chp. 24.2 lecture slides
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CHAPTER 24LECTURE
SLIDES
Prepared by
Brenda LeadyUniversity of Toledo
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Natural selection
Process in which beneficial traits that are heritable become more common in successive generations
Over time, natural selection results in adaptationsChanges in populations of living organisms
that promote their survival and reproduction in a particular environment
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Reproductive successLikelihood of an individual contributing fertile
offspring to the next generationAttributed to 2 categories of traits
Certain characteristics make organisms better adapted to their environment and more likely to survive to reproductive age
Traits that are directly associated with reproduction, such as the ability to find a mate and the ability to produce viable gametes and offspring
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Modern description of natural selection
1. Within a population, allelic variation arises from random mutations that cause differences in DNA sequences
2. Some alleles encode proteins that enhance an individual’s survival or reproductive capability compared to other members of the population
3. Individuals with beneficial alleles are more likely to survive and contribute their alleles to the gene pool of the next generation
4. Over the course of many generations, allele frequencies of many different genes may change through natural selection, thereby significantly altering the characteristics of a population
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Fitness
Relative likelihood that a genotype will contribute to the gene pool of the next generation as compared with other genotypes
Measure of reproductive success Hypothetical gene with alleles A and a
AA, Aa, aa
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Suppose average reproductive successes are…AA produces 5 offspringAa produces 4 offspringaa produces 1 offspring
Fitness is W and maximum is 1.0 for genotype with highest reproductive abilityFitness of AA: WAA = 5/5 = 1.0
Fitness of Aa: WAa = 4/5 = 0.8
Fitness of aa: Waa = 1/5 = 0.2
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Mean fitness of population
Average reproductive success of members of a population
As individuals with higher fitness values become more prevalent, natural selection increases the mean fitness of the population
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Natural selection patterns
Directional selection Stabilizing selection Disruptive/Diversifying selection Balancing selection
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Directional selection
Individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment
InitiatorsNew allele with higher fitness introducedProlonged environmental change
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(a) An example of directional selection (b) Graphical representation of directional selection
Many generations
Light fur Dark fur
Light fur Dark fur
Population of mice in a dimly lit forest
Many generations
Number ofindividuals
Number ofindividuals
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Stabilizing selection
Favors the survival of individuals with intermediate phenotypes
Extreme values of a trait are selected against
Clutch sizeToo many eggs and offspring die due to lack
of care and foodToo few eggs does not contribute enough to
next generation
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Disruptive/Diversifying selection
Favors the survival of two or more different genotypes that produce different phenotypes
Likely to occur in populations that occupy heterogeneous environments
Members of the populations can freely interbreed
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(b) Graphical representation of disruptive selection
Nu
mb
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f in
div
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als
Metal sensitive Metal resistant
Nu
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f in
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Metal sensitive Metal resistant
Many generations(a) Growth of Agrostis capillaris on contaminated soil
Agrostis capillaris
Contaminated soil
a: © Courtesy Mark McNair/University of Exeter
Agrostis capillaris
Contaminated soil
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Balancing selection
Maintains genetic diversity Balanced polymorphism
Two or more alleles are kept in balance, and therefore are maintained in a population over the course of many generations
2 common waysFor a single gene, heterozygote favored
Heterozygote advantage – HS alleleNegative frequency-dependent selection
Rare individuals have a higher fitness
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