Transcript PPT 2 - ap biology

NATURAL SELECTION ALTERS THE EXPECTED
POPULATION DISTRIBUTION
Normal distribution
• AVERAGE IS FAVORED ex: baby birth weight
• ONE EXTREME SELECTED
Ex: Peppered moths
• BOTH EXTREMES SELECTED
Can lead to speciation
Normal curve
Disruptive selection
ex: butterfly mimics
VARIATION ALSO EXISTS BETWEEN POPULATIONS
i.e. due to geographic factors
Island Fox Urocyon littoralis
U. littoralis from separate islands
are physically / genetically distinct.
Channel Islands, CA
Ex: the average number of
caudal (tail) vertebrae
Can be a gradual change, called a cline
Ex: A ‘ring species
Variants
can/do
interbreed
where range
overlaps
But adjacent
variants
do not
interbreed
where the
circle closes
Example:
Ensatina salamanders
NOT ALL VARIATION IS HERITABLE
Phenotype is influenced by environment as well
In mammal species that colonize
islands, big ones have a tendency
to shrink while small ones are apt
to enlarge
mainland
island
island mainland
OTHERS FACTORS leading to EVOLUTION
Genetic Drift
Populations experience unique (and random) mutations,
becoming less alike over time
River divides the population
OTHERS FACTORS leading to EVOLUTION
Genetic Drift
OTHERS FACTORS
leading to EVOLUTION
Founders Effect
A few individuals are the founders
of a new population; this group lacks
all the variations of the original
population
OTHERS FACTORS
leading to EVOLUTION
Founders Effect
Summer 1995: at least 15 iguanas
survived Hurricane Marilyn on a raft
of uprooted trees eventually colonizing
Anguilla. These were the first
Iguana iguana, to reach the island.
Will this population become
distinct in time?
OTHERS FACTORS
leading to EVOLUTION
The Bottleneck Effect
OTHERS FACTORS
leading to EVOLUTION
The Bottleneck Effect
The effect of loss of prairie habitat on
greater prairie chickens in Illinois.
The bottleneck led to reduced
genetic variation, as measured
by number of alleles per locus.
The reduced genetic variation
is also reflected in lowered
egg hatching rates.
Gene Flow
populations become
more alike as genes
are exchanged
hybridization
seeds, pollen
seeds, pollen
Genetic engineering
plasmid exchange
hitch-hikers