Temporal isolationGeographic barrierprevents contactHabitat isolationPostzygotic BarriersPrezygotic Barriers- A population or group of populations whose members have the ability to interbreed- The concept of biological speciesdepends on reproductive isolationEcologicalEvolutionary- Species are identified by having a unique combination of traits - Historically physical traits- Now can use DNA sequencesPhylogenetic Biological- Appearance of new species- Source of biodiversity- When two populations are separated for long enough, they will likely accumulate significant changes that will allow them to become separate speciesSpeciationEX: Sheep and goat mating produces an embryo that doesn’t live past a few weeksFertilized egg cannotprocess past an earlyembryoHybrid inviabilityDisadvantages Can be applied toall organisms Advantages EX: Hybrids between the cotton species:Gossypium barbadense, G. hirsutum, and G. tomentosum appears vigorous and fertile, but their progenies die in seed or early in development, or they develop into sparse, weak plantsSeen mostly in plants,where offspring livesand reproduces but itsown offspring usuallydies early indevelopment or early inlifeHybrid breakdownEX: Male horse + Femaledonkey= Mule or hinnyMule gets 32 chromosomes from the horse and 31 from the donkey = 63 chromosomesInterspecies hybridviable but sterile Hybrid sterility- Gametes fail to unite successfully- Important in species that release gametes into water or airGametic isolationSize or incompatible genitalia prevents mating Mechanical isolation- Behaviors important in mate choice- Change in song Behavioral isolationReproduction atdifferent time of the dayof