Lecture #7 | Genetic Drift

Evolution

Change in allele frequencies in a population over time

Genetic Drift

Change in allele frequencies cause by random sampling in populations

• random process

• always acting at some level in real populations

• represents the constant “background noise” of evolution

How can we observe drift over time?

Tracking the frequency (proportion) of one allele

What causes random sampling in real populations?

Any process that has the effect of randomly adding or subtracting fitness irrespective of genotype

Meiosis lottery

One large source of random sampling

ex: oogenesis

• Which allele becomes the ovum

• ¼ alleles end up in the egg and ¾ do not

Other causes for random sampling

• Random environmental events

• Resources and mates can be randomly encountered or lost

• Natural disasters can kill individuals randomly

What does it mean that drift is always happening?

1. Main driver of allele frequency change (evolution) at the genetic level

2. It is the “null hypothesis” when testing for other evolutionary processes

Buri drift experiment

• began with a balanced 8 males and 8 females that are heterozygous

• This was done 19 times

• Most populations in Buri’s experiment fixed (100% frequency) of one of the two alleles

  • Conclusion: Drift caused genetic diversity to be lost

What populations are impacted most be genetic drift?

Smaller populations

• alleles are fixed more rapidly in small populations resulting in a loss of variation

Census size (Nc)

Count of all the individuals in the population

• However, every individual may not contribute to the gene poll

• Ex: animals where a single male monopolize multiple females results in some males not breeding

• Population size may also be fluctuating through time

Effective population size: Ne

The number of breeding individuals in an idealized populations that would show the same amount of genetic drift as seen in the population being studied

• dictated the strength of drift, not Nc

• reductions in effective population size can cause drift to become stronger

Bottlenecking effect

Only a few individuals contribute to a gene pool before population grows

• magnifies the effect of genetic drift

  • example: cheetahs have very low levels of genetic diversity due to multiple bottlenecks

Founder effects

Occurs when some individuals become isolated from a larger population

• very common on islands

How can we detect the effects of genetic drift

Quantify heterozygosity

• Drifts cause populations to lose genetic variation

• We expect populations that have experienced more drift to have lower heterozygosity

Key points about genetic drift

1. its unbiased

2. Its stringer in smaller populations

3. It causes genetic variability to be lost

4. It causes populations to become different

5. Drift causes alleles to fix

How do large fitness effects impact drift

Large fitness effects can easily overcome drift

Small fitness effects (s) cant overcome drift

Relationship between drift in small populations and deleterious alleles

Drift can cause deleterious alleles to fix