Download Natural Selection in Action

We can see evolution in action.
This usually happens because of one or both of the following:
1) Short Generation times .
a. Reproduce quickly
2) High mutation rates.
3) Selective pressure is very
great.
1) Bacteria reproduce extremely quickly, often
mutate, and many can double their population in
under an hour.
Bacteria can quickly evolve to be resistant to
antibiotics if the drugs are not administered
properly or for the entire duration of the
infection.
If any bacteria are not fully wiped out by
the antibiotics, the more resistant ones
that remain will continue to reproduce.
PBS Microbe Clock
An example of rapid evolution due to very high selection
pressure is taking place among African elephants.
As prized sources of ivory,
they are being hunted closer
and closer to extinction
mainly for their tusks.
• 1. Since the elephants with the largest tusks are being hunted
most, there is a heightened selection pressure (in this case,
humans) that is wiping out large-tusked elephants.
• 2. The remaining population has higher frequency of smallertusked elephants(alleles), and some with no tusks at all!
• 3. So overall, it appears that elephant tusks are shrinking.
1.
Natural Selection is at
work when individuals
reproduce.
2. Finding a mate is part
of the struggle to
reproduce.
Tale of the Peacock Video
Natural Selection in Action
3 Facts, 1 Lie
 A. Salamander of the Sierra
Nevada use camouflage to
adapt to their environment.
 B. Coastal mountain
salamanders use mimicry
to survive their
environment.
 C. Geographic separation
can cause speciation.
 D. Hybrid salamanders are
the best adapted type of
salamanders.
NOVA: Natural Selection in Action Video
II. Speciation
A. Speciation – The
formation of a new
species by the change of
allele frequency (genetic
variation) due to natural
selection.
B. Species: a group of organisms that can interbreed
and produce fertile offspring.
3 Steps of Speciation
1. Separation: part
of a population
becomes isolated
from the rest.
• What events could
cause separation?
3 Steps of Speciation
2. Adaptation:
Natural
selection improves the fit
between species &
environment created
from changes in the gene
pool.
3. Divergence: Enough
differences accumulate no
longer allowing the
populations to interbreed,
which leads to a NEW
SPECIES.
Example #1: Fruit Flies
A population of fruit flies lay their eggs
in some rotting bananas along a beach.
1. Separation - A hurricane washes the
bananas and eggs out to sea and onto a nearby
island.
The flies hatch and are genetically isolated
from the mainland fruit flies.
2. Adaptation - Environmental conditions are
different on the island, so the island flies
evolve different adaptations.
If the two populations were to by chance
meet again they would unlikely mate with
each other due to the differences in their
gene pools creating differences in the
characteristics listed above, especially
mating rituals.
3. Divergence - Natural selection
changes physical features, food
preferences, and mating rituals
Example #1: Fruit Flies
A population of fruit flies lay their eggs
in some rotting bananas along a beach.
1. Separation - A hurricane washes the
bananas and eggs out to sea and onto a nearby
island.
The flies hatch and are genetically isolated
from the mainland fruit flies.
2. Adaptation - Environmental conditions are
different on the island, so the island flies
evolve different adaptations.
If the two populations were to by chance
meet again they would unlikely mate with
each other due to the differences in their
gene pools creating differences in the
characteristics listed above, especially
mating rituals.
3. Divergence - Natural selection
changes physical features, food
preferences, and mating rituals
Example 3: Darwin’s Finches in the
Galapagos Islands
A
A
A
Galapagos
Islands
South
America
a) Founders Arrive
A
B
b) Separation of Populations
c) Changes in the Gene Pool
E
A
B
A
B
d) Reproductive Isolation
B
C
e) Ecological Competition
A
D
B
C
f) Continued Evolution
Factors Contributing to Speciation
Isolating Mechanisms
Genetic Drift / Founders Effect
Isolating Mechanisms
Genetic differences that prevent breeding between two species
(helping to cause speciation) This can happen before or after
mating.
**(Remember) Species: a group of organisms that can
interbreed and produce fertile offspring
Pre-mating Examples - mating rituals, behaviors, songs or
calls, time of mating
Post-mating Examples – hybrids are usually sterile
Genetic Drift
A loss of genetic variation (alleles) brought about by
chance, not adaptation. Usually occurs in small
populations.
Causes less of a chance for speciation since the
species does not have the genetic diversity to adapt
to changes.
Genetic Drift
Cheetahs (decline in population 1,000s yrs
ago during Ice Age); most highly
endangered species due to habitat loss
Mountain Gorilla less than
720 left, due to habitat loss
Founders Effect
Loss of genetic variation due to an isolated small founding
population (could contain some rare variations (alleles)
making the gene pool different from the original)
Examples – Amish (founded by a few couples of German immigrants
in 1770; unusually high frequency of dwarfism & polydactyly (extra
fingers)
Why? Because of continuous marring within the group, recessive
gene diseases are more likely to show up in offspring
At what pace does evolution occur?
GRADUALISM
states that evolution occurs slowly
over time and includes many
transitions (Darwin’s model)
PUNCTUATED EQUILIBRIUM
states that evolution occurs in
short bursts followed by long
periods of stasis (no evolving).
Developed in 1970’s by Stephen
Gould