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Evolutionary Biology
Gene flow
| Question | Answer |
|---|---|
| Genetic rescue example | 1995 Florida Panther N≈25 with inbreeding depression 95% likelihood of extinction in 20 years… 1995 – Eight pumas introduced from Texas |
| Genetic rescue example | - Hz increases! - Survivorship increases! - Inbreeding traits decrease! - population recovers! Texas genotypes in red-orange Example of genetic rescue due to gene flow |
| To understand gene flow – first understand evolution in structured populations | Very few species exist as single, panmictic populations – they are structured Underlying causes: – Natural aggregations – Fragmented habitats |
| So? | • Causes population subdivision • Which leads to genetic differentiation among subpopulations |
| Population structure leads to genetic differences between populations | Random = GENETIC DRIFT and MUTATION & RECOMBINATION • Deterministic = SELECTION and GENE FLOW • These processes influence GENETIC POPULATION STRUCTURE Allele frequencies vary in time and space across groups of individuals - effects evolution |
| Genetic differences exist between populations | Spring tail (Lepidocyrtus) – populations just 10km away diverged for 10 million years Beach mouse, Peromyscus polionotus – intraspecific colour polymorphism caused by single amino acid difference - selected in different habitat types |
| Population substructure changes allele frequencies | Deviation from Hardy Weinberg expectations can result from substructure |
| Wahlund effect | “Reduction in Hz caused by structure” |
| Quantifying genetic structure | • Population structure can be quantified using Wrights F (fixation) statistics • F equals the loss of heterozygosity relative to that expected if all individuals (across the entire sample) were randomly mating (i.e. one single panmictic population) |
| F = (Hexp – Hobs)/Hexp | where Hexp is heterozygosity expected under HWE = (2pq) and Hobs is the heterozygosity actually observed |
| Quantifying genetic structure | • Lower Hobs is produced by subdivision into smaller populations - diverging through genetic drift or selection (and/or non-random mating). • A larger F means there is greater population subdivision (breeding within populations) |
| Quantifying genetic structure | Total F= hierarchical levels FIS - population structure (inbreeding) in individuals relative to subpopulations, FIS = (HS - HI)/HS FIT - population structure (or inbreeding) in individuals relative to the total population, FIT = (HT - HI )/HT |
| Quantifying genetic structure | FST measures - population structure (or inbreeding) in subpopulations relative to the total population, FST = (HT - HS)/HT |
| Population structure (Fst) and genetic drift | With no migration, genetic structure will increase over time due to genetic drift Drift acts more rapidly in small populations genetic structure (FST) will therefore accumulates fastest between small populations |
| Population structure (Fst) and genetic drift | FST = 1 – (1 – 1/2N)t Where N is the effective population size, t = time in generations |
| Population structure and gene flow | • Population structure is a dynamic process • Gene flow = movement of gametes or individuals among populations • Important factor determining the level of population structure Gene flow - opposes the effect of genetic drift! |
| Models of gene flow - A. Isolated population distribution – Island Models | • Discrete populations with individuals migrating from one population to another with equal probability (p) • Model is not realistic in natural populations |
| Models of gene flow - B. Continuous population distribution – Isolation-by-distance Model | • Even distribution of individuals • A series of continuous overlapping populations • Individuals less likely to migrate to distant sites • Closer parts of the population will be more genetically similar • Realistic - used in computer simulations |
| Gene flow changes allele frequencies | Using models we can determine the effect of gene flow on allele frequencies e.g. Island model The magnitude of gene flow can be determined by the migration rate (m) |
| Gene flow changes allele frequencies | p1 = allele frequency in recipient population p2 = allele frequency in donor population m = proportion of alleles entering a population through immigration |
| Gene flow changes allele frequencies | • Changes in allele frequency determined by; – the allele frequency difference between populations (p2 - p1) – the level of gene flow (m) • Expected change in allele frequency (per generation) due to gene flow (∆p): ∆p1 = m(p2 - p1) |
| Gene flow can homogenise allele frequencies | • Starting with p1 = 1 and p2 = 0, and M = 0.01 • ∆p1 = m(p2 - p1) • Over 1 generation g1 p1 g1 = p1 + m (p2 - p1) = p1 + 0.01 (0 - 1) = 0.99 • Over 2 generations g2 p1 g2 = p1 g1 + m (p2 - p1’) = 0.98 |
| Gene flow can homogenise allele frequencies | Change is rapid initially….. but as gene frequencies between populations get more similar i.e. (p2 - p1), the rate of change slows |
| Gene flow can homogenise allele frequencies | • Gene flow is a powerful evolutionary process • High migration over a few generations can have a massive effect on allele frequencies • Gene flow over a small number of generations does not necessarily reflect long term population processes |
| Gene flow versus genetic drift | • Drift has a larger impact on small populations, therefore small populations are more likely to diverge |
| But: | • Migration introduces new alleles into each population • Genetic drift and migration are opposing forces |
| But: | • Population differentiation depends on population size and (effective) migration rate…which can be very high when recipient pop is small (n = small)! • Given enough time, populations reach migration - drift equilibrium |
| Gene flow and population size: island model | Wright showed that under an island model of migration at equlibrium, FST is linked to migration rate: FST = 1/(4Nm + 1) |
| Where: | • m is the fraction of immigrants in the population (migration rate) • N = population size • So Nm = number of immigrants each generation differentiation between populations depends on the product of population size and migration rate = Nm |
| Gene flow and population size: island model | The relationship between Nm and population differentiation (structure) Nm = number of immigrants each generation If Nm = 0 (no migration); FST = 1 (different alleles fixed) – divergence! |
| Gene flow and population size: island model | If Nm = 1 (1 migrant per generation); FST = 0.2, drift is counterbalanced by migration (no divergence or convergence!) = Equilibrium If Nm > 1 populations will homogenise – convergence! |
| Estimating gene flow with Fst | Due to the expected relationship between FST and Nm, FST has been used to estimate migration / gene flow 0.006 - little population subdivision 0.714 - significant population subdivision |
| Estimating gene flow with Fst | Mountain sheep - Continuous distribution within suitable habitat Shows clear pattern of Isolation-by-distance within and among mountain ranges |
| Gene flow, drift and selection | Genome-wide structure between populations on different host plants (different colour dots in the same location on map) is stronger (i.e. FST is higher) than between geographically distant populations on the same plant |
| Gene flow, drift and selection | Suggests local adaptation to different plants, which causes a barrier to gene flow Selection acts on gene flow: Isolation-by-adaptation |
| Key messages | • Population structure can lead to genetic differentiation between subpopulations due to genetic drift • Genetic differentiation is usually measured using F statistics |
| Key messages | • Gene flow introduces novel alleles to populations, can homogenise allele frequencies and counteract drift • Selection can act to counter the effects of gene flow • Population differences prevail! |
Created by:
rose.coo
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