Chapter 16 Population Genetics and Speciation

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Chapter 16Population Genetics and Speciation

• Section 1 Genetic Equilibrium
• Section 2 Disruption of Genetic Equilibrium
• Section 3 Formation of Species

Leopard Seal, Antarctica
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Population Genetics(Microevolution)
Section 1 Genetic Equilibrium

• Study of evolution from a genetic point of view
• Every population has some genetic variation that
  influences fitness
• - Evolution is potentially a continuing process
  in all populations
• Changes in selective factors in the environment
  will almost always be met by evolutionary
  responses
• - leads to shifts in the frequencies of genotypes
  in a population
• Rapid changes will often exceed the capacity of a
  population to respond by evolution
• - decline could lead to extinction

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Darwins finches

• Peter and Rosemary Grant
• The Galapagos Islands normally dry
• El Nino increase rainfall, vegetation flourishes
• La Nina periods of drought
• Reproductive success and survival of individuals
  differed between El Nino and La Nina years
• Caused dramatic evolutionary change

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• Medium ground finch
• Seeds, cracks with beak
• La Nina (drought)
• amount of seeds dropped, seeds became tougher
• population dropped
• 1400 in 1975 to 200 at end of 1977
• larger beaks could crack the larger seeds and
  survived better than those with smaller beaks
• Average beak size increased

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Within a population individuals vary
in observable traits. Traits cover a range that
can be represented by a bell curve
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• El Nino (wet) 1983
• Small seeds in abundance
• Those with smaller beaks handled the smaller
  seeds better, able to survive, produce more
  offspring than those with larger beaks
• Average beak size returned to a lower value

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Population genetics studies the ways in which
populations respond to such selective pressures
with changes in allele frequencies
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Genetic Variation

• Mutation random change in a gene, passed on to
  offspring
• Recombination reshuffling of genes
• Independent assortment
• Crossing-over
• Random Pairing of gametes

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• Gene Pool total genetic information available in
  a population
• When mate at random all combinations of
  different alleles are possible

Peccaries are small, tough relatives of the
modern pig, whose lineage diverged about 40
million years ago. They live in southern Texas,
Arizona, and New Mexico.
The forces of evolution shape and change the
composition of this gene pool and thus the
nature of the population.
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Allele Frequency of a certain allele / total
number of alleles

• B Long bristles on the bodies
• b short bristles
• 15 Individual peccaries in the population? 30
  alleles
• If 6 alleles in this population are b, and 24 are
  B
• Then the frequencies of these alleles are
• 6/30 of the genes in the gene pool are b a
  frequency of 0.2
• 24/30 of the gene in the gene pool are B a
  frequency of 0.8

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Phenotype Frequency of individuals with a
particular phenotype / total number of
individuals in the population
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Hardy-Weinberg Equilibrium

• Set of Assumptions
• No net mutations occur
• Individuals neither enter nor leave the
  population
• The population is large
• Individuals mate randomly
• Selection does not occur
• any exception to these can result in evolution

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No Net Mutations Occur
Section 2 Disruption of Genetic Equilibrium
To be at Equilibrium

• Spontaneous mutations occur constantly at very
  low rates
• If exposed to mutagens, rates can increase
  significantly
• Mutations can produce totally new alleles for a
  trait

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Individuals can neither enter nor leave a
population
To be at Equilibrium

• Size of the population must remain constant
• If individuals move, genes move too
• Immigration movement into a population
• Emigration movement out of a population
• Gene flow process of genes moving from one
  population to another

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Large Population
To be at Equilibrium

• Genetic Drift phenomenon by which allele
  frequencies in a population change as a result of
  random events, or chance

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Individuals Mate Randomly
To be at Equilibrium

• Nonrandom Mating
• Influenced by geographic proximity
• Select a mate with similar traitsAssortative
  Mating
• Sexual selection females tend to choose males
  based on certain traits (Planet Earth Jungles,
  Birds of Paradise)

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Birds of Paradise-Planet Earth
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No Natural Selection
To be at Equilibrium

• Ongoing process in nature
• Some members of a population are more likely than
  others to survive and reproduce and thus
  contribute their genes to the next generation

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Stabilizing Selection individuals with the
average form of a trait have the highest fitness
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Disruptive Selection individuals with either
extreme variation of a trait have greater fitness
than those with the average form of that trait
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Directional Selection individuals that display a
more extreme form of a trait have a greater
fitness than those with an average form
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Black dots represent individuals that die before
passing on their genes.
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Hardy-Weinberg Conditions Animations http//nhscie
nce.lonestar.edu/biol/hwe.html
http//www.accessexcellence.org/AE/AEPC/WWC/1995/h
ardyweinberg.php
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Hardy-Weinberg Equation

• Used to discover the probable genotype
  frequencies in a population and to track their
  changes from one generation to the next
• p2 2pq q2 1 p q 1
• p frequency of the dominant allele (A)
• q frequency of the recessive allele(a)
• p2 is the predicted frequency of homozygous
  dominant people (AA)
• 2pq is the predicted frequency of heterozygous
  people (Aa)
• q2 is the predicted frequency of homozygous
  recessive people (aa)

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Section 3 Formation of Species

• Speciation process of species formation
• Species a single kind of organism.
• -Members are morphologically similar (external
  structure and appearance)
• -Can interbreed to produce fertile offspring.
  (biological species concept)

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How do species give rise to other ones?

• 1. Geographic isolation physical separation of
  members of a population.

Allopatric Speciation happens when species arise
as a result of geographic isolation different
homeland
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No longer experience gene flow, so the gene pools
of each separate population may begin to differ
due to genetic drift, mutations and natural
selection - come in contact with different
environmental pressures
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Reproductive Isolation results from barriers to
successful breeding between population groups in
the same area.

• Pre-zygotic occurs before fertilization
• active at different seasons or times of day
• Post-zygotic occurs after fertilization
• zygote dies
• F1 hybrids have reduced fertility or sterility
• Example mule from horse and donkey

Sympatric Speciation when two subpopulations
become reproductively isolated within the same
geographic area
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Rates of Speciation
Happens at a regular, gradual rate Sudden,
rapid change followed by little to no change
(few million yrs) (few thousand years)