The Evolution of Populations

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Chapter 23

• The Evolution of Populations

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A Common Misconception

• Many people think individuals evolve.
• This is not true.
• Populations evolve as a result of natural
  selection acting on each individual within a
  given population.
• Those individuals better fit to survive are more
  likely to reproduce and pass on genes that will
  benefit future generations.

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Microevolution

• Evolution on a small scale.
• The change in the genetic make up of a population
  from generation to generation.

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Darwins Problem

• Darwins problem was that there were many
  limitations of science at the time.
• He did not have a good explanation for how such
  heritable variations that were required for
  natural selection appeared in a population.
• Nor did he have an explanation for how they were
  transmitted from organisms to their offspring.

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Recall the Blending Hypothesis

• At the time, the blending hypothesis was what
  people used to explain why offspring look like
  both parents.
• Darwin and others realized this was wrong because
  it would eliminate variation within a population.
• Ironically, shortly after Darwin published
  Origin, Gregor Mendel published his paper on
  genetics.

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• Mendels paper went unnoticed for nearly 50
  years.
• In the early 20th century, as scientists
  uncovered the work of Mendel, it became apparent
  that its implications and relatedness to Darwins
  idea were profound.

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Population Genetics

• Scientists so began drawing parallels between
  Darwin and Mendel and melded them into what is
  known as population genetics--the study of how
  populations change over time.

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The Modern Synthesis

• As more was learned about Darwin and Mendel,
  scientists developed the Modern Synthesis--a
  comprehensive theory of evolution that
  incorporates many fields of science.

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Populations

• Populations are groups of individuals that can
  breed with one another and are localized in
  certain regions.
• Some populations are isolated from others.
• Still others can easily mix with other members of
  a population.

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Populations

• Within a population, all of the genes are called
  the gene pool and it consists of all alleles at a
  given locus.
• If only one allele exists in a population, it is
  said to be fixed and all individuals are
  homozygous.
• If more than one allele exists, then individuals
  are either homozygous or heterozygous.

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Allele Frequency

• Consider a population of 500 with 2 alleles, CR
  and CW
• CRCR gives Red
• CWCW gives White
• CRCW gives Pink

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Allele Frequency

• Our Population Breakdown
• 320 red, CRCR
• 160 Pink, CRCW
• 20 White, CWCW
• These numbers suggest a blending hypothesis
• Why cant we use blending?

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The Hardy-Weinberg Theorem

• This theorem is a way to examine how allele
  frequencies change over time when only
  segregation and independent assortment are
  working on the alleles.
• The properties of a non-evolving gene pool--in
  the absence of natural selection.
• The theorem states that the frequencies of the
  alleles will remain constant in a population when
  it is not evolving.

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The Hardy-Weinberg Theorem

• The theorem describes Mendelian inheritance in
  non-evolving populations.
• It also helps us to understand long-term
  evolutionary change--that is, the preservation of
  genetic variation gives the opportunity for
  natural selection to occur.

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Hardy-Weinberg Frequency

• In our population, there are 1000 copies of genes
  (500 individuals, 2 copies).
• 800 of them are CR
• 200 of them are CW
• When we have 2 alleles, by convention we
  represent them as p and q.
• p CR 800/1000 0.8 or 80
• q CW 200/1000 0.2 or 20

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The Hardy-Weinberg Theorem

• To determine the probabilities in our wild flower
  example
• The chance of CRCR is
• p p p2 0.8 0.8 0.64 64
• The chance of CRCW is
• p q 2pq 0.8 0.2 0.32 32
• The chance of CWCW is
• q q q2 0.2 0.2 0.04 4

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The Hardy-Weinberg Equation

• The Hardy-Weinberg Equation becomes p2 2pq
  q2 1
• Again, with a non-evolving gene pool, the
  frequencies of alleles will remain constant if
  mating is random. You can think of it like a
  deck of cards, no matter how many times you
  shuffle them, the types of cards and their
  frequencies remain the same.

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5 Reasons Hardy-Weinberg Doesnt Hold True

• Departure from these 5 conditions results in
  evolution.
• Extremely large population size.
• No gene flow.
• No mutations.
• Random mating.
• No natural selection.

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Mutation and Sexual Recombination

• These provide variety within gene pools.
• Mutations are changes in the nucleotide sequences
  that give rise to new genes and new alleles.
  Sometimes theyre good, usually they are not.
• Most mutations occur in somatic cells and are
  never passed on.
• Only a small percentage of gametes ever get into
  the populations, so any mutation occurring in the
  gametes likely wont get passed on.

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Mutation and Sexual Recombination

• Mutation rates in general are low. The larger
  the organism the less likely a mutation will
  occur and vice-versa.
• For example Plants and animals with long
  generation times are relatively large and have a
  much lower frequency of mutations than do
  microorganism and viruses.

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Mutation and Sexual Recombination

• Sexual recombination is the best way to produce
  variation within a population on a generation to
  generation time scale.
• There are 3 factors which cause the most
  evolutionary change by altering allele
  frequencies
• Natural selection
• Genetic drift
• Gene flow

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1. Natural Selection

• As you know, when organisms are more fit to
  survive, they are more likely to pass on the
  traits that make them better suited for survival
  and this often changes the allele frequency
  within a population.

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2. Genetic Drift

• Genetic drift is an unexpected fluctuation in
  allele frequency from one generation to the next.
  This is often due to a chance event where a
  large proportion of the population is wiped out.

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2. Genetic Drift

• There are two situations which increase the
  likelihood of genetic drift that have a large
  impact on a population
• A. The bottle neck effect
• B. The founder effect

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A. The Bottle Neck Effect

• A sudden change in the environment which
  drastically changes a population can have a
  profound impact on the genetic makeup of the
  population.
• It may change the population in such a way that
  the survivors no longer represent the original
  population.
• The survivors are said to have gone through a
  bottleneck.

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B. The Founder Effect

• When a few organisms become isolated from a large
  population and establish a new population whose
  gene pool is not reflective of the new
  population, we say the founder effect has
  occurred.
• These founders pass through an isolation
  bottleneck and represent a gene pool with altered
  allele frequencies.

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3. Gene Flow

• Gene flow occurs when populations gain or lose
  alleles as organisms come and go within a
  population. Gene flow tends to reduce
  differences between populations.

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Variation

• Variations are heritable differences within a
  population and comprise the raw material for
  diversity and natural selection.
• Only the genetic component of variation can have
  evolutionary consequences as a result of natural
  selection.

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Variation

• Variation within a population comes from either
  discrete characters or quantitative characters
• Discrete-an either or basis determined from a
  single locus.
• Quantitative-comes from 2 or more loci that
  determine the phenotype.

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Fitness

• The adaptive advantage of an organism which
  allows it to make a genetic contribution to the
  gene pool of the next generation.

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Modes of Selection

• Natural selection alters the frequency
  distribution of heritable traits in three ways
• 1. Directional selection.
• 2. Disruptive selection.
• 3. Stabilizing selection.

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Directional Selection

• This is most common when a populations
  environment changes or when members of a
  population migrate to a new habitat with
  different environmental conditions.

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Disruptive Selection

• This occurs when conditions favor individuals in
  both extremes over those of normal average
  phenotypes. It can be important in the early
  stages of speciation.

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Stabilizing Selection

• Acts against extreme phenotypes, it favors the
  intermediates. It reduces variation and
  maintains the status quo of a given phenotype.

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Selection, In General

• Regardless of the mode of selection, selection
  works to favor certain heritable traits through
  differential success.
• Disruptive and stabilizing selection tend to
  reduce variation, but there are methods nature
  uses to preserve it.

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Methods Nature Uses

• 1. Diploidy
• 2. Balancing Selection
• A. Heterozygous advantage
• B. Frequency dependent selection
• 3. Neutral Variation

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Diploidy

• Many eukaryotes are diploid and this hides a lot
  of variation from selection.
• Recessiveness can be transferred from generation
  to generation even if they are harmful because
  they only cause harm when inherited from both
  parents when the zygote is formed.

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Balancing Selection

• Occurs when natural selection maintains stable
  frequencies of two or more phenotypic forms in a
  population.
• Heterozygous advantage--acts in a way that is
  favored by natural selection over either
  homozygous form.
• Frequency dependent selection--the fitness of any
  one morph declines if it becomes too common in a
  population.

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Heterozygous Advantage
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Neutral Variation

• Some of the genetic variation has little or no
  effect on reproductive success. Much of the
  difference we see is found in untranslated parts
  of the genome.
• Confers no advantage--are called pseudogenes.
• Genetic drift can increase or decrease the
  frequency of pseudogenes. Difficult to
  measure--very debatable.

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Sexual Selection

• Sexual selection is natural selection for mating
  success. It can result in sexual
  dimorphism--differences between the sexes in
  secondary characteristics.
• There are two types of sexual selection
• Intrasexual selection.
• Intersexual selection.
• Males are usually the showier sex.

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Intrasexual Selection

• In this, we have direct competition of one sex
  for mates of the opposite sex. A male often
  patrols a group of females and prevents other
  males from mating with her. He is often the
  psychological winner via a ritual that
  discourages competitors. This prevents harm to
  him and increases his own fitness.

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Intersexual Selection

• Individuals of one sex are choosy in selecting
  mates from the other sex. In most cases, a
  females choice depends on the showiness of a
  male.
• Example Peacocks display sexual dimorphism and
  both inter- and intra- sexual selection.

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An Interesting Aside

• Regarding showiness, the most intriguing thing is
  that it is often a hindrance to their survival.
  The benefits, however, seem to outweigh the
  costs. When a female chooses a showier male, she
  is often choosing the healthiest mate with the
  best genes.
• This allows the male to pass his genes on to his
  offspring.

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Natural Selection

• It doesnt fashion perfect organisms
• Evolution is limited by historical constraints.
• Adaptations are often compromises.
• Change and natural selection interact.
• Selection can edit only existing variation.

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1. Evolution is Limited By Historical Constraints.

• Each species comes from a long line of ancestral
  forms.
• Ancestral anatomy isnt scrapped by a new form,
  its a slow change.
• This helps to explain why you dont see an
  example of every species that has ever lived
  preserved in the fossil record.

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2. Adaptations are Often Compromises

• What makes us better in some ways, hinders us in
  others.
• Take the tail of the peacock as an example. It
  makes the bird better in that it allows it to get
  a mate, it hinders it in its ability to evade
  predators.

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3. Chance and Natural Selection Interact

• Chance events can alter a gene pool such as when
  a storm blows birds or insects over an ocean and
  to a new environment. The genes which arrive may
  not be the best in the former population.
• The organisms pass through a bottleneck.

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4. Selection Can Edit Only Existing Variation

• Natural selection favors the fittest phenotype in
  any population, and new variations cant arise on
  demand.