32 - Genetic Drift and Sampling Error, Notas de estudo de Genética

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GENETIC DRIFT:

Sampling error over

single generations

Natural selection is predictable

• Some genotypes have a higher fitness
  • Higher fitness leads to more offspring
  • Genotypes become “overrepresented”
• If the fitness is known, then change by

natural selection is “ predictable ”

• But not all evolutionary change is predictable...

Random chance matters

• Bag of many marbles
• Exactly half are brown
• Exactly half are blue
• What if we picked exactly 4 marbles?
  • How many of each color would we get?

Random chance matters

• Bag of many marbles
• Exactly half are brown
• Exactly half are blue
• What if we picked exactly 4 marbles?
  • How many of each color would we get? ~5% chance we would get all 4 marbles same color

Random chance matters

• Bag of many marbles
• Exactly half are brown
• Exactly half are blue
• What if we picked exactly 2 marbles?
  • How many of each color would we get? ~50% chance we would get both marbles same color

Sampling error!

• Previous example
  • Picking 4 is likely to get you roughly right proportions
  • Picking 2 is not likely to get you roughly right proportions
  • By picking MORE, you get a more representative sample of the original pool

Sampling error is random in

direction over one generation

• Assuming there is more than one allele , any allele is about equally likely to increase or decrease in frequency in one generation by sampling error
• If p=0.6, about equally likely to be p>0.6 or p<0.6 in next generation - But very unlikely to be EXACTLY p=0.6 again
• Allele frequency “drifts” due to sampling error: genetic drift

Small changes are likely.

Big changes are possible but unlikely.

• Imagine tossing a coin 10 times (similar to p=0.5)
• May get 5 heads
  • Getting >5 heads or <5 heads equally likely
  • Getting 0-1 or 9-10 heads very unlikely
    • 10 heads: ~1/1000 chance

Magnitude of change compounds

and relates to the population size

• Greater changes occur in the allele frequency if the sample (population) is smaller
• Population size = 400
• A 1 starts at 0.

Generations: 20 40 60

Frequency of A

1

Figure 7.15 Evolutionary Analysis, 4/e© 2007 Pearson Prentice Hall, Inc.

Magnitude of change compounds

and relates to the population size

• Greater changes occur in the allele frequency if the sample (population) is smaller
• Population size = 40
• A 1 starts at 0.

Generations: 20 40 60

Frequency of A

1

Figure 7.15 Evolutionary Analysis, 4/e© 2007 Pearson Prentice Hall, Inc.

How big are the individual

steps (on average)?

• “Average change” in allele frequency due to one generation of drift = (pq)/(2N)
• p & q are allele frequencies, N is the population size
• For N=4, p=0.5, q=0.5, average change = 0.
  • Likely to go to p=0.53 or p=0.47, on average – could be more or less
• For N=40, p=0.5, q=0.5, average change = 0.
• For N=400, p=0.5, q=0.5, average change = 0.

Take-home messages

• Drift is strongest in small populations
  • “Average change” due to one generation of drift = (pq)/(2N)
• Drift is neither predictable in direction in one generation nor exactly replicable in degree - Under exact same conditions, get different results from genetic drift
• Drift can cause big changes in allele frequency over time

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