RNR 4103 Conservation Genetics: Lect 17 Hybridization

the incorporation of genes from one pop to another through hybridization that results in fertile offspring that further hybridize and backcross to parental populations

•Region where two spp are sympatric & hybridize to form partially fertile progeny
•Often secondary contact between sp pthat diverged in allopatry
•Hybrids generally selected against, but some zones very stable

•Hybrids less fit than parents
–Balance between parental dispersal into hybrid zone and selection against less fit hybrids
–Hybrid zones independent of environment and just the result of 2 pops coming into contact
•Hybrid zone restricted


ex:•Planktonic larvea with long-distance dispersal

Hybrids are fitter than either parent in the hybrid zone but less fit than parents in the parental zone
ex:
Sitka & white spruce
–Distribution along an environmental gradient, linear distribution

–Similar to Bounded Hybrid Superiority Model, but habitat is patchy rather than a gradient or linear
-Parental and hybrid forms each associated with a set of environmental variables; temperature, precipitation, vapour pressure & solar radiation

–E.g. 4 hybridizing western N.Am. oak species
–Hybrids & parental forms selected under different environmental conditions

1.Tension Zone Model
2.Bounded Hybrid Superiority Model
3.Mosaic Model

● Decline & extinction of spp caused by sterility in hybrids, which decreases population growth rate
● Fertile hybrids; genetically distinct populations become mixed
● Heterosis/hybrid vigor or superiority

■ Sheltering of deleterious recessive alleles
■ Increased heterozygote advantage
■ Also possible to observe heterosis in F1 hybrids but outbreeding depression in subsequent generations

hybrids fertile,
parental forms are displaced via hybrid swarms

If hybrids are sterile, no introgression, reduces reproductive potential

•Hybrid swarms: all individuals are hybrids
•Genomic extinction: combinations of genotypes over the entire genome are lost causing loss of an evolutionary lineage
•More difficult than the case of sterile hybrids
•All individuals can be hybrids, so can’t save pure stock
•Introgression may spread even if hybrids have reduced fitness because all progeny of hybrids are hybrids

•Even if no introgression, loss of reproductive effort can cause pops to decline

•Hybrids have decreased performance relative to parental taxa

•Intrinsic: Genetic incompatibilities between hybridizing taxa

•Extrinsic: Reduced adaptation to environmental conditions

•Reduced fitness of hybrids b/c of loss of local adaptation