Frequency of hybridization

• Hybrids occur naturally
  • perhaps 70,000 naturally occurring hybrids
  • is more common in some plant groups than others
    • Sunflowers
    • Birches
    • Oaks
    • Pines
    • Grasses (e.g., Poa)
    • Spleenworts (Asplenium)
    • Columbine (Aquilegia)
    • various Rosaceae (hawthorn, serviceberries, raspoberries, roses)
    • Poplars; aspens
  • Hybridization between different genera occurs, but is rare

Evolutionary outcomes of hybridization

• Reinforces reproductive barriers
  • Selection against less fit intermediate forms
  • Selection reduces "wasting" of potentially more fit progeny
• Formation of hybrid swarms
  • F₁, F₂, etc. generations, plus backcrosses to either or both parents
  • Recombination of parental genotypes
• Hybrid vigor (heterosis) may favor hybrid genotypes in some circumstances, and lead to stabilized hybrid populations
• Introgression: Transfer of genes from one species to another through hybridization and repeated backcrossing
  • May blur the distinctions between species because of gene flow between species (Gilia capitata example)
  • May increase genetic diversity in introgressant
    • New genes (alleles) from foreign species incorporated into plants (Iris example: Fig. 6.20)
    • If gene flow is not extensive enough, species will not merge into one
    • Introgressants may be stabilized, forming a new species
      • Selection for introgressant genotype
      • Reproductive isolation from "pure" parental species
• Hybrid Speciation
  • Many species exhibit characteristics suggesting hybrid origin
  • Most hybrid speciation involves polyploidy, but some examples of hybrid speciation at the diploid level are known
  • Helianthus deserticola
  • Iris nelsonii

Evidence for hybridization

• Intermediate phenotypes
  • Each character exhibits intermediate phenotype
  • Many characters simultaneously exhibit intermediate phenotype
  • Applies to morphology, chemistry, ecology, or most characters controlled by multiple genes (quantitative inheritance)
  • Asplenium flavonoid example (additive characters)
  • Mimulus morphology example (intermediate characters)
• May exhibit parental or novel phenotypes
  • Mostly in phenotypes controlled by single genes
  • Molecular characters
    • Combined parental sequences
    • Chloroplast genome of one species found in plant with mix of parental nuclear markers
• Best evidence
  • Multiple sources of data to avoid falsely attributing intermediate morphology of certain structures to hybridization
  • Presence of both parental species in proximity to putative hybrids
  • Observing reproductive biology and finding mechanisms that make cross-pollination and cross-fertilization possible, if not likely
  • Experimental creation of hybrid by crossing putative parents

Hybrids and Phylogenies