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AP Biology
Macroevolution
Part 2
Important concepts from previous
units
• 1) In order to keep a species extant (living)
reproduction must be possible.
• 2) Genomes of organisms must be similar
enough to allow for reproduction to go to
completion with viable offspring.
• 3) Environments change over time and
distances.
Modes (Ways to) of Speciation
•
Allopatric ( Allo –means “different”; Patric
- means “place”)
–
In this way, a geographic barrier becomes
present within the environment separating the
parent population. This barrier can create two
different environments, which could cause each
species to begin to change/evolve over time
and potentially lead to two different species.
•
•
•
Ring Species – In this type of allopatric
speciation a migrating species moves around
a geographic barrier.
If the barrier is large enough, it can force
each species into different environments.
Each could potentially change to adapt to
the new environment over time and
successive generations until new species
exist.
A ring species is a situation in which two populations which do not
interbreed are living in the same region and connected by a
geographic ring of populations that can interbreed.
A ring species can be best imagined like this:
Consider a species that is geographically distributed in a straight line from
east to west across America: it is possible that the forms in the east and
west are so different that they could not interbreed.
Now imagine taking the line and bending it into a circle, such that the end
points (formerly in the east and west) come to overlap in space.
If they do not interbreed then the geographic distribution of the species
will be in the shape of a ring, and they will be 'ring species':
The extreme forms do not interbreed in the region of overlap.
A ring species has an almost continuous set of intermediates between two
distinct species.
Geographic Range of west coast
salamanders (A ring species)
• Adaptive Radiation– In this type of allopatric speciation, a species also migrates into new
environments.
– As time and successive generations go by, those organisms may begin to
change/evolve to meet the requirements of that new environment. Then
some of the new species population move farther out to the next island
causing the process of change/evolution to occur over time again.
– So ultimately, what happens is that each island has its own species that
evolved in response to that islands environment.
– They all came from the mainland parent population initially and
changed/evolved as time went by on each island. So when you look at all the
species in respect to the parent species, we see a couple of things.
– On the island that is closest to the mainland, we see the fewest changes
from the parent species, because that island environment is probably very
similar to the mainland environment.
– On the island that is farthest from the mainland, we see the most changes
from the parent species, because that environment is most likely very
different than the mainland environment
.
i. Galapagos Islands & Darwin’s finches are great examples
of this process.
ii. If the island is close to the mainland, we see little change
occur.
iii. If the island is farthest from mainland, we see more
changes occur.
Darwin’s Galapagos Island finches
Galapagos Islands
Galapagos Islands close up
– Sympatric (Sym – means “same”) In this
process, a new species evolves out of the parent
species while both remain in the same
environment.
•
For animals – This may occur because of competition
for resources, such as food. The dominant individuals,
having the more favorable traits, will get the resource
that is being competed for. The weaker group, having
the lesser favorable traits, will have to find a different
resource to use. This change in resource utilization
may lead to change/evolution of the weaker group
over time and successive generations until we get two
different species within the same environment.
• For plants – This mainly occurs because of
polyploidy (a condition of having abnormal
chromosomal numbers) because of cross
fertilization between plants that have had
meiosis go awry in the formation of gametes.
– Autopolyploidy - Is the result of self fertilization.
(“auto = “self”; “poly”= many; “ploidy” = “genetic
content”)
– Allopolyploidy - Is the result of different plants cross
fertilizing.
Autopolyploidy
Failure of cell division
in a cell of a growing
diploid plant after
chromosome duplication
gives rise to a tetraploid
branch or other tissue.
2n = 6
Gametes produced by
flowers on this
tetraploid branch are
diploid.
Offspring with
tetraploid karyotypes may be viable
and fertile—a new
biological species.
2n
4n = 12
4n
Allopolyploidy
Unreduced gamete
with 4 chromosomes
Species A
2n = 4
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
Normal gamete
n=3
Modes of Speciation
Allopatric speciation
Sympatric speciation
– Punctuated Equilibrium
•
•
This way of speciation was proposed in 1976 by
Stephen Jay Gould, a famous Harvard professor.
In this method, long periods of stability (this is the
equilibrium) are interrupted suddenly (this is the
punctuated) by a major disruption(such as an asteroid
hitting the earth) that causes a mass extinction of
existing species to occur. Once all disruption has
calmed down(usually after several years), a mass
evolution of new species will occur to occupy all the
new open niches that were created due to the mass
extinction. (These punctuations usually mark/cause
the end of an era.)
Stephen Jay Gould
.
A new species
changes most as it
buds from a parent
species and then
changes little for the
rest of its existence
Time
Gradualism model
Punctuated equilibrium model
•
Microevolution can cause Macroevolution
to occur with enough time and enough
changes in the DNA.
•
Descent with modification is what occurs to
structures or traits in organisms over time.
•
This is the phrase that Darwin used in his
book, not evolution.
•
This modification is best exhibited by the
evolution of the eye.
Example of Descent with Modification
and Natural Selection
• The eye starts out as a collection of light sensing
pigments (called an Oscilli) located on the external
surface of the bell of jellyfish.
– This allows them to tell basic direction (up toward the
sun and down into the dark bottom).
– They can also see shadows (either predators or food
moving).
• You can do the same with your eyes closed.
– The problem is the cells are on the surface where they
can be damaged with a brush against a rock or other
rough surface.
• Nature tried to solve this problem over time by recessing the
cells into a sunken recess (such as we see in the Eye Spots of
Platyhelminthes- flatworms).
• This protected the light sensing cells, but
created a new problem…sediment collection
in the recess.
– Nature tried to solve this problem over time by
sinking the cells down further and reducing the
opening to the recess, such as we see in some
annelids and mollusks.
– Problem solved regarding the sediment in the
recess… but new problem created … lack of light
entering the recess causing decreased vision and
light detection.
– Nature tried to solve this problem over time by
putting a layer of transparent cells over the
opening. This would act as a simple magnifying
glass to increase the amount of light entering the
“eye”. Problem solved … but new problem
created.
• The “vision” is not clear because of the layer
of cells the light passes through.
– Nature tried to solve this problem over time by
forming a lens using a clear protein called
crystalline.
– This would enhance and clarify any image. This is
seen in Arthropods.
– By this time these animals were able to move
quickly within their environments.
– So a new problem… No depth perception, all
vision is basically 2D.
– Nature tried to solve this problem over time by
attaching muscles to the lens to stretch it when
needed.
– This manipulation of the lens allowed for the
ability to detect differences in depth and thus
more 3D to a degree. This is seen in all higher
organisms such as cephalopods, fish, amphibians,
reptiles, birds, and mammals. Problem solved. It
works well and that is why we see it unchanged
essentially in the higher organisms.
Evolution of the eye
Pigmented cells
(photoreceptors)
Pigmented
cells
Epithelium
Nerve fibers
Patch of pigmented cells
Eyecup
Fluid-filled cavity
Epithelium
Optic
nerve
Nerve fibers
Cellular
fluid
(lens)
Pigmented
layer (retina)
Pinhole camera-type eye
Optic nerve
Eye with primitive lens
Cornea
Lens
Retina
Optic nerve
Complex camera-type eye
Cornea
•
Snowball Earth caused the end of the Pre-Cambrian era. 7/8 of
the earth was covered by ice. It took millions of years to thaw
out. Most organisms died. Those that survived were around
deep sea thermal vents, where it was warm enough to support
life. Once the ice melted, the Cambrian explosion of species
occurred to start the beginning of the Paleozoic Era (called the
Age of Fish).
• Pangea, the super continent, caused the end of the Paleozoic era. This
coming together of all the continents caused the earth’s water to be
dramatically displaced. The interior swamps and oceans disappeared and
over time became vast deserts. Most aquatic and terrestrial animal and
plant species went extinct due to loss of water. Those that survived were
around the edge of the supercontinent or in the one big ocean. This mass
extinction allowed for the mass explosion of new reptile species and
desert plants. This began the Mesozoic Era (called the Age of Reptiles).
Pangaea
Cenozoic
0
By the end of the
Mesozoic, Laurasia
and Gondwana
separated into the
present-day continents.
Mesozoic
By the mid-Mesozoic
Pangaea split into
northern (Laurasia)
and southern
(Gondwana)
landmasses.
Paleozoic
65.5
Millions of years ago
By about 10 million years
ago, Earth’s youngest
major mountain range,
the Himalayas, formed
as a result of India’s
collision with Eurasia
during the Cenozoic.
The continents continue
to drift today.
At the end of the
Paleozoic, all of
Earth’s landmasses
were joined in the
supercontinent
Pangaea.
135
251
•
The Asteroid that hit the earth 65 million years ago
caused the end of the Mesozoic Era and the
extinction of the dinosaurs and many plant species.
It caused the sun to be blocked out by soot and ash
for years. The planet became very cold. The
organisms that survived were mainly Mammals,
because of their warm fur. Some reptiles,
amphibians, and fish survived too. Also some
plants. Once the sun returned to the entire earth,
we see the mass explosion of mammals and the
beginning of the Cenozoic Era (called the Age of
Mammals).
Mass extinctions over
time
600
Millions of years ago
400
300
200
500
100
0
100
2,500
80
Number of
taxonomic
families
Permian mass
extinction
2,000
Extinction rate
60
1,500
40
Cretaceous
mass extinction
20
1,000
500
0
Paleozoic
Mesozoic
Cenozoic
Neogene
Paleogene
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Proterozoic eon
0
Asteroid Impact
NORTH
AMERICA
Yucatán
Peninsula
Chicxulub
crater