Topic 12: Isolation by Distance Flashcards
What is isolation by distance?
The accumulation of genetic differentiation between subpopulations when there is geographically restricted gene flow
Gene flow is negatively correlated with geographic distance between subpopulations therefore we expect more geographically distant populations to have less gene flow and higher genetic distances
What is the underlying mutational model for Fst?
If we calculate Fst for a network of populations all at once, the underlying model assumes migration is equally likely between all subpopulations (island model)
What is more realistic than the island model?
The stepping-stone model is more realistic. There is less migration and gene flow and therefore greater genetic distances between more geographically distant populations.
How do we test for isolation by distance and identify which model best fits our data?
We can plot genetic distance versus geographic distance between pairs of subpopulations and examine the relationship. There will be genetic distance (a measure from the last two topics) on the y-axis and then geographic distance on the x-axis. The stepping stone model will have a positive linear slope, and the island model will have a flat slope because there is equal exchange of migrants regardless of distance, this is not the case for the stepping stone model.
The pattern observed on a graph is the result of what?
Result of both gene flow and drift (making populations more similar or more distant)
A positive slope indicates that there is isolation by distance and the steeper the slope the stronger the effect of distance on gene flow. The higher the intercept of the relationship, the lower the average rate of gene flow
What are the interpretations of the genetic distance vs geographical distance graphs?
Linear slope: yes IBD, gene flow decreases with increases distance
low linear slope: less pronounced IBD, high gene flow at all distances
Up and flat slope: IBD until it flattens, gene flow at low distances, the point that the slope changes to 0 is the geographic distance at which there can be no more genetic distance.
High and flat: no IBD and low gene flow
Low and flat: high gene flow and no IBD
What is a Mantel test?
A regression used to estimate the slope and intercept of the relationship between genetic and geographic distance.
The Mantel test tells us the statistical significance of the isolation by distance relationship
What does the Mantel test do?
It assesses whether the pairwise genetic distance matrix is correlated with the pairwise geographic distance matrix at one of the matrices.
A permutation test is used because the elements of each matrix are not independent (if one changes then they all change) as they are pairwise matrices
What is a partial mantel test?
You add a third matrix to the mantel test that contains a new variable.
This allows you to statistically control for this new variable, therefore you can test for correlation between x and y (genetic distance and geographical distance) while controlling for z
What question does the partial mantel test allow you to ask?
Does genetic distance vary as a function of geographic distance, after accounting for z?
Or any variation of that.
