Two main sources of genetic variation

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Transcript Two main sources of genetic variation

Chapter 16
Evolution of Populations
Two main sources of genetic
1. Mutations—change in genes
(DNA sequence) or chromosomes
2. Gene recombination—mixing of
genes that result from meiosis and
sexual reproduction
Gene Pool—the combined
genetic information of all the
members of a particular
Speciation—formation of a new
species through reproductive
Example: Galapagos Island
Ground Squirrels
**Quick Speciation Activity**
Fig. 24-6
A. harrisi
A. leucurus
Types of reproductive isolation
1. Behavioral isolation (sympatric)
2. Geographical isolation (allopatric)
Which type did we demonstrate in our
If one of the Earth’s plates moves 1.9
cm a yr., in 1 million years it would
move 12 miles
Geographic Isolation
Some birds from species A
cross to a second island.
The two populations no
longer share a gene pool.
Changes in the Gene Pool
Seed sizes on the second
island favor birds with large
The population on the second
island evolves into population
B, with larger beaks.
Fig. 22-6
(a) Cactus-eater
(c) Seed-eater
(b) Insect-eater
Species—a group of similar
organisms that can breed and
produce fertile offspring
Fig. 24-2a
(a) Similarity between different species
Exit Slip
• List the conditions required for a
population to become a species.
Types of selection
1. Disruptive Selection
• Selection that splits a population into 2
• Removes individuals with average traits,
but keeps those with more extreme traits.
2. Stabilizing Selection
• Eliminates extreme expressions of a trait
when the average expression leads to
higher fitness.
• Most common form of natural selection.
3. Directional Selection
• An extreme version of a trait makes an
organism more fit.
Height Selection
•Only extremely
tall and extremely
•Only medium
•Only extremely
4. Sexual Selection
• Operates in populations where males and
females look very differently.
• Typically, males will be larger and more
Types of Evolution
Adaptive radiation
(divergent evolution)—
a single species evolves into
several new species that live
in different ways
unrelated species
evolve similarities
when adapting to
Coevolution— two species
evolve in response to changes
in each other over time
Example: mutualism
Moth pollinates the
comet orchid
Rate of Evolution
• Evolution occurs after a catastrophy
• Evolution proceeds in small, gradual steps
Punctuated Equilibrium
• Rapid spurts of genetic change that cause
species to diverge quickly.
• These periods disrupt much longer periods
when the species exhibit little change.
• Instances of abrupt transitions.
Chapter 17
The History of Life
If one of the Earth’s plates moves
1.9 cm a yr., in 1 million years it
would move 12 miles
Fossil record—information about
past life that has been obtained
from fossils-it is incomplete
1. Most organisms are now
2. fossils occur in a particular
3. groups of organisms have
changed over time
Extinct—a species that has
died out
Where do most fossils form?
Most fossils form in sedimentary
rock as weight compresses layers
of sediment in bodies of water
Index Fossil—an easily
recognized species used to
compare the relative ages of
rocks form in
horizontal layers.
When part of Earth’s
crust is compressed,
a bend in a rock
forms, tilting the rock
As the surface
erodes due to water,
wind, waves, or
glaciers, the older
rock surface is
New sediment is
then deposited
above the exposed
older rock surface.
Water carries small rock
particles to lakes and seas.
Dead organisms are buried
by layers of sediment, which
forms new rock.
The preserved remains
may later be discovered
and studied.
Relative dating—the age of a
fossil is determined by comparing
its placement with fossils in other
The oldest layers are on the
Half-life—the length of time
required for half of the
radioactive atoms in a sample
to decay
713 million yrs
Potassium-40 1.3 billion yrs
Uranium-238 4.5 billion yrs
Radioactive dating—scientists
use half-life to calculate the age
of fossils based on the amount of
remaining radioactive isotopes
Microfossils—microscopic fossil
Mass Extinction—many types
of living things become extinct
in a short period of time
Example: Dinosaurs