Hybridization

Hybridization: matings btwn individuals from two pops or groups of pops, which are distinguishable on the basis of one or more heritable characters

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

Has increased bc of intentional and accidental translocation and habitat mod/frag

Has caused extinctions

It’s also a major evolutionary process


Natural Hybridization and evolution

Plants

  • Many taxa have hybrid origins

  • Hybridization an important mechanism for speciation and adaptation

  • At least 23,675 plant spp hybridize

Animals

  • Focus on reproductive isolation leading to speciation

  • Emphasis on reduced fitness of hybrid offspring leading to reproductive isolation and speciation

  • 3759 fish app and about 10% of bird spp


Intraspecific hybridization

  • interbreeding of individuals among pops

  • even small amounts of gene flow promote cohesion among pops (1 migrant per gen)

  • high amounts of gene flow prevent local adaptation

    • genetic swamping: gene flow causes loss of locally adapted alleles/genotypes

    • reduced fitness of intraspecific hybrids


Pop subdivision and fitness

  • subdivision can promote fitness

  • different recessive alleles will drift to high frequencies in different pops

  • progeny from immigrants expected to have greater fitness

  • large impact of some immigrants means that they may be very important to evolution

  • Hybrids may also have a fitness advantage when they are better suited to environments different to those occupied by their parents


Interspecific hybridization

  • interbreeding of individuals among species

  • speciation: process of divergence of lineages that are sufficiently distinct to follow independent evolutionary paths

  • many independent lineages capable of hybridizing and exchanging genes for a long time without losing phenotypic identities

  • hybridizable frog or bird spp separated 20+ mil years ago

  • Hybridizable mammal spp separated 2-3 mya

  • mtDNA esp prone to introgression

    • mtDNA of one sp has completely replaced the mtDNA of another sp without any evidence of nuclear introgression


Hybrid zones

  • region where 2 spp are sympatric and hybridize to form partially fertile progeny

  • often secondary contact btwn spp that diverged in allopatry

  • hybrids generally selected against, but some zones stable

    • dispersal of parental types and selection against hybrids


Persistence of stable hybrid zones

  • Tension Zone Model

    • Balance btwn parental disperal into hybrid zone and selection against less fit hybrids

    • Hybrid zones independent of environment and just the result of 2 pops coming into contact

  • Bounded Hybrid Superiority Model

    • hybrids are fitter than either parent in the hybrid zone but less fit than parents in the parental zone

    • distribution along an environmental gradient, linear dist

  • Mosaic Model

    • similar to ^ but habitat is patchy rather than gradient or linear

    • Hybrids sometimes separated by parental pops by 300 km

    • parental and hybrid forms each associated with a set of environmental variables

    • hybrids and parental forms selected under different environmental conditions


Anthropogenic Hybridization

  • introductions and habitat modification have increased rates of hybridization

    • previously separated spp come together

    • isolating mechanisms break down

    • decline in abundance, individuals can’t find mates of their own sp


Problems with hybridization

  • Decline and extinction of spp caused by

    • sterility in hybrids which decreases pop growth rate

  • Hybridizations without introgression

    • if hybrids are sterile, no introgression but it reduces reproductive potential


Hybridization without introgression

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

  • can selectively remove all hybrids and non-native spp

  • Fertile hybrids; genetically distinct pops become mixed


Hybridization with introgression

  • if hybrids fertile, parental forms are displaced via hybrid swarms

  • hybrid swarms: all individuals are hybrids

  • genomic extinction: combinations of genotypes over the entire genome are lost causing loss of 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 bc all progeny of hybrids are hybrids


Fitness consequences: heterosis

  • Heterosis/hybrid vigor or superiority

    • hybrids have enhanced performance relative to parental taxa

    • cause:

      • sheltering of deleterious recessive alleles

      • increased heterozygote advantage

    • also possible to observe heterosis in f1 hybrids but outbreeding depression in subsequent gens


outbreeding depression

  • hybrids have decreased performance relative to parental taxa

  • Intrinsic: genetic incompatibilities btwn hybridization taxa

  • Extrinsic: reduced adaptation to env conditions

  • Genetic incompatibilities

    • chromosomal

      • diffs in nnumber/structure

    • genic

      • interactions btwn genes from diff taxa


Extrinsic outbreeding depression

  • reduced fitness of hybrids bc of loss of local adaptation


Detecting and describing hybridization

  • morphological characters

    • but not all have a genetic basis

    • dont know filial gen or whether a backcross

    • important for determining whether parental stock is left and salvageable

  • Genetic analyses

    • loci which have diff freqs in parental taxa

    • diagnostic loci are fixed/nearly fixed


Hybridization and conservation

  • natural part of evolution but anthropogenic hybridization is causing extinction

  • protection of hybrids of listed spp

    • 1977: hybrids protected

      • fish and wildlife defined to protect all offspring

    • 1977 and 19983: hybrids of listed spp not protected - the Hybrid Policy

      • listed spp can be harmed by hybridization

    • 1990: Hybrid Policy suspended

      • would lose biodiversity if didnt protect hybrids

    • 1996: intercross policy published but never finalized or approved

    • no official policy for hybrids


Intentional intra-specific hybridization

  • some pops are very small and have little variation left

  • increase variation thru intentional hybridization

  • outbreeding depression least likely when little gentic divergence btwn pops

    • intraspecific hybrid

  • appropriated when pop have lost considerablle genetic variation and effects of inbreeding depression are apparent

  • pops from similar envs should be used as donor pops

  • genetically similar pops should be used

  • florida and texas panthers