleles.
They will be more likely to survive and reproduce and pass their alleles on to the next generation.
Allele combinations that favor survival and reproduction will be more common in a population.
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11. The Role of Natural Selection in Evolution
Natural selection will select for individuals with certain alleles.
When allele frequencies change over time, evolution has occurred.
Natural selection works on individuals, but only populations evolve.
Three factors work together to determine how a population changes over time.
Environmental factors that affect individuals
Sexual reproduction among the individuals
Genetic diversity within the gene pool
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12. Reproductive Success
Individuals that have the combinations of alleles that allow them to successfully reproduce will pass on their alleles.
This success is measured as fitness.
Fitness is a relative measure.
An individual can successfully reproduce, but be less fit than another individual.
Involves the number of offspring produced and their viability
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13. The Importance of Genetic Diversity
A large gene pool with great genetic diversity is more likely to contain genetic combinations that will allow some individuals to adapt to changing environments.
Characteristics that may allow individuals to adapt include
Structural, behavioral, biochemical, or metabolic characteristics
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14. Common Misunderstandings About Natural Selection
Survival of the fittest
Individual survival is not as important as reproductive success (# of descendants).
Struggle for life
Does not refer to conflict
Refers to finding and utilizing resources
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15. Common Misunderstandings About Natural Selection
The significance of acquired characteristics
Phenotypes acquired during the life of the individual will not be passed on.
These will not be important during natural selection.
The relationship between the mechanism of natural selection and the outcomes of the selection process
The effects of natural selection are seen in the population.
The outcome of natural selection (death, reproductive success, mate choice) involves individuals.
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16. What Influences Natural Selection?
Genetic diversity within a species
Genetic recombination as a result of sexual reproduction
Gene expression
The ability of a species to reproduce excess offspring
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17. Mechanisms that Affect Genetic Diversity
Natural selection cannot occur in a population in which all of the individuals are genetically identical.
Genetic diversity is essential for natural selection.
Several mechanisms generate genetic diversity in a population.
Mutation, migration, and sexual reproduction
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18. Mutations and Migration
Spontaneous mutations are changes in DNA that cannot be tied to a specific cause.
Mutations alter existing genes and generate new alleles.
Most mutations are harmful.
Some mutations are helpful.
Mutations are only relevant to natural selection if they happen in cells that give rise to gametes.
Migration results in alleles entering and leaving a population.
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19. Sexual Reproduction and Genetic Recombination
Sexual reproduction generates new combinations of alleles in individuals (genetic recombination).
Crossing over during meiosis generates new combinations of alleles in homologous chromosomes.
Independent assortment generates new combinations of alleles from non-homologous chromosomes.
Random fertilization results in a genetically unique individual.
Genetic recombination can generate a new combination of alleles that would give an individual a selective advantage.
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20. The Role of Gene Expression
In order for genes to be selected for, they must be expressed in phenotype.
Genes are expressed to different degrees in different individuals.
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21. Different Degrees of Expression
Penetrance describes how often an allele is expressed.
Expressivity describes situations in which the allele is penetrant, but not expressed equally in all individuals.
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22. Why Some Genes May Avoid Natural Selection
Some alleles are only expressed during certain stages of life.
If an organism dies before the allele is expressed, then it cannot contribute to fitness.
Some genes need environmental triggers to be expressed.
If the trigger is not encountered, then the gene will not be expressed and will not contribute to fitness.
In heterozygotes, recessive alleles are not expressed.
Some alleles will not be expressed because an unrelated gene is required.
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23. Natural Selection Works on the Total Phenotype
One good allele is not enough to give an individual greater fitness.
Natural selection acts on the total phenotype which involves a combination of characteristics.
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24. The Importance of Excess Reproduction
Successful organisms reproduce at a rate in excess of that necessary to merely replace them when they die.
However, population size remains relatively constant.
A high death rate offsets the high reproductive rates.
Even though a population remains constant in number, the individuals that make up the population change.
Therefore, there are many genetically unique individuals.
Some of these individuals will survive and reproduce, even if the environment changes.
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25. Processes that Drive Selection; Differential Survival
“Survival of the fittest”
Survival is a prerequisite of reproduction.
Individuals that do not survive cannot reproduce.
Darwin’s finches
The finch’s beak size correlated with the type of seeds they ate.
Small beaks - softer seeds
Larger beaks - harder seeds
During drought, as soft seeds became scarce, only birds with larger beaks survived.
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26. Darwin’s Finches
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27. Differential Survival of Herbicide-Resistant Weeds
When herbicides are used on weeds
Only the individual plants that are resistant will survive.
As these reproduce, more and more individuals in the species are resistant to the herbicide.
Over time, the herbicide is ineffective.
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28. Differential Survival of Herbicide-Resistant Weeds
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29. Processes that Drive Selection; Differential Reproductive Rates
Survival is necessary to reproductive success, but does not guarantee reproductive success.
Reproductive success is measured by the number of offspring an individual leaves.
Clover fields
Cows eat taller plants first.
Flowers are at the top of the tall plants.
Tall plants didn’t die, but were not able to reproduce.
Therefore, shorter plants were more reproductively successful.
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30. Processes that Drive Selection; Differential Mate Choice
Sexual selection occurs when some individuals are chosen as mates more often than others.
Relevant only in animal populations
Those that are chosen pass on more genes than those that are not chosen.
More frequently, chosen individuals are usually larger, more aggressive, more colorful, etc.
All of these features attract mates.
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31. Examples of Sexual Selection
Male red-winged blackbirds establish territories in which they only allow females.
The ability to establish and maintain a territory determines which males will mate.
Male peacocks and chickens have elaborate tail feathers.
Males with the most spectacular tails are chosen by females more frequently.
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32. Mate Selection
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33. Patterns of Selection
Natural selection changes allele frequencies in a population.
This actually reduces genetic diversity.
Over time, the individuals become more alike.
Selection can favor different phenotypes in different situations.
This leads to three different forms of selection.
Stabilizing, directional, and disruptive
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34. Stabilizing Selection
Occurs when individuals at the extremes of the range of characteristic are selected against
This means that the “average” individuals are selected for.
Brown mice selected for, white and black selected against
White and black are more likely to be noticed and eaten by predators.
Over time the population will have mostly brown mice.
Stable environments favor stabilizing selection.
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35. Directional Selection
Occurs when individuals of one extreme of the range of characteristic are selected for
Usually occurs when the environment is changing
Insecticide use will select for individuals that are resistant.
Originally there were probably only a few resistant individuals.
After several generations, the population will contain mostly resistant individuals.
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36. Disruptive Selection
Occurs when both extremes of a range of characteristic is selected for
The intermediate is selected against.
Insects that live on plants with dark green or light green leaves
Medium green insects will be noticed and eaten by predators.
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37. Patterns of Selection
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38. Evolution Without Selection — Genetic Drift
Genetic drift involves a significant change in allele frequency that is not a result of natural selection.
Results from chance events
More likely to impact small populations
In a population of 100 plants, 10 of them have red spots on their leaves.
If these 10 are randomly trampled by an animal or are killed by a late frost
The red spot allele would be lost, but not due to natural selection.
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39. Genetic Drift Cougars Divided by human colonization
Small populations became isolated and experienced genetic drift.
Now the populations are endangered because their gene pools are not diverse.
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40. Gene-Frequency Studies and Hardy-Weinberg Equilibrium
G. H. Hardy and Wilhelm Weinberg described a simple mathematical relationship that can be used to study allele frequencies.
They recognized that under certain conditions, allele frequencies would not change over time.
In this case, alleles would mix randomly and the same proportion of genotypes would result.
Their formula uses allele frequencies and genotypic frequencies to detect an equilibrium.
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41. Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
p = frequency of the dominant allele
q = frequency of the recessive allele
p2 = frequency of homozygous dominants
2pq = frequency of heterozygotes
q2 = frequency of homozygous recessives
If the proportion of genotypes matches this equation, then the population is not evolving.
If gene frequencies do not match this equation, then the population is evolving.
Certain conditions have to be met for a population to be in Hardy-Weinberg equilibrium.
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42. The Conditions Necessary for H-W Equilibrium
If these five conditions are met, then gene frequencies will not change.
Random mating
No mutation
No migration
Very large population size
No natural selection
Hardy-Weinberg equilibrium provides a null hypothesis for evolution in a population.
Gives us a way to compare two populations, or two generations of a population to determine if evolution is occurring.
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43. Determining Genotype Frequencies
Genotypic frequencies in a population can be calculated with a modified Punnett Square.
In this Punnett square, allele frequencies must be used to determine the likelihood of particular genotypes occurring in the next generation.
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44. Why Hardy-Weinberg Conditions Rarely Exist
Since the gene pools of most populations are changing over time, H-W equilibrium rarely exists.
Why?
Random mating rarely occurs.
Spontaneous mutations occur.
Immigration and emigration introduce new alleles.
Populations are not infinitely large.
Natural selection does occur.
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45. Using the Hardy-Weinberg Concept to Show Allele-Frequency Changes
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46. The Effect of Changing Allele Frequencies
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47. Summary of the Causes of Evolutionary Change
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