Evolution Part 1 BIOL 1407

Evolution Heritable genetic change in populations or groups of populations over time Changes in gene pool

Evolution Also includes populations diverging from one another over time  may lead to new species

Mechanisms of Evolution Genetic Drift –Population Bottlenecks –Founder Effect Gene Flow –Immigration –Emigration Mutations Natural Selection

Genetic Drift Changes in gene pool of populations due to random chance

Genetic Drift

Genetic Drift Image Credit: UC Museum of Paleontology's Understanding Evolution

Genetic Drift Genetic drift has a bigger effect on small populations.

Bottleneck Effect Changes in gene pool of populations due to some event  drastically reduces population

Bottleneck Effect Survival is random Whether an organism survives does not depend on any characteristic (allele)

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.

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.

Example of Bottleneck Effect Surviving population has an allele frequency different from the original population.

Bottleneck Effect

Bottleneck Effect: Cheetahs 10,000 years ago, cheetah populations worldwide crashed Due to climate change Photo Credit: Courtesy of Smithsonian National nationalzoo.si.edu

Cheetahs Only cheetahs in Africa & Eurasia survived Location, not genes  Survival Photo Credit: Kevin Walsh, 2006, Wikimedia Commons

Cheetahs Cheetahs today: Genetically identical Photo Credit: Lukas Kaffer, 2007, courtesy of Wikimedia Commons

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

Northern Elephant Seals Only animals in a Mexican colony survived Photo Credit: Michael Baird of bairdphotos.com, courtesy of Wikimedia Commons

Northern Elephant Seals Today: >100,000 individuals Very little genetic diversity Photo Credit: Mila Zinkova, 2008, courtesy of Wikimedia Commons

Founder Effect New population established by very small number of individuals (“founders”) By chance, founder group has a different allele mix than original population

Example of Founder Effect 200 German immigrants founded the Old Order Amish of Pennsylvania Photo credit: : Matthew Trump, 2004, Wikimedia Commons

Old Order Amish One couple brought allele polydactyly  Six fingers and toes Photo credit: G. Baujat and M LeMerrer, 2007, Wikimedia Commons

Old Order Amish Inbreeding has  frequency of polydactyly Click on this link: 06/3/l_063_03.html 06/3/l_063_03.html Photo credit: gadjoboy, 2006, Wikimedia Commons

Gene Flow Movement of alleles between populations Immigration: movement of alleles into a population Emigration: movement of alleles out of population

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

Example of Gene Flow Male juvenile Belding’s ground squirrels emigrate from their birthing population to new populations % emigrate as juveniles. Remaining males leave by the end of their first year. Photo Credit: Courtesy of Yathin at FlickR, n/set / n/set /

Mutations Mutations can add new alleles to a population Review this BIOL 1406 concept Photo Credit for black Eastern Fox Squirrel: Jeffrey Pippen, Duke University Photo Credit for wild-type Eastern Fox Squirrel: Calibas, 2007, Wikimedia Commons

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

Natural Selection Environment determines which genes are passed onto the next generation Based on which individuals successfully survive and reproduce

Natural Selection Requires: –Genetic Variation –Overproduction of Offspring –Struggle for Existence –Differential Survival and Reproduction

View video at: Photo credit: Dean E. Briggins, U.S. Fish and Wildlife Service,

Genetic Variation Organisms in the population vary in their characteristics Variations are heritable  Passed from parents to offspring

Variation in Snail Shells

Flower Variations

Natural Selection Flow Chart

How Natural Selection Works

View “Pocket Mouse and Predation” at Photo Credit: Cheryl S. Brehme, USGS at

View “Pocket Mouse Evolution” at Photo Credit: J. Harris, American Society of Mammalogy, at:

Artificial Selection Humans select which genes are passed onto the next generation Based on characteristics that humans find valuable or desirable

View “Dog Breeding” at Photo credit: papillon.cz, courtesy of Wikimedia Commons (

View “Breeding Teosinte” at Photo credit: USDA, 2005, courtesy of Wikimedia Commons

Another Example of Artificial Selection

Sexual Selection Some characteristics influence individual’s chance of mating Photo Credit: Ian Sewell, 2006, Wikimedia Commons

Sexual Selection Individual may have shorter life but will have better chances of mating and producing offspring Photo Credit: David Dennis, 2007, Wikimedia Commons

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

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

Video of Bighorn Sheep Fighting: Photo Credit: Jon Sullivan, Wikimedia Commons

Example of Mate Choice Peacocks display their tails Females choose male with showiest tail Photo credit: Aaron Logan, courtesy of Wikimedia Commons

Example of Mate Choice Large tails decrease survival of peacocks Natural selection favors small tails Photo credit: Jörg Hempel, courtesy of Wikimedia Commons

Example of Mate Choice Peacocks with small tails do not mate Mate choice favors large tails Photo credit: Jyshah, 2007, courtesy of Wikimedia Commons

View the video at: Photo credit: Myukii, courtesy of Wikimedia Commons

The End Unless otherwise specified, all images in this presentation came from: Campbell, et al Biology, 8 th ed. Pearson Benjamin Cummings.