Biol 4802 Evolution
Lecture 15, Chapter 9 (a few points)
Topics for today
- Genetic drift
- Factors that increase its effects
- Impact on population fitness
- Other important aspects of population
of a major event happening to you this year
- Make better
predictions with more information
a specific allele persist in a population?
predict for a specific case
drift – random fluctuations in alleles
- Can estimate
the overall probability of fixation
or loss for any given allele
change due to drift is not adaptive
is the fate of a new mutation?
- Initially at
very low frequency
- What is the
chance that it will occur in a gamete that unites to form a zygote?
- What is the
chance that it will end up in an offspring that survives to reproduce?
- What other factors might matter?
- How big is
the population size?
- Does the
allele have positive, negative or neutral effects?
- Is the
allele dominant or recessive?
frequencies fluctuate at random until one allele becomes fixed
- The fate of
an allele may differ in different populations (different runs on POPULUS)
populations with the same the same initial gene frequencies may evolve by
chance to become different genetic entities
frequency of an allele predicts is likelihood of becoming fixed in a
- New mutation
has a frequency of:
pt = 1
- This is also
the probability of the allele becoming fixed (p=1)
- Drift is
stronger in small populations
- Even alleles
with deleterious effects can become fixed
drift in experimental populations
each generation with 8 males and 8 females for 19 generations
population lost bw75
allele at generation 6
- ~1/3 fixed
at p = 0 or p = 1 by generation 19
Fig. 10.8 (Fig. 10.7old)
impacts of genetic drift
- Measures of
population fitness decline with population size
fitness improved with artificial gene flow
and drift have compounding negative effects
impact of genetic drift
- Form large
behavior between genetically distinct colonies
- Produce a
Not in new (Fig.
do we know population size?
1. Is it census size that really matters?
many genes are individuals transmitting?
is the effective population size?
factors that influence effective population size
Fig. 10.5 old and new
to 50% of offspring not sired by social mate
- Skewed sex
selection favoring higher reproduction in different individuals
individuals have more offspring
more likely to occur
elephant seals – no variation at 24 enzyme loci
reduced population size to 20 individuals in 1890
dominant males do most of the mating
a. Original population
- Fluctuation in populations size
a. Each period of small population size is like
on genetic diversity measured as loss of heterozygosity
1. Pertains to fluctuation in population size, bottleneck and founding
a. Rapid recovery of population size reduces
b. Smaller population size before recovery
increases the effect
c. Slow population recovery has increases the
Fig. 10.7 new (Fig.