Biol 4802 Evolution

Lecture 15, Chapter 9 (a few points) & 10


Topics for today

    1. Genetic drift
    2. Factors that increase its effects
    3. Impact on population fitness
    4. Other important aspects of population size


Fig. 10.2(old and new)

Probability of a major event happening to you this year

  1. Make better predictions with more information

Will a specific allele persist in a population?

  1. Can’t predict for a specific case
  2. Genetic drift – random fluctuations in alleles
  3. Can estimate the overall probability of  fixation or loss for any given allele
  4. Evolutionary change due to drift is not adaptive

What is the fate of a new mutation?

  1. Initially at very low frequency
  2. What is the chance that it will occur in a gamete that unites to form a zygote?
  3. What is the chance that it will end up in an offspring that survives to reproduce?
  4. What other factors might matter?
    1. How big is the population size?
    2. Does the allele have positive, negative or neutral effects?
    3. Is the allele dominant or recessive?

Summary from simulations

  1. Allele frequencies fluctuate at random until one allele becomes fixed
  2. The fate of an allele may differ in different populations (different runs on POPULUS)
  3. Thus, populations with the same the same initial gene frequencies may evolve by chance to become different genetic entities
  4. The frequency of an allele predicts is likelihood of becoming fixed in a population
  5. New mutation has a frequency of:

pt = 1 


  1. This is also the probability of the allele becoming fixed (p=1)
  2. Drift is stronger in small populations
  3. Even alleles with deleterious effects can become fixed

Genetic drift in experimental populations

  1. 107 populations
  2. Propagated each generation with 8 males and 8 females for 19 generations
  3. First population lost bw75  allele at generation 6
  4. ~1/3 fixed at p = 0 or p = 1 by generation 19

Fig. 10.8 (Fig. 10.7old)

Negative impacts of genetic drift

  1. Measures of population fitness decline with population size
  2. Population fitness improved with artificial gene flow
  3. Inbreeding and drift have compounding negative effects

Positive impact of genetic drift

  1. Form large cooperative super-colonies
    1. Aggressive behavior between genetically distinct colonies
    2. Produce a different odor

Not in new (Fig. 10.8 old)

How do we know population size?

1. Is it census size that really matters?

2.   How many genes are individuals transmitting?

3.   What is the effective population size?

Seven factors that influence effective population size

Fig. 10.5 old and new

  1. Breeding systems

a.   Superb fairy wrens

b.   Socially monogamous

c.   Sexually promiscuous

d.   Up to 50% of offspring not sired by social mate

  1. Skewed sex ratios

a.      Environmental sex determination

  1. Natural selection favoring higher reproduction in different individuals

a.   Larger individuals have more offspring

  1. Overlapping generations

a.   Inbreeding more likely to occur

b.   Plants

  1. Population bottlenecks

a.   Northern elephant seals – no variation at 24 enzyme loci

b.   Hunting reduced population size to 20 individuals in 1890

    1. Few dominant males do most of the mating
  1. Founder effects

a.   Original population

  1. Fluctuation in populations size

a.   Each period of small population size is like a bottleneck

Impact on genetic diversity measured as loss of heterozygosity

1. Pertains to fluctuation in population size, bottleneck and founding effects

a.   Rapid recovery of population size reduces effect

b.   Smaller population size before recovery increases the effect

c.   Slow population recovery has increases the effects

Fig. 10.7 new (Fig. 10.6 old)