2 16.2 Evolution as Genetic Change Natural selection can affectphenotypes in a population in 3 waysDirectional SelectionStabilizing SelectionDisruptive Selection
3 Directional Selection Higher fitness at ONE END of curve than at the otherAll phenotypes in population shift toward HIGHER FITNESS# of Individualsin the populationTraits of Population
4 DIRECTIONAL SELECTION Selection Pressure (Against Phenotype)Low FitnessHigh FitnessNew graph shifts in the DIRECTION of Higher Fitness
5 Directional Selection Example: Darwin’s Finches # Birds in Population# Birds in PopulationBeak Size Beak Size Small seeds become scarce. Only large seeds are available.Birds with LONGER beaks gather food, survive and reproduceAverage Beak Size INCREASES
6 Stabilizing Selection Higher fitness at the CENTER of the curveMiddle Stays THE SAMEEnds get NARROWER# of Individualsin the populationEXAMPLE- birth weight of human babiesSmaller than average babies = less likely to be healthyLarger than average babies = less likely to be healthyTraits of Population
7 STABILIZING SELECTION Selection Pressure (Against Phenotype)Low FitnessHigh FitnessLow FitnessNew graph is STABILIZED in the middle
8 Stabilizing Selection Example: Human Birth Weight # Babies in Population# Babies in PopulationBirth WeightBirth WeightSmaller babies are LESS healthy. Larger babies are LESS healthy.Average Sized Babies become Most Common
9 Disruptive Selection EXAMPLE- large seeds and small seeds become more Higher fitness at TWO ENDS of the curveMiddle phenotype DECREASES in frequencyEXAMPLE- large seeds and small seeds become morecommon and there are few medium seedsBoth birds with small beaks and large beaks are best adapted to eat those seedsCan result in 2 subgroups# of Individualsin the populationTraits of Population
10 DISRUPTIVE SELECTION New graph is DISRUPTED in the middle. Selection Pressure (Against Phenotype)High FitnessLow FitnessHigh FitnessNew graph is DISRUPTED in the middle.
11 Disruptive Selection Example: Darwin’s Finches Middle-sized seeds disappear. Only very large and very small seeds are left.# Birds in Population# Birds in PopulationBeak 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.
14 Genetic Drift Coin Flip RANDOM change in allele frequency 1,000 times Happens by CHANCE EVENTSHappens in SMALL POPULATIONSNOT NATURAL SELECTION(Not related to fitness)Coin Flip1,000 timesHow many Heads?10 times
15 Bottleneck EffectA large percentage of a population IS KILLED or prevented from REPRODUCINGINCREASES genetic drift
16 Bottleneck Effects Northern Elephant Seals Bottleneck Event = HUMAN HUNTING (1890s)Population decreased to 20 SealsNow… have have 30,000 sealsWith Reduced VARIATION from Bottleneck
19 The Founder Effect Example: The Cocklebur …and start a NEW POPULATION 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 POPULATIONLet’s get out of here!
20 Now let’s get reproducing! The Founder EffectThey are the FOUNDERS. Their VARIATION gives rise to the variation in the entire NEW POPULATIONWe made it! Woo!Now let’s get reproducing!I miss yellow…
21 Founder EffectA type of Genetic Drift after a SUBGROUP breaks away to form a new populationFrom Your Articles:Amish Communities in PennsylvaniaEllis-van Creveld syndromeEXTRA fingers + toesAbnormal TEETH + nailsA hole in the HEART
22 The Founder Effect Ellis-von Creveld Syndrome A recessive disorderFounders?SAMUEL KING AND HIS WIFE
23 Chance Events and Genetic Drift RANDOM DISASTERSRock SlideTsunamiVolcano EruptionMeteor ImpactNuclear WarEtc.
24 GENETIC EQUILIBRIUMDEFINITION:Hardy-Weinberg PrincipleWhen allele frequencies in a population DON’T CHANGE NO EVOLUTION HAPPENSStates that allele frequencies in a population will remain CONSTANT as long as 5 things are true…
25 Hardy-Weinberg Principle Random MatingEveryone gets an EQUAL chance to pass on allelesNO mate selectingHardy-Weinberg PrincipleStates that allele frequencies in a population will remain constant as long as 5 things are true…
26 Hardy-Weinberg Principle 2. Large PopulationLess effect of GENETIC DRIFTHardy-Weinberg PrincipleStates that allele frequencies in a population will remain constant as long as 5 things are true…
27 Hardy-Weinberg Principle 3. No Movement Into or Out of the PopulationNo MIGRATIONKeep GENE POOL separateHardy-Weinberg PrincipleStates that allele frequencies in a population will remain constant as long as 5 things are true…
28 Hardy-Weinberg Principle 4. No MutationsNo NEW alleles in the populationHardy-Weinberg PrincipleStates that allele frequencies in a population will remain constant as long as 5 things are true…
29 Hardy-Weinberg Principle 5. No Natural SelectionAll genotypes have equal FITNESSNo ADVANTAGES for anyoneHardy-Weinberg PrincipleStates that allele frequencies in a population will remain constant as long as 5 things are true…
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