Chapter 23 The Evolution of Populations

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Chapter 23The Evolution of Populations

• Hmwk Have your lab manual out and your data
  tables ready for the Hardy-Weinberg lab (I hope
  we get to this). I will be collecting the ch 22
  23 study guides today. Est arrival of
  evolution exam is next wed. (ch 22, 23, 24)

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Chapter 22 Study Guide
A Darwin Themed Valentine!
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Evidence for Evolution

• Comparative Anatomy the comparison of body
  structures between species
• The arms, forelegs, flippers, and wings of
  different mammals are homologous structures that
  represent variations on a structural theme that
  was present in their common ancestor

Vestigial Organs homologous structures that
have little if any current importance to the
organism
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More Evidence..

• Comparative Embryology- closely related organisms
  go through similar stages in their embryonic
  development.
• Ontogeny the development of an individual
  organism is a replay of the evolutionary
  history of the species - phylogeny

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Molecular Evidence

• Molecular Homologies
• Same genetic machinery of DNA/RNA
• Because all organisms share the genetic code, it
  is likely that all species descended from a
  common ancestor.
• Organisms (humans/bacteria) share many genes
  these genes have often acquired different
  functions

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What have we learned about evolution from
molecular evidence?

• Tree of Life
• Homologies that evolved more recently are shared
  only within smaller branches of the tree
• Genetic code is shared by all species because
  they date to the deep ancestral past
• Anatomical similarities are generally reflected
  in their molecules in their genes (DNA) and
  gene products (proteins) Hemoglobin (chicken
  has 69 identical to humans)

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The Great Beyond Nature Blog
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Biogeography the geographic distribution of
species

• Islands are showcases of the influence of
  geography on evolution
• Most island species are closely related to
  species from the nearest mainland or neighboring
  island

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The Fossil Record

• Many transitional forms have been found
• Birds
• Whales

Archeopteryx
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Darwin not the first to suggest evolution

• He studied the evidence, made observations, and
  came up with an explanation for how evolution
  occurs natural selection.
• How would you define natural selection?

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What is natural selection?

• the process by which forms of life having traits
  that better enable them to adapt to specific
  environmental pressures, will tend to survive and
  reproduce in greater numbers than others of their
  kind, thus ensuring the perpetuation of those
  favorable traits in succeeding generations.
• only the organisms best adapted to their
  environment tend to survive and transmit their
  genetic characteristics in increasing numbers to
  succeeding generations
• Name several environmental pressures

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A common misconception..

• Individual organisms evolve during their
  lifetimes misconception
• The evolutionary impact of natural selection is
  only apparent in the changes in a population of
  organisms over time.

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What information was Darwin missing?

• Natural selection, requires an understanding of
  hereditary processes that Darwin could not
  explain.
• Mendel was Darwins contemporary, but his
  discoveries went unappreciated at the time
  Darwin never saw Mendels paper on discrete
  heritable units

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Population Genetics

• The early 20th century brought the melding of
  Mendel and Darwins ideas founding population
  genetics the study of how populations change
  genetically over time.

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What is a population?

• a group of individuals belonging to the same
  species.

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How do you know if two organisms are of the same
species?

• a group of individuals that have the potential to
  interbreed and produce fertile offspring in
  nature.

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Who is in your gene pool?

• the total aggregate of genes in a population at
  one time.

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What is wrong with this book title?
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How do you determine allele frequency in a
population?

• An allele frequency is the proportion of one
  allele relative to all alleles at the locus in
  the population
• Example In a population you draw blood samples
  and do the lab work to determine the hemoglobin
  type of the 50 individuals who comprise the
  population.
• You find
• 20 individuals with only type HbA
• 20 individuals with both type HbA and type HbS
• 10 individuals with only type HbS

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What is the allelic frequency of HbA?

• The frequency of the A allele is equal to the sum
  of all of the A alleles divided by the total
  number of hemoglobin alleles
• 40 A alleles (in 20 HbA/HbA individuals) 20 A
  alleles (in 20 HbA/HbS individuals) divided by
  100 total alleles
• 60/100
• 0.60 or 60
• Determine the allelic frequency of HBs

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Allele frequency

• The frequencies of all alleles will always add up
  to 1 (or 100 of the alleles)
• The total number of alleles (not number of forms
  of the gene) for a given population at a given
  locus will be equal to two times the population
  size
• Except for traits on the Y chromosome, where
  population size and allele count would be the same

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Genotype Frequency

• A genotype frequency is the proportion of a
  population that has one genotype relative to all
  genotypes at a specific locus
• In the previous example, we had 10 homozygous
  sicklers, genotype HbS / HbS, out of 50
  individuals for a genotype frequency of 10/50 or
  one-fifth or 0.20 or 20
• A two allele locus will have three genotypes
  (except for Y-linked traits) and the frequencies
  of the three will add up to 1 or 100

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Determining frequencies

• Wildflower population A (Pink) is completely
  dominant to a (white)
• 20 of the 500 plants are white (aa).
• 320 (AA) and 160 (Aa)
• What are the genotypic frequencies?
• What are the allelic frequencies?

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Microevolution

• Microevolution a change in a populations gene
  frequencies from one generation to the next.
• If we calculate the allelic and genotypic
  frequencies of a population over a span of time
  we can determine whether or not it is changing or
  evolving.

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Hardy-Weinberg Theorem

• The Hardy-Weinberg Theorem describes a
  mathematical relationship that allows the
  prediction of the frequency of offspring
  genotypes based on parental allele frequencies
• It also predicts that allele frequencies will not
  change from one generation to the next, i.e., it
  is an equilibrium or non-evolutionary model as
  long as certain factors are present.

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What factors must be present to create a
non-evolving population?

• Very large population less chance of
  fluctuations in the gene pool
• Isolated from other populations no new genes
  enter or old genes leave
• No net mutations gene pool stays the same
• Random mating no traits are preferred
• No natural selection no traits are more
  beneficial to survival

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Create a Population!

• Grab a blank sheet of paper.
• Create a fictional population of organisms
  (common name/scientific name) that you think
  meets all five requirements of the Hardy-Weinberg
  Theorem.
• Draw the habitat, organisms write out how your
  population meets the five requirements.

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Hardy-Weinberg Equilibrium

• The frequencies of alleles and genotypes in a
  populations gene pool remain constant over the
  generations unless acted upon by agents other
  than sexual recombination (meiosis random
  fertilization).
• p dominant allele, q recessive allele
• P2 2pq q2 1

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Use the Hardy-Weinberg Equation to estimate the
percentage of the population carrying the allele
for an inherited disease

• 1/10,000 babies in the U.S. is born with PKU (aa)
• PKU metabolic disorder that results in mental
  retardation if discovered at birth their
  symptoms can be lessened with a
  phenylalanine-free diet
• Assuming the 5 conditions are met What is the
  frequency of q2 (the PKU phenotype)?

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Lab 8 Population Genetics and Evolution

• Purpose
• To learn about the Hardy-Weinberg law of genetic
  equilibrium, and
• study the relationship between evolution and
  changes in allele frequency by using your class
  to represent a sample population

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8A Estimating Allele Frequencies for a Specific
Trait within a Sample Population

• Gene Free hanging or attached earlobes
• Dominant allele Free hanging earlobe (AA or Aa)
• Recessive allele attached earlobe (aa)

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Topics for Discussion

• What is the percentage of heterozygous free
  lobers (2pq) in your class?
• What percentage of the Gig Harbor High School
  Population is heterozygous for the free lobe
  trait?

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The Hardy-Weinberg Theorem

• 1908 Hardy and Weinberg independently suggested
  a scheme whereby evolution could be viewed as
  changes in the frequency of alleles in a
  population or organisms.
• p2 2pq q2 1.0
• If five conditions are met, the populations
  allele and genotype frequencies will remain
  constant from generation to generation.

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Conditions

• The breeding population is large.
• Mating is random.
• There is no mutation of the alleles.
• No differential migration occurs.
• There is no selection.

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The Hardy-Weinberg Equation

• Describes an existing situation.
• Provides a yardstick by which changes in allele
  frequency, and therefore evolution, can be
  measured.
• One can look at a population and ask Is
  evolution occurring with respect to a particular
  gene locus?

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8B Case Studies

• Case I A Test of an Ideal Hardy-Weinberg
  Population
• The entire class will represent a breeding
  population
• To ensure random mating, choose another student
  at random
• Gender and genotype are irrelevant to mate
  selection
• Initial gene frequency of 0.5 for the dominant
  allele A and the recessive allele a . Genotype
  frequencies 0.25 AA, 0.50 Aa, and 0.25aa
• Your initial genotype is Aa
• Each person gets 4 cards (2 A and 2 a)
  representing the products of meiosis

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Procedure

• Turn over cards and shuffle
• Contribute the card on top to your new offspring
  Your partner should do the same
• One will record this genotype as the first
  offspring.
• Shuffle again and draw the other will record
  the second offspring
• You become your new offspring (change your cards!)

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Questions

• What does the Hardy-Weinberg equation predict for
  the new p and q?
• Do the results you obtained in this simulation
  agree? If not, why?
• What major assumptions were not strictly followed
  in this simulation?

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Case II - Selection

• You will modify the simulation to make it more
  realistic.
• In nature not all genotypes have the same
  survival rate.
• If a person inherits two recessive genes for
  sickle-cell anemia they often do not survive to
  reach reproductive maturity.
• You will assume that the homozygous recessive
  individuals never survive.
• Start with the same genotype and frequencies as
  last time.
• If you get an aa you must try again until you
  get something else. Do not record your aa
  offspring.
• Calculate your new class p and q frequencies.

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Questions

• How do the new frequencies of p and q compare to
  the initial frequencies in Case I?
• What major assumptions were not strictly followed
  in this simulation?
• Predict what would happen to the frequencies of p
  and q if you simulated another five generations.
• In a large population would it be possible to
  completely eliminate a deleterious recessive
  allele? Explain.

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Case III Heterozygote Advantage

• Data from many human populations show an
  unexpectedly high frequency of the sickle-cell
  allele in some populations. This is because
  individuals who are heterozygous are slightly
  more resistant to a deadly form of malaria than
  homozygous dominant individuals. There is a
  slight selection against homozygous dominant
  individuals as compared to heterozygotes.

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Procedure

• Keep everything the same as in Case II except
  that if your offspring is AA, flip a coin.
• If the coin is heads the individual does not
  survive if tails, the individual does survive.
• Take a class survey after generation 5 then go
  to generation 10.

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Questions

• Explain how the changes in p and q frequencies in
  Case II compare with Case I and Case III.
• Do you think the recessive allele will be
  completely eliminated in either Case II or Case
  III?
• What is the importance of heterozygotes (the
  heterozygote advantage) in maintaining genetic
  variation in populations?

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Case IV Genetic Drift

• In each generation, some individuals may, just by
  chance, leave behind a few more descendents (and
  genes, of course!) than other individuals. The
  genes of the next generation will be the genes of
  the lucky individuals, not necessarily the
  healthier or better individuals. That, in a
  nutshell, is genetic drift. It happens to ALL
  populationstheres no avoiding the vagaries of
  chance.
• Genetic drift affects the genetic makeup of the
  population but, unlike natural selection, through
  an entirely random process. So although genetic
  drift is a mechanism of evolution, it doesnt
  work to produce adaptations.

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Procedure

• Divide the class into several smaller
  populations.
• Now go through 5 generations as you did for Case
  I.
• Record the new genotypic frequencies and
  calculate the new frequencies of p and q fo reach
  population.

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Questions

• Explain how the initial genotypic frequencies of
  the populations compare.
• What do you results indicate about the importance
  of population size as an evolutionary force?
• Discuss Hardy-Weinberg Problems

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What leads to genetic variation in a population?
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Mutation

• New genes and new alleles originate only by
  changes in the nucleotide sequence of DNA
• Must occur in gametes to be passed on

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Point Mutations

• A change in one base pair
• Usually harmless Why?
• The genetic code is redundant
• Much of the DNA doesnt code for protein
• Even point mutations within a gene may not alter
  amino acid order

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Gene duplication

• Duplications of chromosome segments are almost
  always harmful
• Often smaller pieces of DNA are introduced into a
  genome through the activity of transposable
  elements if it does not have a severe effect,
  it can persist over generations providing an
  expanded genome with new loci that may take on
  new functions by further mutations and selection
• New genes may arise when the coding portions of
  genes are shuffled within the genome
• Early mammals carried a single gene for detecting
  odors has been duplicated humans today have
  about 1,000 olfactory receptor genes, mice 1,300
• 60 of human receptor genes have been inactivated
  by mutations mice have lost only 20

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Mutation Rates

• plants and animals (1/100,000 genes per
  generation)
• Microorganisms and viruses with short generation
  spans mutations can rapidly generate genetic
  variation
• HIV generation span of about 2 days RNA
  genome (more mutations)
• Single-drug treatments will probably never be
  effective
• Even double-drug treatments rarely work for long
  viruses with double mutations conferring
  resistance to both drugs arise daily
• The most effective AIDS treatments at this time
  are drug cocktails combining several medications

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Sexual Recombination

• Far more important than mutation
• Nearly all variations result from recombination
  shuffling of the existing alleles in the gene
  pool during meiosis

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Natural Selection

• Can alter a populations genetic composition
• Individuals in a population exhibit variations in
  their heritable traits, and those with variations
  that are better suited to their environment tend
  to produce more offspring that those with
  variations that are less well suited.

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Genetic Drift

• The smaller the population, the greater the
  change of deviation from the predicted result
• gene frequencies can fluctuate unpredictably from
  one generation to the next these fluctuations
  are called genetic drift

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• Two situations that can increase the impact of
  genetic drift on a population
• The Bottleneck Effect
• Flood, fire, hunting may drastically reduce the
  size of a population the survivors have passed
  through a restrictive bottleneck, and their
  gene pool may no longer be reflective or the
  original populations gene pool
• The Founder Effect
• Isolation of a few individuals from a larger
  population they may establish a new populations
  whose gene pool is not reflective of the source
  population colonizing an island

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Gene Flow

• Genetic additions to and/or subtractions from a
  population resulting from the movement of
  fertile individuals or gametes
• Humans today move much more freely gene flow
  has become an important agent of change in human
  populations that were previously quite isolated.

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Natural Selection

• The primary mechanism of adaptive evolution
• Discrete characters (either/or)
• Quantitative characters (vary along a continuum)
• Polymorphism (when individuals differ in form)

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Relative Fitness

• An organisms fitness the contribution an
  individual makes to the gene pool of the next
  generation, relative to the contributions of
  other individuals
• Population geneticists define relative fitness as
  the contribution of a genotype to the next
  generation compared to the contributions of
  alternative genotypes for the same locus
• If pink white flowers produce equal numbers of
  offspring and red flowers produce 80 as
  many.The top reproducers are given a 1, red
  would be .8 relative fitness

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Natural Selection can alter the frequency
distribution of traits in three ways

• Directional selection
• Favors variants of one
• extreme both dark and light colored
• mice move into an environ with only
• dark rocks.
• Disruptive selection
• Favors variants at both ends
• for instance different colored
• mice in a patchy environ.
• Stabilizing selection
• Removes extreme variants mice
• of a light and dark color living in an
• environment of rocks of intermediate color

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Why doesnt natural selection reduce variation?
Get rid of unfavorable genotypes?

• Diploidy a considerable amount of genetic
  variation is hidden from selection in the form of
  recessive alleles.
• Recessive alleles that are less favorable than
  their dominant counterparts can persist because
  they are propagated in heterozygous individuals.
  
• The rarer the allele, the greater the protection
  from natural selection.

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• Balancing Selection natural selection maintains
  stable frequencies of two or more phenotypic
  forms in a population a state called balanced
  polymorphism. This selection includes
  heterozygote advantage and frequency-dependent
  selection.
• Heterozygote Advantage malaria/sickle-cell
  anemia
• Frequency-Dependent Selection the fitness of
  any one morph declines if it becomes too common
  in the population. Examples predator/prey
  interactions.

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• Neutral Variation some genetic variation has no
  impact on reproductive success much of the
  human genome has no known use or impact
• Sexual Selection natural selection for mating
  success leads to marked differences between the
  sexes in secondary sexual characterizes
• Intrasexual selection selection within the
  same sex direct competition among individuals
  of one sex for mates of the opposite sex
• Intersexual selection mate choice (usually
  females) are are choosy in their mate selection
  some researchers are testing the hypothesis that
  these sexual advertisements (showy plumage)
  reflect overall health because they do pose a
  risk for attracting predators.

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Why sexual reproduction?

• As a method of rapid population expansion, it is
  far inferior to asexual reproduction.
• The processes of meiotic recombination and
  fertilization generate genetic variation
• Genetic variation is important in resistance to
  disease.

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Why Natural Selection Cannot Fashion Perfect
Organisms

• Evolution is limited by historical constraints.
• Birds have two legs and two wings. Wouldnt it
  be handy for some birds to have four legs for
  running and two wings for flight?
• Adaptations are often compromises
• We humans owe much of our versatility and
  athleticism to our prehensile hands and flexible
  limbs, which also make us prone to sprains, torn
  ligaments, and dislocations. Structural
  reinforcement has been compromised for agility.

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• Chance and natural selection interact.
• When a storm blows insects or birds hundreds of
  miles over an ocean to an island, the wind does
  not necessarily transport the species, that are
  best suited to the new environment.
• Selection can edit only existing variations
• Natural selection favors only the fittest
  phenotypes among those currently in the
  population, which may not be the ideal traits.
  New alleles do not arise on demand.

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Practice Write

• 1989 Do the following with reference to the
  Hardy-Weinberg model.
• Indicate the conditions under which allele
  frequencies (p and Q) remain constant from one
  generation to the next.
• b. Calculate, showing all work, the frequencies
  of the alleles and frequencies of the genotypes
  in a population of 100,000 rabbits of which
  25,000 are white and 75,000 are agouti. (In
  rabbits the white color is due to a recessive
  allele, w, and agouti is due to a dominant
  allele, W.)
• c. If the homozygous dominant condition were to
  become lethal, what would happen to the allelic
  and genotypic frequencies in the rabbit
  population after two generations?

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Practice Write

• Discuss how each of the following contributes
  evidence that evolution has occurred
• a. Paleontology
• b. Geographical distribution
• c. Biochemical studies

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Paleontology

• The succession of forms observed in the fossil
  record is consistent with the major branches of
  descent in the tree of life.
• The oldest known fossils are prokaryotes
• Stromatolites thrived in warm aquatic
  environments and built reefs much the same way as
  coral does today.
• Paleontologists have discovered fossils of many
  transitional forms that link ancient organisms to
  modern species
• fossil evidence that birds descended from one
  branch of dinosaurs
• have found fossilized whales linking these
  aquatic mammals to their terrestrial ancestors

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Geographical DistributionBiogeography

• Closely related species tend to be found in the
  same geographic region
• Australia the marsupials distinct from other
  mammals
• Two organisms may adapt to similar environments
  in similar ways though they evolved from
  different ancestors
• Sugar gliders (marsupials) are similar to flying
  squirrels (placental)
• Islands are showcases of biogeography
• Contain endemic plants and animals
• Most island species are closely related to
  species form the nearest mainland or neighboring
  island
• Two islands with similar environments in
  different parts of the world are populated not by
  closely related species but by species that
  resemble those of the nearest mainland where
  the environment is often quite different
• The finches of the Galapagos (island chains)
  see speciation from island to island

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Biochemical Studies

• DNA if the species are closely related, the
  sequences likely differ at only one or a few
  sites.
• In contrast, comparable nucleic acid sequences in
  distantly related species usually have different
  bases at many sites and may even have different
  lengths
• Over long periods of time insertions and
  deletions accumulate
• Amino Acid Sequences (Proteins)
• mtDNA sequences in humans

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Ch 24 The Origin of Species

• Speciation is the appearance of new species
  the source of biological diversity
• Its not enough to explain how adaptations evolve
  in a population microevolution
• Evolutionary theory must also explain how new
  species originate and develop
• Macroevolution Evolutionary change above the
  species level the cumulative effect of
  speciation over vast tracts of time the
  appearance of feathers during the evolution of
  birds from one group of dinosaurs

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Two Patterns of Evolutionary Change

• Anagenesis the accumulation of changes that
  gradually transform a given species into a
  species with different characteristics.
• Cladogenesis the splitting of a gene pool into
  two or more separate pools, which each give rise
  to one or more new species only cladogenesis
  can promote biological diversity by increasing
  the number of species

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How to define the term species

• The biological species concept a population or
  group of populations whose members have the
  potential to interbreed in nature and produce
  viable, fertile offspring, but are unable to
  produce viable, fertile offspring with members of
  other populations.

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Reproductive Isolation

• The biological species concept hinges on
  reproductive isolation biological factors
  (barriers) that impeded members of two species
  from producing viable, fertile hybrids

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Prezygotic barriers p.474

• Habitat Isolation
• Temporal Isolation
• Behavioral Isolation
• Mechanical Isolation
• Gametic Isolation

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Postzygotic Barriers

• Reduced Hybrid Viability the genes of different
  parent species may interact and impair the
  hybrids development
• Reduced Hybrid Fertility offspring may be
  vigorous but sterile meiosis may be affected
  by the number of chromosomes
• Hybrid Breakdown some first-generation hybrids
  are viable and fertile, but when they mate with
  one another or with either parent species,
  offspring of the next generation are feeble or
  sterile

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Take notes on the rest of chapter 24 noting all
bold faced terms
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Possible Essay Questions

• Describe the modern theory of evolution and
  discuss how it is supported by evidence from two
  of the following three areas a. Population
  genetics b. Molecular biology c. Comparative
  anatomy and embryology
• Define, discuss, and give an example of how each
  of the following isolating mechanisms contributes
  to speciation in organisms. a. Geographical
  barriers b. Ecological (including seasonal)
  isolation c. Behavioral isolation d. Polyploidy