Chapter 16: Population Genetics

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Chapter 16Population Genetics Speciation

• Ch 16 will link the understanding of the theories
  of natural selection evolution with principles
  of genetics.

http//sps.k12.ar.us/massengale/population_genetic
s_notesbi.htm
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I. Genetic Equilibrium

• Traits vary within a population
• Population Genetics
• the study of evolution from a
• genetic point of view.
• Population biologists study
• many different traits in
• populations -such as size and color.

Charles Darwin's first sketch of an evolutionary
tree from his First Notebook on Transmutation of
Species (1837) http//en.wikipedia.org/wiki/Specia
tion
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2. Microevolution

• a small- scale change in the collective genetic
  material (alleles) of a population.
• Microevolution can be contrasted with
  macroevolution which is the occurrence of
  large-scale changes in gene frequencies, in a
  population, over a geological time period (i.e.
  consisting of lots of microevolution).

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• remember
• - alleles -are the variations in genes that code
  for traits)
• population - is a group of individuals of the
  same species that routinely interbreed.
• A population is the smallest group in which
  evolution is observed.
• Individuals do not evolve, populations do.

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3. Standard Bell Curve

• Traits vary among individuals can be mapped
• Shows that most individuals have average traits
• a few individuals have extreme traits.

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4. Causes of Variation of Traits

• Variations in genotype arise by
• -mutation (random change in a gene)
• -recombination (reshuffling of genes in an
  individual- remember meiosis- crossing over,
  Independent assortment)
• -random pairing of gametes (many gametes, chance
  union)

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B. The Gene Pool

• The total genetic information available in a
  population.
• Imagine a pool with all the possible genes for
  the next generation in it. - then make a set of
  rules to predict expected genotypes
• So a gene pool is the sum of all
• the individual genes in a given
• population.

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1. Allele frequency

• The frequency of an allele is the number of
  occurrences of that allele in that population
• -Within a gene pool, every allele or gene variant
  has a particular ratio or frequency.
• -is determined by dividing the total number of a
  certain allele by the total number of alleles of
  all types in the population.

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2. Review of phenotype genotype

• Genotype- is the actual genetic information (the
  combo of alleles for traits.)
• Three different genotypes
• BB (homozygous dominant)
• Bb (heterozygous)
• bb (homozygous recessive)
• Phenotype- what is seen.
• 3 genotypes , in complete dominance- show 2
  phenotypes
• long bristles (BB and Bb),

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3. Predicting Phenotype

• Phenotype frequency
• Predicting Phenotype
• Phenotype frequency is equal to the number of
  individuals with a particular phenotype divided
  by the total number of individuals in the
  population.
• b. Counting calculating
• 1. Count the alleles of each type in each
  generation. Example- 12 R, 4r total 16
  alleles in 8 individual in 1st generation
• 2. Divide the type of each allele by the total
  number of alleles.
• Example- 12/16 R 0.75 4/16 r 0.25

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Phenotype Frequency
The four oclock flower illustrates how phenotype
changes from generation to generation. Compare
1st 2nd generations. Note that although the
phenotypes change the allele frequencies remain
the same.
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Gene pool
EXAMPLE
15 individuals in the population (each organism
has 2 alleles per trait), thus 30 alleles for
trait - if 6 alleles in this population are of
the b variety, 24 are of the B variety, then
frequencies of alleles are 6/30 of the genes
in the gene pool are b - a frequency of 0.2
6/24 of the gene in the gene pool are B - a
frequency of 0.8. Together, 0.2 0.8 1.0 (all
the genes, 100)
http//www.brooklyn.cuny.edu/bc/ahp/LAD/C21/C21_Ge
nePool.html
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Law of probability The chances of 1 gamete
having an allele meeting with any other allele
is expressed

• frequency of R X frequency of R frequency of
  RR pair
• (example 0.75 X 0.75 0.5625)
• Frequency of r X frequency of r frequency of rr
  pair
• (example 0.25 X 0.25 0.0625)
• So that the frequency of Rr can be figured out by
  subtracting the sum of RR rr from 1.0.
• (example 1.0 (0.5625 0.0625) 0.375 (Rr
  pairing)

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C. Hardy-Weinberg Genetic Equilibrium

• Englishman HARDY German WEINBERG
• Showed frequency of alleles in a population stays
  constant for generations if certain conditions
  are fulfilled.
• In other words-Allele frequencies in the gene
  pool do not change unless acted upon by certain
  forces.
• Hardy-Weinberg genetic equilibrium is a
  theoretical model of a population in which no
  evolution occurs
• the gene pool of the population is stable.

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Five conditions for hypothetical H-W population

• 1. No net mutations occur ( alleles remain the
  same)
• 2. No Individuals enter or leave the population
  (Immigration or Emigration)
• 3. The population is LARGE
• 4. Individuals must mate randomly
• 5. Natural selection does not occur.
• Genetic Equilibrium is a theoretical state.
  Real populations probably do not meet all these
  conditions. Use equation to see causes of
  DISRUPTION of genetic equilibrium

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II. Disruption of Genetic Equilibrium

• Evolution may take place when populations are
  subject to genetic mutations, gene flow, genetic
  drift, nonrandom mating, or natural selection.
• Mutation
• Changes in the DNA.

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B. Gene Flow

• 1. Immigration movement of individuals into
  the group
• 2. Emigration-movement of individuals out of the
  group
• Emigration and immigration cause gene flow
  between populations and can thus affect gene
  frequencies.
• Example- males of baboon troops- fight for
  dominance of group of females. Females tend to
  stay in troop born into. Less dominant or
  younger males move to a different troop. This
  ensures gene flow.

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Genetic Drift

• Genetic Drift- the phenomenon by which allele
  frequencies in a population change as a result of
  random events or chance.
• Genetic drift refers to the expected population
  dynamics of neutral alleles (those defined as
  having no positive or negative impact on fitness)
• (Natural selection describes the tendency of
  beneficial alleles to become more common over
  time (and detrimental ones less common), genetic
  drift refers to the tendency of any allele to
  vary randomly in frequency over time due to
  statistical variation alone.)

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C. Large populations

• Large populations tend to stabilize allele
  frequencies.
• Genetic drift is more pronounced in small
  populations where failure of even a single
  individual to reproduce can change allele
  frequencies in the next generation.
• See graph page 322

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D. Non- random mating (Sexual selection)

• Mating is nonrandom whenever individuals may
  choose partners.
• Sexual Selection
• Sexual selection occurs when certain traits
  increase an individuals success at mating.
• Sexual selection explains the development of
  traits that improve reproductive success but that
  may harm the individual.

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• Females are the limiting sex
• - invest more in offspring than males
• -many females are unavailable for fertilization
  (because they are carrying for young or
  developing young)
• -males tend to be in excess
• Sexual selection arises in response to either
• 1. Female Choice Intersexual selection, in which
  females choose males based upon elaborate
  ornamentation or male behaviors, or
• 2. Male Competition Intrasexual selection, in
  which males compete for territory or access to
  females, or areas on mating grounds where
  displays take place. Male-male competition can
  lead to intense battles for access to females
  where males use elaborate armaments (e.g., horns
  of many ungulates).

http//bio.research.ucsc.edu/barrylab/classes/ani
mal_behavior/SELECT.HTM
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E. Natural selection

• One of the most powerful agents of genetic change
• can influence evolution in one of three general
  patterns
• 1. Stabilizing selection- favors the formation
  of average traits.
• 2. Disruptive selection -favors extreme traits
  rather than average traits.
• 3. Directional selection -favors the formation
  of more-extreme traits.

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Stabilizing Selection

• Reduces variation
• Favors individuals with an average phenotype over
  the extremes.
• Example
• very large fish cannot hide under rocks
• very small fish move too slowly
• predators eat both of these extremes
• average sizes survive best

Next 3 diagrams http//bio.research.ucsc.edu/b
arrylab/classes/animal_behavior/SELECT.HTManchor2
69237
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Disruptive Selection

• Selects for phenotypes at both extremes
• can creative two distinct distributions from a
  single distribution.
• Example
• large small seeds available to eat
• Birds with very large and very small beaks
  survive best
• Average sizes not best suited for survival.

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Directional Selection

• -A response to a change in the environment can
  select for traits above or below average
• - we see a shift in the mean for the trait
  (either up or down)

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III. Formation of Species

• A. Definition of species
• 1. Morphological- a species is a populations
  of organisms that look alike (same structures
  appearance)
• 2. Biological -a species is a population of
  organisms that can successfully interbreed but
  cannot breed with other groups.
• Combined definition- a species is a group of
  organisms that look alike can successfully
  interbreed to create fertile offspring.

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B. Isolation Speciation

• 1. Geographical Isolation Allopatric
  speciation
• Results from the separation of population
  subgroups by geographic barriers.
• Geographical Isolation may lead to allopatric
  speciation (Happens when species arise as a
  result of geographical isolation)

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2. Reproductive Isolation

• results from the separation of population
  subgroups by barriers to successful breeding.
• a. Prezygotic isolation occurs before
  fertilization.
• examples- different sizes-body structure
  prevents mating, different mating
  ritual or behavior, different
  breeding time, not recognizing songs or calls.
• b. Postzygotic isolation occurs after
  fertilization.
• examples- embryo does not develop or
  creates a hybrid organism that is
  infertile or weaker

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Sympatric speciation

• Reproductive isolation within the same geographic
  area is known as sympatric speciation.
• May occur to give adaptive advantage to organisms
  that use slightly different niches.

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C. Rate of Speciation

• Gradualism
• -The gradual model of speciation
• -species undergo small changes at a constant
  rate.
• 2. Punctuated Equilibrium
• - new species arise abruptly
• - differ greatly from their ancestors, and then
  change little over long periods.

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The illustration below shows two contrasting
models for rates of speciation.

• Which model of speciation rates is illustrated by
  model A in the graph?
• F. gradualism
• G. sexual selection
• Gradualism Punctuated
  equilibrium

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Questions

• 1. Which type of selection
• is modeled in the illustration?
• What might cause this ?
• 2. What is the term for the total
• genetic information in a population?
• 3. Saint Bernards and Chihuahuas (two breeds of
  domestic dogs) cannot normally mate because they
  differ so much in size. Thus, they are
  reproductively isolated to some extent. What type
  of isolating mechanism is operating in this case?

Directional, change in the environment.
Gene pool prezygotic
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Hardy Weinberg Equation

• The gene frequency of a population in
  Hardy-Weinberg Equilibrium is written as pp
  2pq qq
• where p the frequency of the dominant allele,
  and q the frequency of the recessive allele. It
  follows that p q 100 of all the genes in the
  gene pool.
• When you have allele frequencies, you can then
  calculate genotype frequencies using the H-W
  equation, (AA) p2, (Aa) 2pq, and (aa) q2.

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Example

• 16 pigs with 4 of them black (recessive aa).
• 16 pigs are 100
• 4 pigs are 25 (aa 25) therefore q20.25
  -------gt q0.5 p q 1 ----------gtp 0.5 1
  -----gt p 0.5
• AA (homozygous) are p2----gt0.5X0.5 0.25 25
• 2Aa (heterozygotus) are 2pq----gt 2 x (0.5) x
  (0.5) 0.5 50 so the equation is AA 2Aa
  aa 1 25 50 25 1