Zoology101: Animal Biology Last Lecture Outline Lecture 37 1. Evolutionary trap2. Speciation and Macroevolution Current Lecture 1. Allopatric and Sympatric Speciation2. Adaptive Radiation and Mass extinctions Allopatric and Sympatric Speciation• Allopatric: ▪ geographic separation of population (rivers, islands)▪ Divergence • greatly reduced gene flow• genetic drift (founder effect)• Natural selection• Even if population is restored, interbreeding may no longer be possible ▪ Channel Islands• Founder effect likely help set the stage• Isolation from gene flow → migration greatly reduced • Unique selective environment on island → probably helps promote divergence • Islands and conservation◦ islands usually have most biodiversity for species to form, geographic isolation must be followed by reproductive isolation • Sympatric speciation◦ speciation without geographic isolation◦ most common mechanism: Polyplpoidy (many chromosomes)▪ Error during meiosis changes number of chromosomes in gametes → offspring; offspring may be viable and fertile ▪ divergence occurs within one generation ◦ Most common in plants (wheat, oat, potatoes, cotton)◦ Can occur in animals- less common▪ reproductive isolation with in the same population (different mating preference, habitat preference, disruptive selection) • How much genetic change is required for speciation?◦ Impossible to generalize ▪ cumulative divergence (slow)▪ single/few mutations leading to reproductive isolation • How quickly does speciation occur?◦ Patterns can often be inferred from fossil records, morphological or molecular data◦ Punctuated equilibrium: theory in evolutionary biology which proposes that most specieswill exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis. ◦ Gradualism: continued divergence until reproductive isolation is complete ◦ The interval between speciation events can range from 4,000 to 40 million years. Ave: 6.5 million years Adaptive radiation and Mass extinction • Adaptive radiation: periods of evolutionary change in which groups of organisms form many new species◦ can occur when species colonize in new environments (silversword on Hawaiian islands)◦ Can arise due to evolution of a key adaptation (hard body parts, flowers)• Key events in earth's history ◦ Life is old and it has taken a long time to get where we are today ▪ On an hour scale, humans have been around .2 seconds ◦ Several key events are linked with evolution of novel adaptations▪ photosynthesis, eukaryotic organisms, colonization on land ◦ Boundaries between geological divisions are often marked by big changes in dominant organisms • Mass extinctions ◦ Boundaries between geological divisions often marked by mass extinction events ◦ 5 mass extinctions → loss of 50% or more of the population ◦ often associated with big environmental changes (onset of ice age, meteor, volcanic activity)▪ Cretaceous mass extinction event• 65 million years ago• lost 50-80% of marine species • several hypothesis: asteroid impact/collisions, volcanic activity, climate change • took 5-10 million years to restore diversity • Reduction in competition brought rise to
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