Lecture 21BIOEE 1780 1st Edition Lecture 21 Outline of previous lectureI. Speciation videoII. SpeciesIII. Speciation A) Barriers of gene flowOutline of current lectureI. SpeciationII. Reproductive barriersA) PrezygoticB) PostzygoticIII. Selection and speciationIV. Hybrid zoneA) Secondary hybrid zoneB) Primary hybrid zoneCurrent lectureI. Speciation *Term review: sympatric speciation, allopatric speciation, reproductive barriers, metapopulationsII. Reproductive barriers*There is an excellent speciation flow chart in your text bookA) Prezygotic barriers (occur before and during mating)*Premating: Isolating barriers that impede gene flow-Behavioral isolation (such as lack of attraction)-Ecological isolation (isolating barriers that are by-products of the ecology of different species, includes: habitat isolation (breed in different places), temporal isolation (breed at different times), and pollinator isolation (rely on different pollinators or deposit pollen on different parts of the insect))-Mechanical isolation (the parts don’t fit together, example: different insects pollinate black sage and white sage)*Postmating: Occurs after sperm or pollen has been transferred, but before fertilizationB) Postzygotic*Isolating barriers that act after a zygote begins to develop-Intrinsic (development problem in hybrids that is relatively independent of the environment)-Extrinsic (depends on the environment)*Example: behavioral sterility of hybrid males in acoustically communicating grasshoppers… one species uses a low frequency, the other a high frequency, and none of the ladies like the hybrids’ song)III. Selection and speciation*Reinforcement: the increase of reproductive isolation between populations through selection against hybrid offspring*Clearly can only happen if the hybrids are ACTUALLY PRODUCED*Natural selection can favor those who do not create hybrid offspring*Natural selection leads from postzygotic to prezygotic*SummaryGeographic isolating variablesVarianceDispersalReproductive isolating barriersPrezygotic barriersPre-matingPost-matingPostzygotic barriers ReinforcementIntrinsicExtrinsic*The idea of natural selection playing a role in speciation is relatively newIV. Hybrid zones *Hybrid zone exists where the ranges of two species meet, interbreed, and create hybrid offspringA) Secondary hybrid zone*Arises from a split in a population, but they then come back into contactB) Primary hybrid zone*Formed by selection on a previously homogenous species across an environmental gradient*Hypothetical hybrid zoneAllele frquencyWest
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