Week 11 Flashcards
population genetic structure is caused by
Geographic barriers to gene flow/habitat patchiness/low dispersal
Historic events
Natural selection
which type of structure is determined by
Interaction of landscape structure and population history evens and natural history of the organism (such as dispersal)
It can be very different for organisms even in the same habitat
ephemeral vs stable populations
Ephemeral - metapopulations, population units interconnected by gene flow, extinction/recolonisation
Stable - continuous population structure and isolation by distance
genetic markers
Genetic variation is sampled appropriately
Population genetic models linking genetic variation and processes
Statistical tests of which models best fit
models should be
Simplify complicated systems so we can see important factors
Allow predictions that can be tested with experiments or observations
discrete subpopulations
island, stepping stone
isolation by distance
continuous
island model
All subdivisions can exchange migrants
Distance does not impede gene flow
Aphids are excellent dispersers
stepping stone
Dispersal is more likely between adjacent subpopulations
Distance does impede gene flow
Gallwasps is an example
isolation by distance (Wright)
Series of overlapping neighbourhoods
Distance does impede gene flow
Many organisms show this pattern
Virtually any species in continuous habitat with somewhat restricted gene flow/dispersal
how can we know which population genetic structure applies?
Different models make different predictions about the relationship between genetic drift and geographic distance
So with genetic data sampled over a range of distances, we can infer what model applies
Fst properties and inferences
A common measure of genetic difference between subpopulations
It cannot be negative - any slight negative values are just estimates near 0
Increases with genetic difference in allele composition between subpopulations
Ranges from 0 to 1
If it is 0, there is no meaningful difference between subpopulations
Most similarity comes from commoner alleles rather than very rare ones
Fst will be larger for a pair of subpopulations when they differ more in their expected heterozygosities for the same alleles
two attributes of Fst
Amount (effect size)
Statistical significance
biological factors that change Fst
neutral: genetic drift/dispersal/habitat structure/time
Small populations diverge faster due to drift
Fst increases as gene flow among demes decreases
Non-neutral: natural selection
Fst vs geographical distance
Steeper means stronger spatial population genetic structure for less-good dispersers (less gene flow)
Is steeper in fragmented than unfragmented habitat
Fragmented habitat has less gene flow so Fst increases with distance
Fst vs natural selection
Increases with diversifying selection strength (selection works to make demes different). Ne makes a lot of difference when selection is stronger
Reduces under balancing selection - works to make demes similar
what are genetic distance models concerned with?
Genetic distance models are fundamentally concerned with connectivity of individuals and genes among populations
More connected populations have a more flat genetic pattern
Connectivity helps with survival
metapopulation
Population of populations
United by extinction and recolonisation
population connectivity
Demographic connectivity + genetic connectivity
Demographics looks at growth rates affected by immigration, emigration, death and birth rate between populations
Genetic connectivity depends on the absolute number of genetically effective dispersers (Nm)
High levels of dispersal might not lead to demographic connectivity if they are countered by low recruitment and survival
Demographic connectivity is very hard to measure in the field (watching immigration, births, deaths…)
Cannot estimate demographic from genetic tho (not always anyway)
gene flow
Matters because it can speed up or slow down evolution
Can alleviate inbreeding
Two types of genetic connectivity are characterised by gene flow and Fst: adaptive connectivity (bring in useful new alleles) and inbreeding connectivity (avoid harmful inbreeding)
