BIOL204 Cons Gen Session 4

Question 1

Wrights F statistics-
Fis
Fst
Fit

Answer 1

An integrated way of viewing and calculating genetic diversity at three hierarchical levels of population structure. Assessed using HW.

individual diversity within sub-populations

sub-population diversity relative to the total population

individual diversity relative to the total population

Question 2

What is Fis?

Answer 2

FIS (= the ‘inbreeding coefficient’)

The heterozygosity observed among individuals compared to what we’d expect for the subpopulation as a whole (He)
varies from -1 to 1
-1 = excess of homozygotes
1 = excess of heterzygotes

Question 3

What is IBD?

Answer 3

Isolation by distance - the more km distance between populations of the same species the populations will be more genetically distant. not always the case, if there is an obstruction, the pop may have gene flow with a further pop that’s easier to get to.

Question 4

what is the The Wahlund Effect?

Answer 4

A signature of lower heterozygosity than expected might indicate that you have not correctly delineated separate populations within the total population to start with.

Question 5

what is Panmixia

Answer 5

a randomly breeding population

Question 6

What is Fst

Answer 6

Fst shows connectivity between sub populations

High Fst i.e 1 means the populations are not connected
Low Fst i.e 0 means the populations are homogeneous, very connected
An indermediate Fst value shows the degree of connectivity
if Fst sig diff to 0 indicates population structuring

Question 7

What is population sub-structuring?

Answer 7

any deviation from random-mating, and includes phenomena such as inbreeding, associative mating (where reproduction is stratified among genotypes), and geographical subdivision

Question 8

The detection of population sub-structure through Fst may not mean there is no gene flow, what 2 scenarios could be happening?

Answer 8

a) populations have been recently and suddenly isolated from each other = no recent gene flow
b) they are large stable populations but there may be low and on-going gene flow

Question 9

How can we calculate the number of migrants in a population? (Nm) (5)

Answer 9

Indirectly
1) using Fst stats
2) Maximum likelihood estimates - coalescent estimators
3) Distribution of private alleles
Directly
4) Catch-Mark–Recapture
5) Assignment tests - identify ‘foreign’ genotype signatures

Question 10

what are the pitfalls of suing F stats to measure gene flow and how can we get around this?

Answer 10

• you make assumptions before doing your task
• very dependent on the genetic markers you choose. If you choose less variable markers then your Fst value may be too high suggesting there is not much gene flow & may be misinterpretive. Choose more & more variable markers i.e. microsatellites or SNPs.

Question 11

What are the assumptions of F stats? (3)

Answer 11

• An infinite number of populations- this means theoretically could not be capped
• No mutation
• Migrants and residents have same likelihood of reproducing