Presentation on theme: "Evolution as Genetic Change. 16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection."— Presentation transcript:
Evolution as Genetic Change
16.2 Evolution as Genetic Change Natural selection can affect phenotypes in a population in 3 ways A.Directional Selection B.Stabilizing Selection C.Disruptive Selection
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
Selection Pressure (Against Phenotype) Low Fitness High Fitness DIRECTIONAL SELECTION New graph shifts in the DIRECTION of Higher Fitness
Directional Selection Example: Darwin’s Finches 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 EXAMPLE- birth weight of human babies Smaller than average babies = less likely to be healthy Larger than average babies = less likely to be healthy # of Individuals in the population Traits of Population
Low Fitness STABILIZING SELECTION New graph is STABILIZED in the middle High Fitness Selection Pressure (Against Phenotype)
Stabilizing Selection Example: Human Birth Weight Birth Weight Smaller babies are LESS healthy. Larger babies are LESS healthy. Average Sized Babies become Most Common
Disruptive Selection 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
High Fitness DISRUPTIVE SELECTION New graph is DISRUPTED in the middle. Low Fitness Selection Pressure (Against Phenotype)
Disruptive Selection Example: Darwin’s Finches 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. Middle-sized seeds disappear. Only very large and very small seeds are left.
Types Of Selection With Bird Beaks http://player.discoveryeducation.com/index.cfm?guidAssetId=7016E4 B0-0335-42A9-BF3E-BB21D2A4D9C8
Which Type of Selection Is It?
Genetic Drift 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 How many Heads?
Bottleneck Effect A large percentage of a population IS KILLED or prevented from REPRODUCING INCREASES genetic drift
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
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!
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 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
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: When allele frequencies in a population DON’T CHANGE NO EVOLUTION HAPPENS Hardy-Weinberg Principle States that allele frequencies in a population will remain CONSTANT as long as 5 things are true…
Hardy-Weinberg Principle 1.Random Mating – Everyone gets an EQUAL chance to pass on alleles – NO mate selecting 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 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 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 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 States that allele frequencies in a population will remain constant as long as 5 things are true…