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Lecture 17


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Western University
Biology 1001A
Tom Haffie

BIOLOGY LECTURE 17 Read text sections Chapter 19 up to and including 196 SpeciationAchieving reproductive isolationSplitting off of one specie into different speciesEvolution will still happen but species diversity wouldnt happenWhat is the process by which two populations become separated from one another and diverge so much that they can no longer successfully interbreed How is this achievedUsually occurs in allopatry geographical separationisolationdivergencesecondary contact and reinforcement they have been separated for so long that when they are put back together they no longer interbreed AllopatryBasically the geographical ranges of the two populations separatedont overlapPhysical barrier arises glacier highway riverColonizingphysical barriers such as oceansPrevents gene flowOver evolutionary time two population will diverge Helps test BSCbecause of separating barrier Island Hotbeds of speciationAllopatric speciation happensIsolation small populations founder effects different selection pressuresDifferent selection pressureseffects on island populations different Secondary contactDoesnt always happenWhen it happens two resultsdepend on how much divergence took placeIf populations havent diverged much Usually if the physical barrier didnt last long or selection processes werent too differentSpeciation collapses and populations resume interbreeding hybrid swarmIf more divergence happenedSeparation last for long periodsGenetic driftstrongly operating in different directions Selection process differentleads to genetic variation Differences in chromosomal compatibilityAlternatively postzygotic isolating mechanisms may have evolvedhybrids less fit than parental formsdirect selection favoring prezygotic isolating mechanisms reinforcementControversial whether postzygotic or prezygotic mechanism evolves first
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