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Evolution Part 2
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Evolution Changes in gene pool over time
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Evolution Includes populations diverging from one another over time
may lead to new species
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Mechanisms of Evolution
Genetic Drift Population Bottlenecks Founder Effect Gene Flow Immigration Emigration Mutations Natural Selection
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Genetic Drift Changes in gene pool of populations due to random chance
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Genetic Drift
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Genetic Drift Image Credit: UC Museum of Paleontology's Understanding Evolution
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Genetic Drift Genetic drift has a bigger effect on small populations.
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Bottleneck Effect Changes in gene pool of populations due to some event drastically reduces population
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Bottleneck Effect Survival is random
Whether an organism survives does not depend on any characteristic (allele)
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Example of Bottleneck Effect
Population of plants on a mountain side An avalanche wipes out all but a patch of plants behind the shelter of a large outcrop.
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Example of Bottleneck Effect
Plants behind the outcrop survived only because they were out of the path of the avalanche. There was nothing about the plants’ genes that influenced survival.
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Example of Bottleneck Effect
Surviving population has an allele frequency different from the original population.
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Bottleneck Effect
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Bottleneck Effect: Cheetahs
10,000 years ago, cheetah populations worldwide crashed Due to climate change Photo Credit: Courtesy of Smithsonian National nationalzoo.si.edu
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Cheetahs Only cheetahs in Africa & Eurasia survived
Location, not genes Survival Photo Credit: Kevin Walsh, 2006, Wikimedia Commons
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Cheetahs Cheetahs today: Genetically identical
Photo Credit: Lukas Kaffer, 2007, courtesy of Wikimedia Commons
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Bottleneck Effect: Northern Elephant Seals
Once numerous in the northern Pacific. 1800s: Hunted extensively for blubber Photo Credit: Michael Baird of bairdphotos.com, courtesy of Wikimedia Commons
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Northern Elephant Seals
Only animals in a Mexican colony survived Photo Credit: Michael Baird of bairdphotos.com, courtesy of Wikimedia Commons
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Northern Elephant Seals
Today: >100,000 individuals Very little genetic diversity Photo Credit: Mila Zinkova, 2008, courtesy of Wikimedia Commons
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Founder Effect New population established by very small number of individuals (“founders”) By chance, founder group has a different allele mix than original population
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Example of Founder Effect
200 German immigrants founded the Old Order Amish of Pennsylvania Photo credit: : Matthew Trump, 2004, Wikimedia Commons
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Old Order Amish One couple brought allele polydactyly Six fingers and toes Photo credit: G. Baujat and M LeMerrer, 2007, Wikimedia Commons
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Old Order Amish Inbreeding has frequency of polydactyly
Click on this link: /3/l_063_03.html Photo credit: gadjoboy, 2006, Wikimedia Commons
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Gene Flow Movement of alleles between populations
Immigration: movement of alleles into a population Emigration: movement of alleles out of population
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Example of Gene Flow Transfer of pollen from one population of sunflowers into another population of sunflowers Photo Credit: Sunflower Pollen, Courtesy of Wikimedia Commons
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Example of Gene Flow Male juvenile Belding’s ground squirrels emigrate from their birthing population to new populations. 40-70% emigrate as juveniles. Remaining males leave by the end of their first year. Photo Credit: Courtesy of Yathin at FlickR, /set /
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Mutations Mutations can add new alleles to a population
Photo Credit for black Eastern Fox Squirrel: Jeffrey Pippen, Duke University Photo Credit for wild-type Eastern Fox Squirrel: Calibas, 2007, Wikimedia Commons
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Asexual Populations Mutations are primary source of genetic variation in asexual populations Photo Credit for E. coli 0157 colonies: Centers for Disease Control, 2005, Wikimedia Commons
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Natural Selection Environment determines which genes are passed onto the next generation Based on which individuals successfully survive and reproduce
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Natural Selection Requires: Genetic Variation
Overproduction of Offspring Struggle for Existence Differential Survival and Reproduction
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View video at: http://www. pbs. org/wgbh/evolution/library/11/2/e_s_4
Photo credit: Dean E. Briggins, U.S. Fish and Wildlife Service,
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Genetic Variation Organisms in the population vary in their characteristics Variations are heritable Passed from parents to offspring
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Variation in Snail Shells
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Flower Variations
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Natural Selection Flow Chart
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How Natural Selection Works
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View “Pocket Mouse and Predation” at http://www. hhmi
Photo Credit: Cheryl S. Brehme, USGS at
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View “Pocket Mouse Evolution” at http://www. hhmi
Photo Credit: J. Harris, American Society of Mammalogy, at:
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Artificial Selection Humans select which genes are passed onto the next generation Based on characteristics that humans find valuable or desirable
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View “Dog Breeding” at http://www. hhmi
Photo credit: papillon.cz, courtesy of Wikimedia Commons (
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Photo credit: USDA, 2005, courtesy of Wikimedia Commons
View “Breeding Teosinte” at Photo credit: USDA, 2005, courtesy of Wikimedia Commons
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Another Example of Artificial Selection
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Sexual Selection Some characteristics influence individual’s chance of mating Photo Credit: Ian Sewell, 2006, Wikimedia Commons
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Sexual Selection Individual may have shorter life but will have better chances of mating and producing offspring Photo Credit: David Dennis, 2007, Wikimedia Commons
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Types of Sexual Selection
Mate Competition One sex competes for access to the other sex Other sex always “chooses” winner Mate Choice One sex chooses mate based on characteristics of other sex
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Example of Mate Competition
Male bighorn sheep fight for mates Male with largest horns usually wins Photo Credit: Alan D. Wilson, naturespicsonline.com, courtesy of Wikimedia Commons
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Photo Credit: Jon Sullivan, Wikimedia Commons
Video of Bighorn Sheep Fighting: Photo Credit: Jon Sullivan, Wikimedia Commons
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Example of Mate Choice Peacocks display their tails
Females choose male with showiest tail Photo credit: Aaron Logan, courtesy of Wikimedia Commons
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Example of Mate Choice Large tails decrease survival of peacocks
Natural selection favors small tails Photo credit: Jörg Hempel, courtesy of Wikimedia Commons
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Example of Mate Choice Peacocks with small tails do not mate
Mate choice favors large tails Photo credit: Jyshah, 2007, courtesy of Wikimedia Commons
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Photo credit: Myukii, courtesy of Wikimedia Commons
View the video at: Photo credit: Myukii, courtesy of Wikimedia Commons
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The End Unless otherwise specified, all images in this presentation came from: Campbell, et al Biology, 8th ed. Pearson Benjamin Cummings.
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