Evolution as Genetic Change

16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways Directional Selection Stabilizing Disruptive

Directional Selection Higher fitness at ONE END of curve than at the other All phenotypes in population shift toward HIGHER FITNESS # of Individuals in the population Traits of Population

DIRECTIONAL SELECTION Selection Pressure (Against Phenotype) Low Fitness High Fitness New graph shifts in the DIRECTION of Higher Fitness

Average Beak Size INCREASES Directional Selection Example: Darwin’s Finches # Birds in Population # Birds in Population Beak Size  Beak Size  Small seeds become scarce. Only large seeds are available. Birds with LONGER beaks gather food, survive and reproduce Average Beak Size INCREASES

Stabilizing Selection Higher fitness at the CENTER of the curve Middle Stays THE SAME Ends get NARROWER # of Individuals in the population EXAMPLE- birth weight of human babies Smaller than average babies = less likely to be healthy Larger than average babies = less likely to be healthy Traits of Population

STABILIZING SELECTION Selection Pressure (Against Phenotype) Low Fitness High Fitness Low Fitness New graph is STABILIZED in the middle

Stabilizing Selection Example: Human Birth Weight # Babies in Population # Babies in Population Birth Weight Birth Weight Smaller babies are LESS healthy. Larger babies are LESS healthy. Average Sized Babies become Most Common

Disruptive Selection EXAMPLE- large seeds and small seeds become more Higher fitness at TWO ENDS of the curve Middle phenotype DECREASES in frequency EXAMPLE- large seeds and small seeds become more common and there are few medium seeds Both birds with small beaks and large beaks are best adapted to eat those seeds Can result in 2 subgroups # of Individuals in the population Traits of Population

DISRUPTIVE SELECTION New graph is DISRUPTED in the middle. Selection Pressure (Against Phenotype) High Fitness Low Fitness High Fitness New graph is DISRUPTED in the middle.

Example: Darwin’s Finches Disruptive Selection Example: Darwin’s Finches Middle-sized seeds disappear. Only very large and very small seeds are left. # Birds in Population # Birds in Population Beak Size  Beak Size  Average-sized beaks are least common. Birds with VERY LARGE beaks and VERY SMALL beaks are best adapted. This can result in 2 subgroups.

Types Of Selection With Bird Beaks http://player.discoveryeducation.com/index.cfm?guidAssetId=7016E4B0-0335-42A9-BF3E-BB21D2A4D9C8

Which Type of Selection Is It?

Genetic Drift Coin Flip 1,000 times How many Heads? 10 times RANDOM change in allele frequency Happens by CHANCE EVENTS Happens in SMALL POPULATIONS NOT NATURAL SELECTION (Not related to fitness) Coin Flip 1,000 times How many Heads? 10 times

Bottleneck Effect A large percentage of a population IS KILLED or prevented from REPRODUCING INCREASES genetic drift

Northern Elephant Seals Bottleneck Effects Northern Elephant Seals Bottleneck Event = HUMAN HUNTING (1890s) Population decreased to 20 Seals Now… have have 30,000 seals With Reduced VARIATION from Bottleneck

The Founder Effect

The Founder Effect Example: The Cocklebur Main population with LOTS OF VARIATION (many different colors) A FEW hitch a ride to an area where there are no cockleburs. …and start a NEW POPULATION Let’s get out of here!

Now let’s get reproducing! The Founder Effect They are the FOUNDERS. Their VARIATION gives rise to the variation in the entire NEW POPULATION We made it! Woo! Now let’s get reproducing! I miss yellow…

Founder Effect From Your Articles: Amish Communities in Pennsylvania A type of Genetic Drift  after a SUBGROUP breaks away to form a new population From Your Articles: Amish Communities in Pennsylvania Ellis-van Creveld syndrome EXTRA fingers + toes Abnormal TEETH + nails A hole in the HEART

Ellis-von Creveld Syndrome The Founder Effect Ellis-von Creveld Syndrome A recessive disorder Founders? SAMUEL KING AND HIS WIFE - 1744

Chance Events and Genetic Drift RANDOM DISASTERS Rock Slide Tsunami Volcano Eruption Meteor Impact Nuclear War Etc.

GENETIC EQUILIBRIUM DEFINITION: Hardy-Weinberg Principle When allele frequencies in a population DON’T CHANGE NO EVOLUTION HAPPENS States that allele frequencies in a population will remain CONSTANT as long as 5 things are true…

Hardy-Weinberg Principle Random Mating Everyone gets an EQUAL chance to pass on alleles NO mate selecting Hardy-Weinberg Principle States that allele frequencies in a population will remain constant as long as 5 things are true…

Hardy-Weinberg Principle 2. Large Population Less effect of GENETIC DRIFT Hardy-Weinberg Principle States that allele frequencies in a population will remain constant as long as 5 things are true…

Hardy-Weinberg Principle 3. No Movement Into or Out of the Population No MIGRATION Keep GENE POOL separate Hardy-Weinberg Principle States that allele frequencies in a population will remain constant as long as 5 things are true…

Hardy-Weinberg Principle 4. No Mutations No NEW alleles in the population Hardy-Weinberg Principle States that allele frequencies in a population will remain constant as long as 5 things are true…

Hardy-Weinberg Principle 5. No Natural Selection All genotypes have equal FITNESS No ADVANTAGES for anyone Hardy-Weinberg Principle States that allele frequencies in a population will remain constant as long as 5 things are true…