Download 16.2 Evolution as Genetic Change

16.2 Evolution as Genetic Change
Natural selection can affect phenotypes in a population in 3 ways
1. Directional Selection
 higher fitness at ONE END of the curve than at the other
 all phenotypes in the population shift towards HIGHER FITNESS
 Example
bird population where food becomes scarce

Birds with LONG beaks gather food and survive &
reproduce

Average beak size of the population INCREASES &
shifts right
new graph shifts in the DIRECTION of higher fitness
2. Stabilizing Selection
a. Higher fitness at the CENTER of the curve
b. Middle Stays THE SAME
c. Ends get NARRROW
d. Example- birth weight of human babies
i. Smaller than average babies = _LESS likely to be
healthy
ii. Larger than average babies = LESS likely to be healthy
e. New graph shifts toward middle and is STABILIZED
3. Disruptive Selection
a. Higher fitness at TWO ends of the curve
b. Middle phenotype DECREASES in frequency
c. Example- large seeds and small seeds become more
common and there are few medium seeds
i. Both birds with SMALL beaks and LARGE beaks are best adapted to eat those seeds
ii. Can result in TWO subgroups
d. New graph is DISRUPTED in the middle.
Genetic Drift
o RANDOM change in allele frequency
 Happens by CHANCE EVENTS and happens in SMALL POPULATIONS
 NOT NATURAL SELECTION
o Bottleneck Effect
 A large percentage of a population is KILLED or prevented from REPRODUCING
 INCREASES genetic drift
 Event in which the population is GREATLY REDUCED
 Example- Northern Elephant Seals
o Bottleneck Event = HUMAN HUNTING
o Population decreased to 30 seals & now have 30,000 seals with reduced VARIATION
o Founder Effect
 Example- The cocklebur
main population has lots of VARIATION
a FEW hitch a ride and start a new POPULATION
they are the FOUNCERS and their VARIATION gives rise to new POPULATION
 A type of GENETIC DRIFT
 After a SUBGROUP breaks away to form a new population
 Example- Amish community in Pennsylvania with Ellis-von Creveld Syndrome
o high numbers of EXTRA fingers & toes, abnormal TEETH, hole in HEART
o Founders = Samuel King and his wife
o Chance events = _________________________________________________________