Download Phenotypic Variation

소진화: 개체군 내에서의
유전적변화Microevolution: Genetic
Changes within Populations
Chapter 20
안지훈
20.1 Variation in Natural Populations
 Evolutionary biologists describe and quantify
phenotypic variation
 Phenotypic variation can have genetic and
environmental causes
 Several processes generate genetic variation
 Populations often contain substantial genetic
variation
Microevolution
 Microevolution
• Heritable variation in genetics of a population
 Population
• Individuals of species at same time and place
 Microevolution under strong natural selection
• Example: Antibiotic resistance
Selling Penicillin
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 420.
Phenotypic Variation
 Phenotypic variation
• Heritable variation in appearance and/or function
• Phenotypic plasticity
 Quantitative variation
• Characteristics with a range of variation
• Controlled by multiple genes
 Qualitative variation
• Characteristics with distinct states
• Polymorphisms distinct variants of character
Phenotypic and Quantitative Variation
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 420.
Variation
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 421.
Hardy-Weinberg Principle
 Null models
• Conceptual models that serve as theoretical
reference points to observation
 Hardy-Weinburg principle: Conditions where
diploid organisms have genetic equilibrium
• If conditions met, microevolution does not occur
• If observations do not match null, microevolution
occurring
• Mechanism for persistence of recessive alleles
Hardy-Weinberg Conditions
 No mutations occurring
 Population closed to migration
 Population infinite in size
 All genotypes free of selection
 Random mating with respect to genotype
Population Bottleneck
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 428.
Natural Selection
 Increase in traits that enhance reproduction from
one generation to the next
• If a phenotype is successful, all alleles in
organism are selected
Relative Fitness
 Number of successful offspring compared to
others in population
• Allele frequency increases if individuals carrying it
have more offspring than others
• Relative reproductive success is natural selection
Types of Natural Selection
 Directional selection favors individuals near one
end of phenotypic spectrum
 Stablizing selection favors individuals with
intermediate phenotypes
 Disruptive selection favors individuals with
extreme phenotypes
Directional Selection
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 430.
Stabilizing Selection in Humans
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 431.
Directional to Stabilizing Selection (1)
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 432.
Directional to Stabilizing Selection (2)
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 432.
Disruptive Selection
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 433.
Sexual Selection
 Favors traits that increase mating succes
• May cause sexual dimorphism
• Intersexual selection (structures to lure mates)
• Intrasexual selection (structures to combat samesex rivals)
 May result in structures that reduce survivability
but increase mating success
Sexual Selection
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 434.
Nonrandom Mating
 Occurs when individuals select mates with
similar genotypes
 Inbreeding
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Individuals select genetically related mates
Self-fertilization in many plants and some animals
Small populations
Increased frequency of homozygotes and
recessive phenotypes
20.4 Maintaining Genetic and
Phenotypic Variation
 Diploidy can hide recessive alleles from the
action of natural selection
 Natural selection can maintain balanced
polymorphisms
 Some genetic variations may be selectively
neutral
Diploidy and Recessive Alleles
 Most natural populations have high genetic and
phenotypic variation
 Diploidy prevents harmful, recessive alleles from
leaving populations
• When recessive alleles are rare, most copies are
in heterozygotes
Recessive Allele Frequencies
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 435.
Balanced Polymorphisms
 Balanced polymorphism
• Two or more phenotypes in stable proportions
 Heterozygote advantage
• Maintains high proportions of recessive alleles
• Heterozygotes selectively favored even if
homozygotes are selected against
Heterozygote Advantage
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 436.
Balanced Polymorphisms
 Varying environments favor different phenotypes
• Camouflage from predators varies with
background
 Frequency-dependent selection favors rare
phenotypes until they become common
• Predators often focus on most common prey
Habitat Variation
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 437.
Frequency-Dependent Selection
Russell, Wolfe, Hertz, Starr and McMillan (2008) Thompson Higher Education, 1st ed., p. 438.
Neutral Selection
 Neutral variation hypothesis
• Some genetic variation is selectively neutral
 Not all genetic variation preserved by natural
selection
• Genetic variation proportional to population size
• Bottlenecks
• Genetic drift
20.5 Adaptation and
Evolutionary Constraints
 Scientists construct hypotheses about the
evolution of adaptive traits
 Several factors constrain adaptive evolution
Evolution of Adaptive Traits
 Adaptive traits
• Products of selection that increase relative fitness
 Adaptation
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Accumulation of adaptive traits over time
Adaptation hypotheses must be tested
Current structures came from previous structures
Some traits arise by chance and not selection
Constraints on
Adaptive Evolution
 Adaptations are usually compromises
• Most environments have competing selective
pressures
• Environments constantly change over time
 Adaptation lags environmental change
• Each generation adapted to environment of
parents
• Natural selection never anticipates environmental
change