Transcript Table of Contents - Milan Area Schools

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Charles Darwin’s Theory of Evolution
• Observations made on his 5 year trip helped
Darwin formulate his theory of evolution, which
had two major components.
 First, species are not immutable, but change,
or adapt, over time.
 Second, the agent that produces the changes
is natural selection.
Figure 23.1 Darwin and the Voyage of the Beagle (Part 2)
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Charles Darwin’s Theory of Evolution
• Fourteen years after Darwin first made the
observations, Alfred Russel Wallace came to
similar conclusions independently.
• On July 1, 1858, Darwin’s and Wallace’s ideas
were presented to the Linnaean Society of
London.
• In 1859 Darwin published The Origin of Species.
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Charles Darwin’s Theory of Evolution
• Darwin observed that slight variations among
individuals can significantly affect the chance that
a given individual will survive and the number of
offspring it will produce.
• Darwin called this differential reproductive
success of individuals natural selection. (See
handout)
• Darwin clearly understood a fundamental principle
of evolution—that populations, not individuals,
evolve and become adapted to the environments
in which they live.
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Charles Darwin’s Theory of Evolution
• The rediscovery of Gregor Mendel’s publications
gave rise to the study of population genetics
which provides a major underpinning for Darwin’s
theories.
• What is population genetics?
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Genetic Variation within Populations
• Population Genetics studies variation within and
among species in order to understand the
processes that result in evolutionary changes in
species through time.
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Genetic Variation within Populations
• For a population to evolve, its members must
possess heritable, genetic variation, which is the
raw material on which agents of evolution act.
• We observe phenotypes in nature, the physical
expressions of genes.
• The genetic constitution that governs a trait is
called its genotype.
• A population evolves when individuals with
different genotypes survive or reproduce at
different rates.
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Genetic Variation within Populations
• Genes have different forms called alleles.
• A single individual has only some of the alleles
found in the population to which it belongs.
• The sum of all the alleles in a population is the
gene pool.
• The gene pool contains the variation (different
alleles) that produces the differing phenotypes on
which agents of evolution act.
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Genetic Variation within Populations
• Natural populations possess genetic variation.
• For example, selection for traits in a wild mustard
has produced many important crop plants.
• What happened with this?
Figure 23.4 Many Vegetables from One Species
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Genetic Variation within Populations
• The study of the genetic basis of evolution is
difficult because genotypes do not uniquely
determine phenotypes.
• Dominance can lead to a particular phenotype
being expressed by more than one genotype.
• Different phenotypes can also be produced by a
given genotype, depending on environmental
conditions encountered during development.
• Ex. Leaf size
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Genetic Variation within Populations
• A locally interbreeding group within a geographic
population is called a Mendelian population.
• The relative proportions, or frequencies, of all
alleles in a population are a measure of that
population’s genetic variation.
• Biologists can estimate allele frequencies for a
given locus by measuring numbers of alleles in a
sample of individuals from a population.
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Genetic Variation within Populations
• Measurements of allele frequencies range from 0
to 1, and the sum of all allele frequencies at a
locus is 1.
• An allele’s frequency (p) is calculated by dividing
the number of copies of the allele in a population
by the sum of alleles in the population.
• If only two alleles (A and a) for a given locus are
found among the members of a diploid population,
they may combine to form three different
genotypes: AA, Aa, and aa.
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Genetic Variation within Populations
• Allele frequencies can be calculated using
mathematics with the following variables:
 NAA = the number of individuals that are
homozygous for the A allele (AA)
 NAa = the number of individuals that are
heterozygous (Aa)
 Naa = the number of individuals that are
homozygous for the a allele (aa)
 Note that NAA + NAa + Naa = N, the total number
of individuals in a population.
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Genetic Variation within Populations
• The total number of alleles in a population is 2N
because each individual is diploid (in this case,
either AA, Aa, or aa).
• p = the frequency of allele A.
• q = the frequency of allele a.
• For each population, p + q = 1.
Figure 23.6 Calculating Allele Frequencies
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Genetic Variation within Populations
• The two populations in this example have the
same allele frequencies for A and a, but they are
distributed differently. Therefore, the genotype
frequencies of the two populations are different.
• Genotype frequency is the number of individuals
with the genotype divided by the total number of
individuals in the population.
• The frequencies of different alleles at each locus
and the frequencies of different genotypes in a
Mendelian population describe its genetic
structure.
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Homework
•Read pp 465- 470.
•Answer problems on hand
out.
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The Hardy–Weinberg Equilibrium
• A population of sexually reproducing organisms in
which allele and genotype frequencies do not
change from generation to generation is said to be
at Hardy–Weinberg equilibrium.
• Five assumptions must be made in order to meet
Hardy–Weinberg equilibrium.
 Mating is random.
 Population size is very large.
 There is no migration between populations.
 There is no mutation.
 Natural selection does not affect the alleles
under consideration.
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The Hardy–Weinberg Equilibrium
• If the conditions of the Hardy–Weinberg
equilibrium are met, two results follow.
• The frequencies of alleles at a locus will remain
constant from generation to generation.
• After one generation of random mating, the
genotype frequencies will not change.
• The second result can be stated in the form of the
Hardy–Weinberg equation: p2 + 2pq + q2 = 1.
Figure 23.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 1)
Figure 23.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 2)
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The Hardy–Weinberg Equilibrium
• The most important message of the Hardy–
Weinberg equilibrium is that allele frequencies
remain the same from generation to generation
unless some agent acts to change them.
• The equilibrium also shows the distribution of
genotypes that would be expected for a population
at genetic equilibrium.
• The Hardy–Weinberg equilibrium allows scientists
to determine whether evolutionary agents are
operating and their identity (as evidenced by the
pattern of deviation from the equilibrium).
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Evolutionary Agents and Their Effects
• Evolutionary agents cause changes in the allele
and genotype frequencies in a population.
• These are observed as a deviations from the
Hardy–Weinberg equilibrium.
• The known evolutionary agents are mutation,
gene flow, random genetic drift, nonrandom
mating, and natural selection.
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Evolutionary Agents and Their Effects
• The origin of genetic variation is mutation. A
mutation is any change in an organism’s DNA.
• Most mutations appear to be random and are
harmful or neutral to their bearers.
• Some mutations can be advantageous.
• Mutation rates are low; one out of a million loci is
typical.
• Although mutation rates are low, they are
sufficient to create considerable genetic variation.
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Evolutionary Agents and Their Effects
• One condition for Hardy–Weinberg equilibrium is
that there is no mutation.
• Although this condition is never met, the rate at
which mutations arise at single loci is usually so
low that mutations result in only very small
deviations from Hardy–Weinberg expectations.
• If large deviations are found, it is appropriate
to dismiss mutation as the cause and look for
evidence of other evolutionary agents.
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Evolutionary Agents and Their Effects
• Gene flow results when individuals migrate to
another population and breed in their new
location.
• Immigrants may add new alleles to the gene pool
of a population, or they may change the
frequencies of alleles already present if they come
from a population with different allele frequencies.
• No immigration is allowed for a population to be in
Hardy–Weinberg equilibrium.
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Evolutionary Agents and Their Effects
• Genetic drift is the random loss of individuals
and the alleles they possess.
• In very small populations, genetic drift may be
strong enough to influence the direction of change
of allele frequencies even when other evolutionary
agents are pushing the frequencies in a different
direction.
• Organisms that normally have large populations
may pass through occasional periods when only a
small number of individuals survive (a population
bottleneck).
Figure 23.8 A Population Bottleneck
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Evolutionary Agents and Their Effects
• During a population bottleneck, genetic variation
can be reduced by genetic drift.
• Populations in nature pass through bottlenecks for
numerous reasons; for example, predation and
habitat destruction may reduce the population to a
very small size, resulting in low genetic variation.
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Evolutionary Agents and Their Effects
• When a few pioneering individuals colonize a new
region, the resulting population will not have all
the alleles found among members of the source
population.
• The resulting pattern of genetic variation is called
a founder effect.
• Ex. Polydacdaly.
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Evolutionary Agents and Their Effects
• Nonrandom mating occurs when individuals mate
either more often with individuals of the same
genotype or more often with individuals of a different
genotype.
• The resulting proportions of genotypes in the following
generation differ from Hardy–Weinberg expectations.
• If individuals mate preferentially with other individuals
of the same genotype, homozygous genotypes are
overrepresented and heterozygous genotypes are
underrepresented in the next generation.
• Conversely, individuals may mate preferentially with
individuals of a different genotype.
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• Start 9-27-07
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Evolutionary Agents and Their Effects
• For adaptation to occur, individuals that differ in
heritable traits must survive and reproduce with
different degrees of success.
• Alleles that help an organism in a particular
environment would more likely be passed on and
change the frequency of the allele.
• This process is known as natural selection.
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Evolutionary Agents and Their Effects
• The reproductive contribution of a phenotype to
following generations relative to the contributions
of other phenotypes is called its fitness.
• The fitness of a phenotype is determined by the
average rates of survival and reproduction of
individuals with that phenotype.
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The Results of Natural Selection
• Most traits (Characters) are influenced by alleles
at more than one locus.
• For example, the size of individuals in a
population is influenced by genes at many loci,
and distribution of sizes is likely to be a bellshaped curve.
• Natural selection can act on traits with quantitative
variation in three ways:
 Stabilizing selection
 Directional selection
 Disruptive selection
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The Results of Natural Selection
• Stabilizing selection preserves the
characteristics of a population by favoring
average individuals. Ex. Human Birth Weight
Figure 23.13 Human Birth Weight Is Influenced by Stabilizing Selection
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The Results of Natural Selection
• Directional selection changes the characteristics
of a population by favoring individuals that vary in
one direction from the mean of the population.
• Directional selection occurs when one extreme of
a population contributes more offspring to the next
generation.
Figure 23.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 2)
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The Results of Natural Selection
• Disruptive selection changes the characteristics
of a population by favoring individuals that vary in
both directions from the mean of the population.
• Disruptive selection occurs when individuals at
both extremes of a population are simultaneously
favored.
Figure 23.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 3)
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The Results of Natural Selection
• Sexual selection was Darwin’s explanation for the
evolution of apparently useless but conspicuous
traits in males of many species. such as bright
colors, long tails, etc.
• Ex. In widowbirds, males with longer tails attract
significantly more females than do males with
shorter tails.
• Source of non-random mating.
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Assessing the Costs of Adaptations
• Adaptations generally are good and bad.
• Recombinant DNA techniques allow investigators
to compare individuals that differ only in the
genetically based adaptation of interest.
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Assessing the Costs of Adaptations
• Plasmid transfer techniques have been used to
measure the cost associated with the resistance
to an herbicide conferred by a single allele in
Arabidopsis thaliana.
• Plants with the resistance allele produce 34
percent fewer seeds than nonresistant plants,
indicating a high cost for resistance to the
herbicide.
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Maintaining Genetic Variation
• Genetic drift, stabilizing selection, and directional
selection all tend to reduce genetic variation.
• However, when organisms reproduce sexually,
variation is amplified.
• Random assortment of chromosomes during
meiosis, crossing over, and the cellular
component of each gamete contribute to the
diversity of offspring.