Genetic Variation

1
Genetic Variation Evolution

• Chapter 23

2
What you need to know!

• How mutation and sexual reproduction each produce
  genetic variation

3
Variation

• Variations exist between individuals in a given
  population. The following are heritable
• Mutation mostly point mutations (other
  mutations are possible)
• Sexual Reproduction crossing over, independent
  assortment
• Diploidy recessive alleles can be hidden (from
  natural selection) but stay around

4
Variation

• Outbreeding mating with non-related partners
  increases new allele combinations
• Behavioral incest is blocked in most higher order
  species
• Balanced polymorphism different phenotypes in a
  single population propagate natural selection
• Heterozygous advantage advantage to
  heterozygous alleles despite deadly recessive
  alleles (ex. Sickle cell anemia)

5
Variation

• Hybrid Vigor superior quality of offspring when
  crossing highly inbred strains
• Frequency dependent selection predators form
  search images fitting most common prey
  phenotypes
• http//evolution.berkeley.edu/evolibrary/article/h
  appyface_11
• Neutral Variation no selection pressure for
  certain phenotypes
• Silent Mutations mutations that do not impact
  phenotypes (no selection pressure)

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Natural Selection

• Differential reproductive success of different
  phenotypes as a result of the interaction with
  the environment
• Due to genetic variations certain phenotypes are
  better or worse adapted
• Survival depends on the adaptations (fitness)
• Reproduction depends on survival

7
Microevolution

• Def Natural selection causes changes in
  relative frequencies of alleles in the gene pool
  (Hardy Weinberg)
• Adaptation means genes are selected to
• Increase (good)
• Decrease (bad)
• Stay the same (neutral)
• Evolution is the accumulation of advantageous
  traits

8
Stabilizing Selection

• Extreme phenotypes are bad adapatations
• Example primate birth weight
• Too small and the baby dies
• Too large and the mother dies
• Draw

9
Directional Selection

• A single extreme adaptation is good
• Example peppered moths
• Draw

10
Diversifying Selection

• Selection against the common phenotype
• Selection for both extremes
• Example weeds are either very tall (difficult
  to uproot) or very short (survive lawn mowers)

11
Sexual Selection

• Selection for sexual phenotypes
• Male competition fittest male mates most
  (antlers, horns, body size)
• Female choice attractiveness and mating
  behavior
• Sexual dimorphism diifferent appearance of
  males and females in certain species

12
Artificial Selection

• Directional selection carried out by humans
• Humans select a desireable trait and only breed
  animals with the selected trait (dogs, cats,
  sheep, horses, cows, etc.)

13
Imperfection

• Natural selection does not fashion the perfect
  organism because
• Selection only edits existing variation
• Evolution is limited by historical constraints
• Adaptations are often compromises
• Change, natural selection and the environment
  interact

14
Homework

• Concept Checks 23.1 (pg. 458)
• What did Mendels findings about genetics add to
  Darwins theory of evolution by natural
  selection?
• Suppose a population or organisms with 500 gene
  loci is fixed at half of these loci. How many
  alleles are found in its gene pool? Explain.
• Which parts of the Hardy-Weinberg equation (p2
  2pq q2 1) correspond to the frequency of
  individuals that have at least one PKU allele?

15
Homework

• Concept Checks 23.2 (pg. 460)
• Of all the mutations that occur, why do only a
  small fraction become widespread in a gene pool?
• How does sexual recombination produce variation?

16
Homework

• Concept Checks 23.3 (pg. 462)
• In what sense is natural selection more
  predictable than genetic drift?
• Distinguish genetic drift and gene flow in terms
  of (a) how they occur and (b) their implications
  for future genetic variation in a population.

17
Homework

• Concept Checks 23.4 (pg. 470)
• Does nucleotide variability in a population
  always correspond to phenotypic polymorphism?
  Why or why not?
• What is the relative fitness of a sterile mule?
  Explain.
• How does sexual selection lead to sexual
  dimorphism?
• Explain what is meant by the reproductive
  handicap of sex.