Evolution of Populations

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Evolution of Populations

• Unit V
• Chapter 16

Nothing in Biology makes sense except in the
light of evolution Th. Dobzhansky (1973)
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Genes Variation

• While developing his theory of evolution, Darwin
  did not know how heredity worked
• Without understanding heredity, Darwin was unable
  to explain 2 important factors
• The source of variation central to his theory
• How hereditable traits were passed from one
  generation to the next
• Today, genetics, molecular biology, and
  evolutionary theory work together to explain how
  evolution takes place

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Gene Pools

• A gene pool is the combined genetic information
  of all the members of a particular population
• Recall that a population is a collection of
  individuals of the same species in a given area
  which share a common group of genes
• The relative frequency of an allele is the number
  of times that allele occurs in a gene pool
  compared to the number of times other alleles
  occur

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Relative Frequency of Alleles
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Sources of Genetic Variation

• The two main sources of genetic variation are
  mutations and the genetic shuffling that results
  from sexual reproduction
• A mutation is any change in a sequence of DNA
• Most inheritable differences are the result of
  gene shuffling that occurs during sexual
  reproduction

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Mutations

• Mutation is the ultimate source of variation
  within a population
• Mutation is also a source of evolution
• Mutation Video

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Sexual Recombination

• Most inheritable differences are due to gene
  shuffling that occurs during the production of
  gametes

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Single-Gene and Polygenic Traits

• The number of phenotypes produced for a given
  trait depends on how many genes control the trait
• A single-gene trait is controlled by a single
  gene that has 2 alleles (and thus 2 possible
  phenotypes) Example petal color in flowers
• Most traits are controlled by 2 or more genes and
  are called polygenic traits each gene of a
  polygenic trait often has 2 or more alleles (and
  thus many possible phenotypes) Example heigth

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Natural Selection on Single-Gene Traits

• Natural selection on single-gene traits can lead
  to changes in allele frequencies and thus to
  evolution

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Natural Selection on Polygenic Traits

• Natural selection on polygenic traits can affect
  the distribution of genotypes in any of three
  ways
• Stabilizing selection
• Directional selection
• Disruptive selection

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Stabilizing Selection

• selection is against phenotype with arrows
• selection is against both extreme phenotypes
• intermediate survives and reproduces at a higher
  rate than others
• phenotypic extremes are eliminated, variance has
  decreased
• population has stabilized around mean
• Butremember that mutation and gene flow
  can increase variance by counteracting selection

This type of selection occurs in stable
environments most often
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Directional Selection

• selection against 1 extreme in favor of the
  other extreme
• after time we see a shift in the direction of
  the population

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Disruptive Selection

• selection is against phenotype with arrows
• selection is against intermediate phenotype in
  favor of BOTH extremes
• number of intermediates after a few generations
  is low, but variation is maintained here
• in the real world, this can lead to speciation
• if this occurs long enough and there is barrier
  to gene flow, speciation can occur

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Genetic Drift

• In addition to natural selection, genetic drift
  is a way by which allele frequencies can change
• In the real world, population sizes fluctuate
• Because populations fluctuate in size, sometimes
  there can be changes in allele frequencies due to
  random chance
• These changes are called random genetic drift
• In small populations, individuals that carry a
  particular allele may leave more descendants than
  other individuals, just by change
• Over time, a series of chance occurrences of this
  type can cause an allele to become common in a
  population

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The Power of Genetic Drift

• Genetic drift is a powerful force when a
  population size is very small
• Can and does lead to allele fixation
• Depends on starting frequency (which allele
  becomes fixed)

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Consequences of Genetic Drift

• Consequences of genetic drift
• Can and does lead to fixation of alleles
• Effect of chance is different from population to
  population
• Small populations are effected by genetic drift
  more often than larger ones
• Given enough time, even in large populations
  genetic drift can have an effect
• Genetic drift reduces variability in populations
  by reducing heterozygosity

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Real World Examples of Genetic Drift

• The Bottleneck Effect
• Occurs when only a few individuals survive a
  random event, resulting in a shift in allele
  frequencies within the population
• Small population sizes facilitate inbreeding and
  genetic drift, both of which decrease genetic
  variation

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Real World Examples of Genetic Drift

• The Founder Effect
• Occurs when individuals from a source population
  move to a new area and start a new population
• This new population is often started by
  relatively few individuals that do not represent
  the population well in terms of all alleles being
  represented

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Genetic Drift

• What determines which variants survive the event
  or get to the new location?
• Random chance
• Genetic drift has the larges effect on small
  populations (10-100 individuals)

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Population Genetics

• Hardy-Weinberg Principle states that allele
  frequencies tend to remain constant in
  populations unless something happens
• This situation in which allele frequencies remain
  constant is called genetic equilibrium
• If allele frequencies do not change, the
  population will not evolve
• Hardy-Weinberg is a mathematical model that
  describes the changes in allele frequencies in a
  population
• Allows us to predict allele and genotype
  frequencies in subsequent generations (testable)

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Hardy-Weinberg Principle

• Model assumptions (conditions required to
  maintain genetic equilibrium from generation to
  generation)
• Panmictic population random breeding population
• Large population size n gt 100
• No evolution no selection, no gene flow, no
  genetic drift, no mutation
• These are mathematical assumptions, NOT
  biological assumptions

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Hardy-Weinburg Principle

• The Hardy-Weinburg Principle is neat because it
  can serve as a null hypothesis for evolution
• It can show that evolution is occurring within a
  population

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Hardy-Weinburg Principle

• Let p frequency of allele A
• Let q frequency of allele a
• Let p2 frequency of genotype AA
• Let 2pq frequency of genotype Aa
• Let q2 frequency of genotype aa
• Law says, given assumptions, that within 1
  generation of random mating, the genotype
  frequencies are found to be in the binomial
  distribution p22pqq21 (genotype frequencies)
  and pq1 (allele frequencies)

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Hardy-Weinburg Principle

• P2 2pq q2 1
• Cannot prove the negative if population remains
  in HWE, this doesnt mean evolution hasnt
  occurred
• This may result from the fact that the gene you
  are studying has not changed

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Speciation

• A species is a basic evolutionary unit
• A species is a population that exchanges genes
• A species will have distinguishable
  characteristics that are measurable and
  comparable to other groups
• Evolutionary Species Concept defines a species
  as an individual lineage of ancestor/descendent
  populations which maintain a unique identity
  separate from other such groups and pursue their
  own evolutionary path historical fate.
• Biological Species Concept defines a species as
  a group of actually or potentially interbreeding
  natural populations, which are reproductively
  isolated from other such groups.

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Mechanisms of Speciation

• As new species evolve, populations become
  reproductively isolated from each other
• When the members of 2 populations cannot
  interbreed and produce fertile offspring,
  reproductive isolation has occurred
• Reproductive isolation can develop in a variety
  of ways, including
• Behavioral isolation
• Geographic isolation
• Temporal isolation

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Behavioral Isolation

• Behavioral isolation occurs when two populations
  are capable of interbreeding but have differences
  in courtship rituals or other types of behavior
• this type of isolation will prevent individuals
  of the 2 populations from attempting to mate

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Geographic Isolation

• With geographic isolation, two populations are
  separated by geographic barriers such as rivers,
  mountains, or bodies of water
• Over time, changes that occur in one population
  may be different than those that occur in the
  other population which can lead to speciation

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Temporal Isolation

• Temporal isolation occurs when 2 or more species
  reproduce at different times
• Example many species of orchid all live in the
  same rain forest, however, each species releases
  its pollen only on a single day, preventing
  interbreeding between the different species of
  orchid

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Speciation in Darwins Finches

• Speciation in the Galapagos finches occurred by
• Founding of new populations
• Geographic isolation
• Gene pool changes
• Reproductive isolation
• Ecological competition

Small groups of finches moved from one island to
another, became reproductively isolated, and
evolved into new species
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Isolating Mechanisms Concept Map
Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
produced by
produced by
produced by
which result in
Independentlyevolving populations
which result in
Formation ofnew species