Observed examples of Evolution Special Lecture Recent well documented examples of evolutionary changes o Stephanodiscus Centric diatom found only in Yellowstone lake Wyoming Defined by details of silica cell wall patters Descended from common diatom stephanodiscus niagarae found in many north American lakes Sediment record goes back 1400 years Number of ribs and spikes changed slowly over time Lake began after the last glacier cycle Pollen record in sediment shows rapid warming periods from 13700 to 10000 years ago Diatom records in sediments Original was S niagarae Began to see a shift in morphology starting about 13000 years ago Rapid shift between 13000 and 10000 o Intermediates well documented From 10000 to present little change to morphology Lessons learned We saw speciation in action through the intermediate fossils Directional selection Extremely rapid evolution in about 3000 years followed by no change Rapid speciation perhaps due to the rapid climatic shift at the end of the Ice Age Questions Why only in this lake Did climatic shift drive the rapid speciation How Does this apply to other diatoms or protists o Rhagoletis apple maggot Parasitic fly that lays eggs on Hawthorne tree Maggots eat fruit pupate over winter and emerge as adult flies to mate and lay eggs on new fruit Timing of mating egg laying depends on when the tree sets fruit Shift in host availability Humans brought and distributed apples all over north America Rhagoletis was infesting apples by mid 19th century 2 different populations now identified Hawthorne vs apple population Timing of mating egg laying and life span of larval stage are very different now Could be in the process of becoming genetically isolated Little hybridization Populations differ at several loci Populations have shifted mating times to match the host tree species 999 Both examples in natural populations with good documentation
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