Download Natural Selection

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Chapter 23
Population genetics
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Population: a localized group of individuals
belonging to the same species
Species: a group of populations whose individuals
have the potential to interbreed and produce fertile
offspring
Gene pool: the total aggregate of genes in a
population at any one time
Population genetics: the study of genetic changes
in populations
Modern synthesis/neo-Darwinism
“Individuals are selected, but populations evolve.”
Hardy-Weinberg Theorem
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Serves as a model for
the genetic structure of
a nonevolving
population (equilibrium)
5 conditions:
1- Very large
population size;
2- No migration;
3- No net mutations;
4- Random mating;
5- No natural selection
Hardy-Weinberg Equation
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p=frequency of one allele (A);
q=frequency of the other allele (a);
p + q=1.0
(p=1-q & q=1-p)
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P2=frequency of AA genotype;
2pq=frequency of Aa plus aA genotype;
q2=frequency of aa genotype;
p2 + 2pq + q2 = 1.0
Microevolution, I
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New definition of Evolution at the population
level.
 Evolution is a generation to generation
change in a population’ s frequencies of
alleles.
This also can be called microevolution: A
change in the gene pool of a population over
a succession of generations
1- Genetic drift: changes in the gene pool of
a small population due to chance (usually
reduces genetic variability)
Figure 23.4 Genetic drift
Microevolution, II
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The Bottleneck
Effect: type of
genetic drift
resulting from a
reduction in
population (natural
disaster) such that
the surviving
population is no
longer genetically
representative of the
original population
Microevolution, III
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Founder Effect:
a cause of genetic
drift attributable to
colonization by a
limited number of
individuals from a
parent population
Microevolution, IV
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2- Gene Flow:
genetic exchange
due to the migration
of fertile individuals
or gametes between
populations
(reduces differences
between
populations)
Microevolution, V
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3- Mutations: a change in an organism’s DNA
(gametes; many generations); original source
of genetic variation (raw material for natural
selection)
4- Nonrandom mating: inbreeding and
assortive mating (both shift frequencies of
different genotypes)
5- Natural Selection: differential success in
reproduction; only form of microevolution that
adapts a population to its environment
Population variation
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Polymorphism:
coexistence of 2 or
more distinct forms
of individuals
(morphs) within the
same population
Geographical
variation: differences
in genetic structure
between populations
(cline)
Figure 23.8 Clinal variation in a plant
Two Random Processes that
generate genetic variation
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Mutation – new alleles originate only by
mutation. Rare and random events and
usually occur in somatic cells and are not
passed on to the offspring.
Sexual Recombination combines old
alleles with new and fresh assortments
every generation.
Variation preservation
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Prevention of natural selection’s
reduction of variation
Diploidy 2nd set of chromosomes hides
variation in the heterozygote
Balanced polymorphism 1- heterozygote
advantage (hybrid vigor; i.e.,
malaria/sickle-cell anemia);
2- frequency dependent selection
(survival & reproduction of any 1 morph
declines if it becomes too common; i.e.,
parasite/host)
Natural selection
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Fitness: contribution an individual makes
to the gene pool of the next generation
3 types:
A. Directional
B. Diversifying
C. Stabilizing
Figure 23.12 Modes of selection
Sexual selection
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Sexual dimorphism:
secondary sex
characteristic
distinction
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Sexual selection:
selection towards
secondary sex
characteristics that
leads to sexual
dimorphism
16.3 Maintenance of Diversity
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Genetic Variability
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Populations with limited variation may not be able to
adapt to new conditions
Maintenance of variability is advantageous to the
population
Only exposed alleles are subject to natural
selection
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Maintenance of Diversity
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Natural selection causes imperfect adaptations
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Depends on evolutionary history
Imperfections are common because of necessary
compromises
The environment plays a role in maintaining
diversity
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Disruptive selection due to environmental differences
promotes polymorphisms in a population
If a population occupies a wide range, it may have
several subpopulations designated as subspecies
The environment includes selecting agents that help
maintain diversity
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Subspecies Help Maintain Diversity
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Pantheropsis obsoleta obsoleta
Pantheropsis obsoleta quadrivittata
Pantheropsis obsoleta lindheimeri
Pantheropsis obsoleta rossalleni
Pantheropsis obsoleta spiloides
(E.o. lindheimeri, E.o. quadrivittata): © Zig Leszczynski/Animals Animals/Earth Scenes; (E.o. spiloides): © Joseph Collins/Photo Researchers, Inc.;
(E.o. rossalleni): © Dale Jackson/Visuals Unlimited; (E.o. obsoleta): © William Weber/Visuals Unlimited
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Maintenance of Diversity
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Recessive alleles:
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Heterozygotes shelter recessive alleles from selection
Heterozygotes allow even lethal alleles to remain in the
population at low frequencies virtually forever
Sometimes recessive alleles confer an advantage to
heterozygotes
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The sickle-cell anemia allele is detrimental in homozygote
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However, heterozygotes are more likely to survive malaria
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The sickle-cell allele occurs at a higher frequency in malaria
prone areas
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Maintenance of Diversity
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Heterozygote Advantage
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Assists the maintenance of genetic, and
therefore phenotypic, variations in future
generations.
In sickle cell disease heterozygous individuals
don’t die from sickle-cell disease, and they
don’t die from malaria.
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