Notes: Evolution as Genetic Change
Natural selection can affect phenotypes in a population in 3 ways
1. Directional Selection
 higher fitness at _____ _______ of the curve than at the other
 all phenotypes in the population shift towards __________ ___________
 Example
bird population where food becomes scarce

Birds with ___________ beaks gather food and survive &
reproduce

Average beak size of the population _______________ & shifts
right
new graph shifts in the ___________ of higher fitness
2. Stabilizing Selection
a. Higher fitness at the _____________of the curve
b. Middle Stays _____________
c. Ends get ______________
d. Example- birth weight of human babies
i. Smaller than average babies = _______ likely to be healthy
ii. Larger than average babies = ________ likely to be healthy
e.
New graph shifts toward middle and is __________________
3. Disruptive Selection
a. Higher fitness at ________ends of the curve
b. Middle phenotype ______________ in frequency
c. Example- large seeds and small seeds become more
common and there are few medium seeds
i. Both birds with _________ beaks and _________ beaks are
best adapted to eat those seeds
ii. Can result in _______ subgroups
d.
New graph is ______________ in the middle.
Genetic Drift
o ___________ change in allele frequency
 Happens by ___________ ___________ and happens in _________ _______________
 NOT _____________ _______________
o Bottleneck Effect
 A large percentage of a population is _______________ or prevented from _________________
 ______________ genetic drift
 Event in which the population is __________________________.
 Example- Northern Elephant Seals
o Bottleneck Event = ___________________________
o Population decreased to ____ seals & now have __________ seals with reduced ____________
o Founder Effect
 Example- The cocklebur
main population has lots of ____________
a _____ hitch a ride and start a new ____________
they are the ____________ and their __________ gives rise to new ______________
 A type of __________ ____________
 After a _______________ breaks away to form a new population
 Example- Amish community in Pennsylvania with Ellis-von Creveld Syndrome
o high numbers of _______ fingers & toes, abnormal ___________, hole in _____________
o Founders = ________________________
o Chance events = _________________________________________________________
Evolution versus Genetic Equilibrium
o Genetic Equilibrium = When allele frequencies in a population __________ ____________
o Hardy-Weinberg principle = says allele frequencies in a population will remain ____________ (don’t change) if 5 things are
true…
1) Random Mating = Everyone gets an ________ chance to pass on alleles. _______ mate selecting
2) Large Population = Less effect of ____________ ___________
3) No Movement Into or Out of the Population = No ______________. Keep ___________ separate
4) No Mutation = No ________ alleles in the population
5) No Natural Selection = All genotypes have equal ___________. No ________________ for anyone