1. NATURAL SELECTION ALTERS THE EXPECTED
POPULATION DISTRIBUTION
Normal distribution

2. • AVERAGE IS FAVORED ex: baby birth weight

3. • ONE EXTREME SELECTED Ex: Peppered moths

4. • BOTH EXTREMES SELECTED ex: butterfly mimics
Can lead to speciation
Normal curve
Disruptive selection

5. i.e. due to geographic factors
VARIATION ALSO EXISTS BETWEEN POPULATIONS
i.e. due to geographic factors
Island Fox Urocyon littoralis Channel Islands, CA
Ex: the average number of
caudal (tail) vertebrae
U. littoralis from separate islands are physically / genetically distinct.

6. 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

7. Example:
Ensatina salamanders

8. 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

9. OTHERS FACTORS leading to EVOLUTION
Genetic Drift
Populations experience unique (and random) mutations,
becoming less alike over time
River divides the population

10. OTHERS FACTORS leading to EVOLUTION
Genetic Drift

11. 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

12. 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?

13. OTHERS FACTORS
leading to EVOLUTION
The Bottleneck Effect

14. 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.

15. Gene Flow populations become more alike as genes are exchanged
hybridization
seeds, pollen
seeds, pollen
Genetic engineering
hitch-hikers
plasmid exchange