Download File - Mr. Shanks` Class

Lesson # 8: Evolution
(Speciation)
Introductory Terms
- In order for one population to become very different from
another, they must be reproductively isolated
- This means that there will no longer be a free exchange of
alleles between the two populations
What is a Species?
Includes all members of a population that can interbreed under
natural conditions
Speciation
Occurs when the gene pool of a population become split
The formation of new species
The process by which reproductively isolated populations
become distinct species
Modes of Speciation
- New species can form as a result of the evolution of a
reproductive isolating mechanism that prevents members of
two populations from interbreeding
- New species can evolve under a variety of circumstances
Reproductive Isolating Mechanism
- Any behavioural, structural, or biochemical trait that
prevents individuals of different species from reproducing
successfully together
Types of Barriers
Prezygotic Barriers
- A reproductive isolating mechanism that prevents interspecies
mating and fertilization
Examples
1) Behavioural isolation
2) Temporal isolation
3) Ecological isolation
4) Mechanical isolation
5) Gametic isolation
Postzygotic Barriers
- A reproductive isolating mechanism that prevents reproduction
in offspring from interspecies reproduction
Examples
1) Zygotic mortality
2) Hybrid inviability
3) Hybrid infertility
Prezygotic Barriers
1) Behavioural Isolation
- Different species use different courtship and other mating clues
to find and attract a mate
- Usually in the form of mating signals and rituals
Examples
- Male frogs of different species have unique calls that attract only
females of their own species
- Fireflies showing distinct patterns of flashes attract various mates
Prezygotic Barriers
2) Temporal Isolation
- Different species breed at different times of the year
Examples
- Often used by plants because plants cannot move so they must
use mechanisms such as timing to reproduce at specific times
One might flower in the early spring, and by the time that is
done flowering another species might just beginning to flower
(certain flowers mate in the spring, others in the fall)
Prezygotic Barriers
3) Ecological Isolation
- Very similar species may occupy different habitats within a region
- Species utilize different resources in the habitat
- Individuals mate in their preferred habitat, and therefore do not
meet individuals of other species with different ecological
preferences.
Example
- The mountain bluebird lives at high elevations, while the eastern
bluebird prefers lower elevations and does not encounter the
mountain species
Prezygotic Barriers
4) Mechanical Isolation
- Differences in morphological features may make two species
incompatible
- The structure of the genitals is so distinct that it is often used as
a character in insect classification
- Only one male reproductive organ will fit into the specific
female reproductive tract of a species
Example
- Many insects have modifications on their genitalia such that the
male and female parts are a perfect mechanical fit
- Orchids are shaped so that only certain beetles can reach the
point of pollination, and therefor pick up the pollen. That beetle
will then go to another orchid of the same type to deposit the
pollen
Prezygotic Barriers
5) Gametic Isolation
- Male gametes may not be able to recognize and fertilize an egg
of a different species
- Two populations exchange sperm and egg but chemical markers
prevent the eggs from being fertilized because they do not
recognize them
Example
- Many marine animals including corals, clams, and sea
cucumbers release their sperm and eggs into open water. The
sperm recognizes the eggs of their own species through
chemical markers on the surface of the eggs
Scenarios
Two snake types share an over – wintering
burrow and eat the same diet, but mate at
different optimal temperatures
Two turtles share a pond and general diet, but
the female of one has a tall shell and the other a
flat shell; the female of the first has a hallow in
its lower shell, the male has a flat lower shell
Two sparrows share a similar diet, winter
in different places, but share mating and
breeding grounds with one singing from
the tops of trees and one from small bushes
Two voles hibernate in different
places, occasionally mate but the
sperm and egg do not fuse
Two fireflies share the same diet,
mate at the same time throughout
the year, but create various patterns
of flashes to attract their mate
Post Zygotic Barriers
1) Zygotic Mortality
- Mating and fertilization are possible, but genetic differences result in
a zygote that is unable to develop properly
- Results in death of zygote
Example
- Some species of sheep and goat are able to mate, but the zygote will
die
2) Hybrid Inviability
- A hybrid individual develops but either dies before birth or, if born
alive, cannot survive to maturity
Example
- When tigers and leopards are crossed, the zygote begins to develop
but the pregnancy ends in a miscarriage or stillborn offspring
3) Hybrid Infertility
- Hybrid offspring remain healthy and viable but are sterile
Example
- Mules are the sterile hybrid offspring of a horse – donkey cross
Other Mechanisms
1) Allopatric Speciation
- The formation of a new species as a result
of evolutionary changes following a
period of geographic isolation
Example
- River, volcano, mountain range
2) Sympatric Speciation
- The evolution of populations within the
same geographic area into separate
species
Example
- Various types of tress in a forest