SBI3U1
Recall:
Microevolution:change within the frequency of
alleles in a population.
Speciation: the formation of a new species
from an existing species.
A.K.A. “macroevolution”
*Note: Two different
species of zebra.
When two populations have become
reproductively isolated over time, there is
“No gene flow between them”

Two types of reproductive isolating
mechanisms:
1. Pre-Zygotic
2. Post-Zygotic
Species are not able to mate or when eggs
cannot be fertilized.
(A.K.A.- Pre-fertilization Barriers)
 5 pre-zygotic isolating mechanisms:

-Behavioural
-Habitat
-Temporal
-Mechanic
-Gametic

Species have a specific signal or behaviour
that prevents closely related species from
interbreeding.
Both birds are very
similar in
appearance. Due to
the differences in
their song, these
birds do not
interbreed.
Western
Meadowhalk
Eastern
Meadowhalk

Two species live in the same area but
different habitats, they rarely encounter and
interact with one another.
Garter snake
(thamnophis sirtalis)
perfers open areas.
Garter snake
(thamnophis
ordinoides) commonly
found near water.
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Temporal conditions refers to time of day,
year and season.
Some organisms mate at different times
which limits their interaction.
Very common in flower species.


Some species are anatomically
incompatible and unable to
fertilize.
Ex: The genital in insects
operate like a lock & key model


Occurs when a species’ gametes (egg & sperm)
are not able to fuse to form a zygote
Ex: Sperm is unable to survive within female
reproductive tract

Sperm and egg meet, but zygote cannot
develop into a viable offspring.
There are 3 post-zygotic mechanisms.
-Hybrid Inviability
-Hybrid Sterility
-Hybrid Breakdown

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
When genetic information is not compatible
between species.
Zygote does not undergo mitosis, and
zygote cannot develop further.
E.g. hybrid embryo between sheep and goats
die before birth

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Two species can mate and produce hybrid
offspring. (ie. Mule + donkey)
However, The hybrid offspring is sterile.
Meiosis fails to produce normal gametes in
the hybrid offspring.

Two organisms are able to mate and produce
viable and fertile offspring.
However, when the hybrid species mate,
( 2nd generation) their offspring are weak and
sterile.

Cotton plants produce
fertile hybrids, but
their offspring die as
seeds
Section 9.2
1) Sympatric Speciation
2) Allopatric Speciation

Populations in same
geographical area become
reproductively isolated

More common in plants than
animals

chromosomal changes in
plants or non-random mating
in animals alters gene flow.

The result is reproductive
incompatibility without
geographical isolation.



Occurs when populations are
separated by a geographical
barrier and diverge
genetically.
Gene flow is interrupted
Separation can occur
due to river, water level
change, geological
remodelling etc.

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Type of allopatric speciation
Diversification of a common ancestral species
into a variety of differently adapted species
Example:
◦ A few seed eating birds occupy an empty niche with
fruit
◦ Mutation makes the birds good fruit eaters
◦ Fruit and seed eaters no longer mate
Divergent Evolution:
Species that were similar to the
ancestral species diverge and become
increasingly distinct.

Convergent Evolution:
 Similar
traits arise because different
species have independently adapted to
similar environmental conditions.
Two models have been proposed to explain the speed at
which evolution occurs.
Gradualism
•
views evolutionary change as slow and steady, before and
after a divergence
• supported since Darwin’s time
Punctuated Equilibrium
•
views evolutionary history as long periods of stasis, or
equilibrium, that are interrupted by periods of divergence
UNIT 3 Chapter 9: Evolution and Speciation
Section 9.2
It is now accepted that both models of evolutionary change are at
work. While many species have evolved rapidly during periods in
Earth’s history, the fossil record also shows very gradual change
for some species over extended periods of time.
Two models have been
proposed to explain the
speed of evolution: (A)
gradualism and (B)
punctuated equilibrium.
UNIT 3 Chapter 9: Evolution and Speciation
Section 9.2
Human activities have had great impact on speciation,
population decline, and extinction.
Here are some examples :
• conversion of wilderness into cropland
• development of areas for tourism
• building urban subdivisions and roads
UNIT 3 Chapter 9: Evolution and Speciation
Section 9.2
Mass extinctions decrease the number of
species.
 However, the overall trend is an overall
increase in species over the course of Earth’s
history.

Example of Mass Extinction: Cretaceous
Dinosaur
A global trigger, likely a large asteroid
impacting Earth, caused the extinction of
dinosaurs and more than 50% of the marine
species.
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Upcoming Evolution Unit test- TBA
Read and make note 9.2
Complete pg. 373 Q #1-4, 6, 8