lecture 10 - genetic drift and pop size Flashcards
what is census pop size
Census population size:
○ Everyone in the population
what is breeding pop size
Only reproductive adults
how do you find breeding pop size?
census pop size/ 3
why is census pop divided by 3?
Census pop size divided by 3 because it is based on the assumption that
there are 3 main groups in a population
a. Pre-reproductive: children
b. Reproductive
c. Post-reproductive: elderly
- This also applies to everyone not just humans
eg. neaderthal vs human cranium size
g: paper concluded that difference in cranium size between neanderthals and
humans is because of genetic drift
■ Bigger facial features
- Elongated cranium supports the bigger facial features, control
large bodies
■ Brain functioning not good, no proper social instinct
northern seal ex?
Eg: northern elephant seals
■ Reduced genetic variation because of what humans have done to them
■ Hunting reduced population size to 20 individuals at the end of 1970s
- They have returned to about 30,000 but still lack variation and
genes showing effects of bottleneck still
- When compared to southern elephant seals, they have
less genetic variation
what is Ne?
■ Effective population size
■ Translates census sizes into size of an idealized population showing
same rate of loss of genetic diversity as the real population under study
why is the formula with Nm and Mf not used for humans? when is it used?
This formula is used when there is sex ratio
■ Not applicable to humans as our ratio is mostly balanced
why did cheetas have a bottleneck
heetah: bottleneck reduced genetic variation
■ Not good in reproduction, immune system issues
■ Susceptible to physiological and reproductive impairment
(inbreeding depression)`
what is the average time back to a common ancestor with neutral evolution?
“If the gene is evolving neutrally (that is, with no selection) in a diploid organism,
the mathematics of probability tell us that the average time back to this common
ancestor is 2Ne generations
○ Eg: . if the effective population size is 1000, the coalescence of two genes
occurred about 2000 generations ago
who came up with the coalescent theory?
JFC Kingsman and his theory showed that joining up lineages to
ancestors is called n-coalescence
■ 2 copies merge from common ancestor
describe coalescent theory
■ Population genetic models that focus on the probability that
members of a population share a common ancestor in the past
■ Shows how any two copies of an allele share a common ancestor
in the past; that is, they coalesce to a common ancestor
■ Provides bridge between population genetics models and
molecular data
● Describes how demography, recombination and other
factors affect shapes of gene trees and provides tools for
making statistical inferences from molecular population
genetic data
what is MRCA?
Most recent common ancestor – the first allele that is ancestral to
later alleles
○ To calculate number of generations until coalescence (how far back did
the mutation arrive?)
when is there no heterozygosity? what happens as t increases?
0nly 1 allele: no heterozygosity
As t increases, heterozygosity goes down (until there is fixation, etc)
what impacts heterozygosity?
Number of individuals havve an effect on heterozygosity
■ Lower number of population: more severe drop in heterozygosity due to
genetic drift
