Lecture #2 � Evolution of Populations by 3r8jZs1

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									Lecture #2 – Evolution of
      Populations




                            1
              Key Concepts:
•   The Modern Synthesis
•   Populations and the Gene Pool
•   The Hardy-Weinberg Equilibrium
•   Micro-evolution
•   Sources of Genetic Variation
•   Natural Selection
•   Preservation of Genetic Variation

                                        2
  Some preliminary definitions
• Species – individual organisms capable of
  mating and producing fertile offspring
• Population – a group of individuals of a
  single species
• Community – a group of individuals of
  different species

   Images – species, population, community


                                             3
          The Modern Synthesis
     integrates our knowledge about
                 evolution
•   Darwin’s natural selection
•   Mendel’s hereditary patterns
•   Particulate transfer (chromosomes)
•   Structure of the DNA molecule

    All explain how the genetic structure
     of populations changes over time
                                            4
              KEY POINT

    Environmental factors act on the
individual to control the genetic future of
              the population

Individuals don’t evolve…..populations do
      * ** ** **   ** ** * * * * * *** **
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                       * * *                5
                           * *
Population = a +/- localized group
  of individuals of one species



         Image – population of iris




                                      6
          Critical Thinking
• How do we determine the boundaries of a
  population???




                                            7
           Critical Thinking
• How do we determine the boundaries of a
  population???
  Boundaries are scale dependent
  Some sub-populations overlap
  Some are more isolated
  We can look at populations at many different
   scales – micro to meta



                                                  8
 Recall basic genetic principles:
• In a diploid species (most are), every
  individual has two copies of every gene
  One copy came from each parent
• Most genes have different versions = alleles
• Diploid individuals are either heterozygous
  or homozygous for each gene
  Heterozygous = Aa
  Homozygous = AA or aa

                                             9
  Recall basic genetic principles:
• The total number of alleles for any gene in
  a population is the number of individuals in
  the population x 2
  If the population has 10 individuals, there are
   20 copies of the A gene – some “A” alleles
   and some “a” alleles
• All these alleles comprise the “gene pool”


                                                     10
    Hardy-Weinberg Theorem
• Gene pool = all alleles in a population
• All alleles have a frequency in the
  population
  There is a percentage of “A” and a
   percentage of “a” that adds up to 100%
• Hardy-Weinberg Theorem demonstrates
  that allele frequencies don’t change
  through meiosis and fertilization alone

                                            11
    Hardy-Weinberg Theorem
• A simple, mathematical model
• Shows that repeated random meiosis and
  fertilization events alone will not change the
  distribution of alleles in a population
  Even over many generations

                p2 + 2pq + q2 = 1

      we will not focus on the math – you’ll
                work on this in lab            12
    Hardy-Weinberg Theorem
• Meiosis and fertilization randomly shuffle
  alleles, but they don't change proportions
  Like repeatedly shuffling a deck of cards
  The laws of probability determine that the
   proportion of alleles will not change from
   generation to generation
• This stable distribution of alleles is the
  Hardy-Weinberg equilibrium

         Doesn’t happen in nature!!!            13
Conditions for H-W Equilibrium:
•   No natural selection
•   Large population size
•   Isolated population
•   Random mating
•   No mutation

         Doesn’t happen in nature!!!
    The violation of each assumption acts as
           an agent of microevolution          14
       The value of H-W???
• It provides a null hypothesis to compare to
  what actually happens in nature
• Allele frequencies DO change in nature
• BUT, they change only under the
  conditions of microevolution
  In nature, all the H-W assumptions are violated
• Result – populations DO evolve

                                                15
           Critical Thinking
• What are the limitations of the Hardy-
  Weinberg theorem???




                                           16
           Critical Thinking
• What are the limitations of the Hardy-
  Weinberg theorem???
• The H-W model considers just one trait at a
  time, and assumes that just one gene with 2
  alleles (one completely dominant) controls
  that trait
• Recall your basic genetics – is this
  realistic???

                                          17
            Critical Thinking
• Reality is much more complex for most traits
  in most organisms
  Incomplete dominance or codominance
  More than 2 alleles for many genes
  Pleiotropy – one gene affects multiple traits
  Polygenic traits – multiple genes affect one trait
  Epistasis – one gene affects expression of
   another gene
  Environmental effects on phenotypic expression
• Reproductive success depends on the way
  all genes and traits interact         18
      Individuals Do Not Evolve
• Individuals vary, but populations evolve
• Natural selection pressures make an
  individual more or less likely to survive and
  reproduce
• But, it is the cumulative effects of selection
  on the genetic makeup of the whole
  population that results in changes to the
  species
The environment is a wall; natural selection is a gate
                                                    19
The environment is the wall;
natural selection is the gate

  * ** ** **   ** ** * * * * * *** **

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                  *****      *****      20
              Micro-evolution:
    population-scale changes in allele
               frequencies

•   Natural Selection
•   Genetic Drift
•   Gene Flow           Image – natural variation in
                        flower color; same image for
•   Selective Mating    all these summary slides

•   Mutation

                                                       21
Natural Selection – the essence of
         Darwin’s theory
Differential reproductive success is the only
  way to account for the accumulation of
       favorable traits in a population




                                                               More on this later….
More on this later….




                       Cartoon – beaver with chainsaw paws 
                       “natural selection does not grant
                       organisms what they “need””




                                                                                  22
              Micro-evolution:
    population-scale changes in allele
               frequencies

•   Natural Selection
•   Genetic Drift
•   Gene Flow
•   Selective Mating
•   Mutation

                                         23
Genetic Drift – random changes in allele
frequency from generation to generation
 • Reproductive events are samples of the
   parent population
    Larger samples are
     more representative
     than smaller samples
     (probability theory)        Parent pop = 10% blue



                            1
                            2




                                                         24
Larger pop = ~29% blue      Smaller pop = 100% blue
Genetic Drift – random changes in allele
frequency from generation to generation
 • More pronounced in smaller and/or more
   segregated populations
    Bottleneck effect
    Founder effect
                                  Parent pop = 10% blue



                             1
                             2




                                                          25
Segregated pop = ~29% blue   Segregated pop = 100% blue
Bottlenecking = extreme genetic drift



         Diagram – bottlenecking




                                   26
          Critical Thinking
• What events could cause a bottleneck???




                                        27
            Critical Thinking
• What events could cause a bottleneck???
  Bottlenecks occur when there is an extreme
  and indiscriminate reduction in the
  reproducing population
  Disease
  Herbivory
  Malnutrition
  Major disturbance (flood, fire)
  Human intervention
                                          28
Conservation implications – cheetahs are a bottlenecked species




                    Image – cheetah




                                                           29
                                       Extreme range
                                      reduction due to
                                     habitat destruction
Maps – historic and current range      and poaching
of cheetahs

                                             +
                                       Cheetahs were
                                    naturally bottlenecked
                                     about 10,000 years
                                    ago by the last major
                                     ice age (kinked tail)

                                    The species is at risk
                                        of extinction 30
 Australian Flame Robin, California Condor,
              Mauritian Kestrel
…..and many more, all driven nearly to extinction…..




      Images – bottlenecked and now endangered species




                                                                           31
   Some colorful results of a quick web search on “bottlenecked species”
Founder Effect = extreme genetic drift
 • Occurs when a single individual, or small
   group of individuals, breaks off from a
   larger population to colonize a new habitat
   Islands
   Other side of mountain
   Other side of a river…
 • This small group may not represent the
   allele distribution of the parent population

                                                  32
Founder Effect   33
34
35
Long distance dispersal events can lead to
            the founder effect



        Image – a founding population of
        seeds; possibly also the bird if it’s a
        gravid female




                                                  36
           Critical Thinking
• What do you think follows long distance
  dispersal to a new ecosystem???




                                            37
           Critical Thinking
• What do you think follows long distance
  dispersal to a new ecosystem???
• Adaptive radiation frequently leads to
  many new, closely related species as the
  organisms adapt to new habitat zones in
  their new home
                            1

               Founding     2
               Population   3

                            4                38
              Micro-evolution:
    population-scale changes in allele
               frequencies

•   Natural Selection
•   Genetic Drift
•   Gene Flow
•   Selective Mating
•   Mutation

                                         39
                 Gene Flow
• Mixes alleles between populations
  Immigration
  Emigration
• Most populations are NOT completely
  isolated




                                        40
           Critical Thinking
• Will gene flow tend to increase or
  decrease speciation???




                                       41
          Critical Thinking
• Will gene flow tend to increase or
  decrease speciation???
• Gene flow tends to preserve species by
  shuffling alleles between all sub-
  populations




                                           42
Gene Flow   43
              Micro-evolution:
    population-scale changes in allele
               frequencies

•   Natural Selection
•   Genetic Drift
•   Gene Flow
•   Selective Mating
•   Mutation

                                         44
   Selective Breeding


Image – peacock with mating display




                                      45
          Critical Thinking
• Animal behaviors are obvious examples
• Can you think of others???




                                          46
           Critical Thinking
• Animal behaviors are obvious examples
• Can you think of others???
• Proximity is important even in species that
  do not have mating behaviors
  Many plants and fungi are randomly fertilized
   or pollinated…..but generally the exchange is
   between closer neighbors
                                   Image –
                                   fungi spores


                                                  47
              Micro-evolution:
    population-scale changes in allele
               frequencies

•   Natural Selection
•   Genetic Drift
•   Gene Flow
•   Selective Mating
•   Mutation

                                         48
      Mutations
• Random, rare, but
  regular events                        Diagram – mutations
• The only source of
  completely new traits


                      just for fun…..
Cartoon - jackalope




                                                              49
  Evolution =
random events
              x
    “the gate”
* ** ** **   ** ** * * * * * *** **
               *
               * *                    50
                * * *
        Review: Micro-evolution:
    population-scale changes in allele
               frequencies

•   Natural Selection
•   Genetic Drift
•   Gene Flow
•   Selective Mating
•   Mutation

                                         51
  Sources of Genetic Variation
• Natural selection acts on natural variation
• Where does this variation come from???
  Meiosis
  Mutation
• Additional mechanisms help preserve
  variation (later)



                                                52
Meiosis = key source of variation



        Diagram – meiosis I




                                    53
Diagram – meiosis II




                       54
Random, Independent Assortment
 of Homologous Chromosomes
                     n=2




        Diagram – results of meiosis with n=2




                                                55
 Probability theory reveals that for
  random, independent events:
• If each event has 2 possible outcomes
  In this case, one side of the plate or the other
• The possible number of distribution
  combinations = 2n, where n = the number of
  events
  In this case, the distribution event is the
   distribution of chromosomes to the gametes
  n = the haploid number of chromosomes
• If n is 2, then combinations are 22 = 4
                                                  56
    Random, Independent Assortment
     of Homologous Chromosomes

   n=2


                Diagram – results of meiosis with n=2




    Four
  possible
distributions                                           57
 Probability theory reveals that for
  random, independent events:
• If each event has 2 possible outcomes
  In this case, one side of the plate or the other
• The possible number of distribution
  combinations = 2n, where n = the number of
  events
  In this case, distribution refers to the distribution
   of chromosomes to the gametes
  n = the haploid number of chromosomes
• If n is 23, then combinations are 223 = 8.4
  million!
                                                     58
   Probability is Multiplicative:
   8.4 million x 8.4 million > 70 trillion!!!

That is the number of possible combinations
  of maternal and paternal chromosomes in
  the offspring of a randomly mating pair of
                    humans



                                                59
Recombination     Diagram – recombination
 increases the
    potential
   variation to
     infinity


                                            60
           Critical Thinking
• Can meiosis produce totally new traits???




                                          61
           Critical Thinking
• Can meiosis produce totally new traits???
• No – remember, normal meiosis just
  shuffles the alleles
• Only mutation can make entirely new
  alleles




                                          62
Natural Selection as a Mechanism
   of Evolutionary Adaptation
• Natural selection acts on the variation
  produced by meiosis and mutation
• Selection increases the “fitness” of a
  population in a given environment
• Fitness = ???



                                            63
Natural Selection as a Mechanism
   of Evolutionary Adaptation
• Natural selection acts on the variation
  produced by meiosis and mutation
• Selection increases the “fitness” of a
  population in a given environment
• Fitness = reproductive success
  NOT big, NOT smart, NOT strong
  The production of successful offspring is the
   key
                                                   64
    Natural selection has limits
• Individuals vary in fitness
  Natural selection promotes the most fit
• Selection acts on the phenotype – the
  whole, complex organism
  Results from the combination of many different
   genes for any organism
  These genes are expressed in the whole,
   complex environment
• Selection is always constrained by the
  whole, complex evolutionary history of the
  species                                   65
          Critical Thinking
• Can evolution respond to “needs”???




                                        66
           Critical Thinking
• Can evolution respond to “needs”???
• NO!!!
• Evolution is a combination of random
  events + successful reproduction in a
  given environment
• The environment is the wall; natural
  selection is the gate!!!!
  If the phenotype “works”, the genotype
   passes through the gate
                                            67
   Patterns of Change by Natural
             Selection
• Directional Selection
• Diversifying Selection (AKA disruptive)
• Stabilizing Selection


                   Diagram –
                   patterns of
                   natural
                   selection



                                            68
Remember, all populations exhibit a
    range of natural variation




     Diagram – patterns of natural selection




                                               69
        Directional Selection
• Phenotypes at one extreme of the range
  are most successful
  Color
  Pattern
  Form
  Metabolic processes
                                   Diagram –
• The population shifts to favor   directional
                                   selection
  the successful phenotype
                                                 70
       Diversifying Selection
• Multiple, but not all, phenotypes are
  successful
  Patchy environments
  Sub-populations migrate to new habitats
• The population begins to fragment and
  new species begin to diverge   Diagram –
                                    diversifying
                                    selection



                                                   71
         Stabilizing Selection
• The intermediate phenotypes are most
  successful
  Homogenous environments
  Stable conditions
• The range of variation within the
  population is reduced             Diagram –
                                      stabilizing
                                      selection




                                                    72
           Critical Thinking
• Which selection mode will most quickly
  lead to the development of diversity???




                                            73
           Critical Thinking
• Which selection mode will most quickly
  lead to the development of diversity???
• Diversifying selection tends to produce
  multiple species, and the parent species
  may also persist




                                             74
              Diagram – patterns of selection
directional                                     diversifying




                                                        75
           Critical Thinking
• Can you think of a real-life example of an
  adaptive phenotype???




                                               76
           Critical Thinking
• Can you think of a real-life example of an
  adaptive phenotype???
• Everything!
  Variation is random
  Selection is adaptive




                                               77
 Preservation of Natural Variation
• Diploidy
• Balanced Polymorphism
• Neutral Variation



       Images – natural variation in flower color




                                                    78
 Diploidy – 2 alleles for every gene
• Recessive alleles retained in heterozygotes
  Not expressed
  Not eliminated, even if the recessive trait is
  aa may be eliminated, while Aa is preserved in
   the population
• Recessive alleles function as latent
  variation that may prove helpful if
  environment changes

                                               79
     Balanced Polymorphism
• Heterozygote advantage
• Frequency dependent selection
• Phenotypic variation




                                  80
   Balanced Polymorphism – heterozygote
                advantage
 a mutation in the gene that codes for hemoglobin causes a single amino acid
  substitution in the protein, RBC shape changes from round to sickle shape

Sickle-cell Anemia

                                   Map – global distribution of sickle cell allele

                                   Images – normal and sickled red blood cells




                                                                               81
Balanced Polymorphisms – Frequency
        Dependent Selection
           rare clone is less infected




        Graph – frequency dependent selection results




                                                        82
Balanced Polymorphisms – Phenotypic Variation
 multiple morphotypes are favored by heterogeneous
                (patchy) environment




            Images – balanced polymorphisms in asters and snakes




                                                                   83
          Neutral Variation
• Genetic variation that has no apparent
  effect on fitness
• Not affected by natural selection
• May provide an important base for future
  selection, if environmental conditions
  change



                                             84
Key Concepts: QUESTIONS???
•   The Modern Synthesis
•   Populations and the Gene Pool
•   The Hardy-Weinberg Equilibrium
•   Micro-evolution
•   Sources of Genetic Variation
•   Natural Selection
•   Preservation of Genetic Variation

                                        85

								
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