Population Structure Flashcards
What is the definition of a genetically structured population?
A population that is subdivided in some way so that individuals are more likely to breed with a near neighbour rather than a more distant individual - i.e. deviate from random mating with respect to location
- Can have structure in discrete units - e.g., subpopulations (demes)
- Or can be continuous - e.g., allele frequencies change due to geographical distance
Why is population structure important?
- It allows allele frequencies to vary in diff places - for populations to evolve apart - primarily due to genetic drift
- Is important for local adaption
What effect does gene flow have?
- Gene flow homogenises allele frequencies (limits local adaption) - counteracts population structure effects
What is genetic differentiation?
Gentic differentiation is the accumulation of allele frequencies that differ among sub-populations
What are some of the different models that describe population structure?
- Island - number of discrete pops - equal probability of migration between any population
- Stepping stone / (2D) - sequential stepwise migration between population - move one-by-one
- Continuous - migration throgh continuous range limited by dispersal distance - higher probability of breeding with nearer neighbours
- Source-sink - through the core of a species range to the edge
How can we think of population structure in a hierarchical way?
- Individuals
- Sub-populations
- Regions - groups of sub-pops
- Total population
How can we measure/quantify population structure / genetic differentiation?
F statistics:
- Can be seen as the probability of Identity by Descent (IBD) between 2 sequences or 2 alleles
- With random mating - doesn’t matter where you pick sequences/alleles from, but does matter for this (non-random mating)
What are the 3 types of F statistic?
- Fis = probability of IBD of 2 gene copies in a single individual (‘inbreeding coefficient’)
- Fst = probability of IBD of 2 genes in the same deme
- Fit = probability of IBD of 2 randomly picked genes in whole pop
What does Identity by Descent (IBD) mean?
Means that they have been derived from the same allele copy in the previous generation
What is inbreeding and when does it arise?
- Inbreeding occurs when two individuals that are related mate
- Can be relatives, or because they share ancestry in past - e.g., result of bottleneck or founder effect
What is the result of inbreeding?
- Means that the progeny of inbreeding matings is more likely to be homozygous than if individuals were randomly mating - or from a pop with large Ne
- Recessive deleterious mutations are therefore much more probable to occur in homozygous form
- Therefore more likely to show physical and health defects and reduced fertility/fecundity
- Causes less viable/lower fitness than individuals that are not the result of inbreeding
What is inbreeding depression?
Inbreeding depression is the name given to the reduction in fitness relative to outbred individuals
Where can inbreeding often be observed in?
- Domestic animals - result of selective breeding
- Royalty and Nobility or human ethic groups with consangineous marriages
- Populations of conservation concern
How can you measure inbreeding?
Fis statistic:
- If inbreeding occurs, gametes dont meet at random so the 2 gene copies in an individual have a higher probability of IBD (higher Fis)
- Fis = 0 - Hardy-Weinberg equilibrium
- Fis > 0 : deficity of heterozygots relative to HW expectations - may indicate inbreeding or departure from HW e.g.m pop structure
- Fis < 0 : excess of heterozygotes relative to HW expectations
How is pop structure related to inbreeding?
- Pop structure generates deviation from HW equilibrium and can be considered as type of inbreeding - reducing heterozygosity compared to expected under random mating
- Individuals in sub-pop ate more likely to mate within sub-pop than member of neighbouring sub-pop
