Population Genetics and Speciation

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Chapter 16

• Population Genetics and Speciation

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Variation of Traits

• In nature many quantitative traits in a
  population tend to follow a bell curve pattern
• Average (ex height and weight)

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• The report, Mean Body Weight, Height, and Body
  Mass Index (BMI) 1960-2002 United States, shows
  that the average height of a man aged 20-74 years
  increased from just over 5'8" in 1960 to 5'9½" in
  2002, while the average height of a woman the
  same age increased from slightly over 5'3" 1960
  to 5'4" in 2002.
• Meanwhile, the average weight for men aged 20-74
  years rose dramatically from 166.3 pounds in 1960
  to 191 pounds in 2002, while the average weight
  for women the same age increased from 140.2
  pounds in 1960 to 164.3 pounds in 2002.

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Causes of Variation

• Environment
• heredity
• Mutation
• Recombination of genes
• Random pairing of gametes

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• For humans, the number of different gametes is
  223 223, or 8,388,6082, giving
  70,368,744,177,664 (70 trillion) possible
  combinations.

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The Gene Pool

• Total genetic information available in a
  population

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• Allele frequency
• Divide number of certain allele by the total
  number of alleles

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• Phenotype frequency- number of individuals with a
  particular phenotype divided by the total number
  of individuals in the population
• Frequency of a pair? Multiply the individual
  frequency numbers.

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Hardy Weinberg

• Genotype frequencies tend to remain the same in
  a population unless acted on upon outside forces.
  (genetic equilibrium)
• 5 things must happen for this to occur

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• No Mutations
• No individuals enter of leave
• Population is large
• Mating is random
• Natural Selection does not occur
• Where on Earth does this happen?

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Hardy Weinberg Equation

• Calculate frequencies of alleles in population
• P dominant allele
• Q recessive allele
• Problems 1, 2, 3 go over together

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Disrupting Genetic Equilibrium

• Mutation
• Can increase with mutagens
• Spontaneous mutations
• Usually remain fairly low in number
• Introduce new alleles into pop.
• Many are harmful, some are beneficial some are
  neither

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Mutations

• Provide new alleles in gene pool
• Adaptations

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Gene Flow

• Emigration- out
• Immigration- in
• This maintains gene flow
• Ex.- Male lions chase away maturing cubs, they
  must go find another group
• Gene flow.

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Genetic Drift

• Allele frequencies change as a result of random
  events
• This is more applicable to a smaller population
  than to a larger one.

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Causes of Evolution

• Genetic Drift change due to chance
• Bottleneck effect (fire, earthquake, flood)

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Non random mating

• Not good
• Geographic proximity
• Amplify certain traits
• Some select mates based on similarities, this is
  assortative mating

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• Founder effect

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• Small population breaks away from a larger one.
  Rarer alleles are over represented..
• Amish 1770s German population, extreme
  isolation, and intermarriage

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Sexual Selection

• Females choose males, based on certain traits
• Peacock Male is brightly colored (may attract
  predators, but gets the girls to notice you)

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Sexual Selection

• sexual dimorphism bestows upon the females a
  large, pink, pillowy buttocks

sexual dimorphism bestows upon the females a
large, pink, pillowy buttocks

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Natural Selection

• Some members are more likely to survive and
  reproduce- thus their genes will be passed on to
  the next generation
• When Natural Selection is at work, distribution
  of properties may change over time

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Stabilizing Selection

• Its Best to have the average traits
• The ones that survive are lets say, not the
  biggest , or the smallest

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Types of Selection

• Stabilizing

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Disruptive Selection

• Being one extreme or another is better than being
  average

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Directional Selection

• One extreme form of a trait is better for survival

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Formation of Species

• Is called speciation
• Over time animals can change a lot, or a little,
  from ancient ancestors

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Morphological Concept of Species

• Saysspecies are animals that look alike
• Over time, we found that dissimilar animals bred
  and interacted in nature so we had to change
  that..

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Biological Concept of Species

• Species are members that can interbreed

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Modern Concept of Species?

• Look alike
• And, can interbreed

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How can new species arise?

• Geographic Isolation
• Allopatric Speciation
• Reproductive Isolation
• Sympatric Speciation

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Geographic Isolation

• Physical Isolation
• Gene flow between them stops
• Genetic drift
• Mating cannot occur

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Allopatric Speciation

• 2 different species due to geographic isolation

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Reproductive Isolation

• Successful mating is prevented
• Prezygotic- before fertilization
• Post-zygotic- after fertilization

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Reproductive Isolation

• Anatomical incompatibility
• Small male dog and large female dog cannot mate
• Prezygotic barriers- prevent mating
• Postzygotic barriers- mating occurs but offspring
  is infertile

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Sympatric Speciation

• 2 sub populations become reproductively isolated
  within the same geographic area.

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Rates of Speciation

• Sometime millions of years, sometimes a lot less
  (thousand)
• Theory millions of years gradualism
• Theory- sudden bursts- punctuated equibrium