evolution D2 - HS Biology IB

Assessment Statements
• Define allele frequency
• State that evolution involves a change in allele frequency in a
population’s gene pool over a number of generations.
• Discuss the definition of the term species
• Describe three examples of barriers between gene pools
(allopatric,sympatric and polyploidy).
• Explain how polyploidy can contribute to speciation.
• Compare allopatric and sympatric speciation
• Outline the process of adaptive radiation
• Compare convergent and divergent evolution
• Discuss ideas on the pace of evolution, including gradualism and
punctuated equilibrium.
• Describe one example of transient polymorphism.
• Describe sickle-cell anemia as an example of balanced polymorphism
Why Biology is Awesome!
The 2012 Nobel Prize for Physiology or Medicine was awarded
Monday to Sir John B. Gurdon and Shinya Yamanaka for work that
revolutionized the understanding of how cells and organisms
They discovered that mature adult cells can also differentiate just
like stem cells.
So….what we learned about stem cells and what makes them so
unique and special when it comes to future treatments is now no
longer true, any cell can do that!
Cool Adaptations
Tripod fish, Bathypterois grallator
Pacific Barreleye Fish
Not eyes,
water intake
Star Nosed Mole
68–65.5 million years ago
Largest flying animal of all time.
35ft wing span!
Temporal Isolation
Over generations, a population can undergo a great deal of change from its
original state. But all members of that population are still belonging to the
same species unless some members become REPRODUCTIVELY ISOLATED
from the original population.
Speciation is the separation of two previously interbreeding populations into
two populations that can no longer mate to produce fertile, viable offspring.
Bufo americanus and Bufo fowleri . The members of these species can be
successfully crossed in the laboratory producing healthy, fertile hybrids.
However, mating does not occur in the wild even though the geographical
distribution of the two species overlaps. The reason for the absence of interspecies mating is that B. americanus mates in early summer and B. fowleri in
late summer.
Another barrier between gene pools can occur due to polyploidy.
A polyploidy organism has more than two sets of chromosomes.
The normal number of sets is 2, known as diploid (2n), polyploidy organism could
contain 3 or more sets. 3n triploid, 4n tetraploid.
Polyploidy is widespread in plants but rare in animals. This happens when sets
of chromosomes are not completely separated during cell division.
If chromosomes aren’t separated properly in mitosis and the cell fails to
divide after telophase then the cell becomes tetraploid.
Each chromosome will have a matching pair and will be able to undergo
meiosis to form fertile gametes.
A tetraploid can cross with another tetraploid to form fertile offspring.
If a tetraploid crosses with a diploid plant they would produce triploid plants that
would be sterile.
In this case, polyploidy acts as a barrier between the diploid and tetraploid
The populations may become so different that they develop into new species.
Tetraploidy can occur through hybridization of diploid species
T.dubius 2n
T.pratensis 2n
T.miscellus 4n
T.dubius 2n
T. porrifolius 2n
T. mirus 4n
Hybrid Infertility
Barriers between gene pools can also occur as a result of hybrid infertility,
although this doesn’t lead to speciation.
Most plant and animal hybrids are infertile.
Ligers, mules (donkey and horse) they are infertile, so they can not lead to
the development of a new species.
Comparing Sympatric and Allopatric Speciation
Sympatric Speciation
Allopatric Speciation
A new species arises from
existing species that is living
in the same area.
A new species arises
because a physical barrier
separates it from other
members of an existing
Temporal or behavioral
isolation can produce
significant changes in the
genetic make-up within a
species so that a new
species is formed.
Physical barriers may include
mountain ranges, valleys or
bodies of water, or humanmade features such as roads,
canals or built up areas.
Convergent and Divergent Evolution
As the process of speciation takes place, new species form that no longer
resemble one another.
When this occurs, species are said to have diverged from their common
ancestor known as divergent evolution.
New species evolve as organisms exploit new habitats and reduce
competition by occupying their own specific niche.
Over generations, physical and behavioral adaptations develop through
natural selection that help organisms survive and reproduce in their
particular habitat.
Example of this is the pentadactyl limb. Many forms have evolved from a
common ancestral form.
Adaptive Radiation
Adaptive radiation is a form of divergent evolution, and occurs as new species develop from a
common ancestor as they adapt to new environments.
Darwin’s finches on the Galapagos Islands is a good example.
The ancestral finch species that first arrived on the islands had a small but thick beak for eating
small seeds.
On the different islands many other food sources were available
Any birds with beaks that were a slightly different shape, making them better at using a new food
source, would be more likely to survive and pass on their genes.
The change in beak shape would be reinforced with every new generation, eventually producing
different groups of birds that were adapted to feeding on new foods.
Convergent Evolution
Other situations, organisms that have
different ancestors may evolve similar
characteristics, because conditions favor
these particular features.
They make it more likely that an organism
will survive and reproduce successfully in
the environment.
This is known convergent evolution.
These wings are a good example, they
serve a similar function but are derived
from completely different structures.
Convergent Evolution
Similar characteristics but
Different ancestors.
Divergent Evolution
Different characteristics
but same ancestor
Nature’s Various Ways to answer the question:
How to Survive?
Texas Horned Lizard
Hippo Sweat
Blob Fish
Yeti Crab
380 mya
33ft long (10 meters)
3.6 tons
The Pace of Evolution: Gradualism and Punctuated Equilibrium
Darwin viewed evolution as a slow, steady process called gradualism, where
changes slowly accumulate over many generations and led to speciation.
The evolution of the whale to about 52 million years.
Punctuated Equilibrium
In some cases, the fossil record does not contain any intermediate stages between one species and
the next.
One explanation is that fossilization is such a rare event the intermediate fossils simply have not been
In 1972c Stephen J Gould suggested that the fossils had not been found because they don’t exist. He
proposed punctuated equilibrium.
The driving force for evolution is selection pressure, so if the selection pressure is very mild or nonexistent then species will tend to remain the same (equilibrium)
When there is a sudden, dramatic change in the environment, there will be new intense selection
pressures which lead to rapid development of new species.
Example of Punctuated Equilibrium
A good example of intense selection pressure in modern times is the use of antibiotics,
which has resulted in the appearance of resistant species of bacteria in a very short
period of time.
Multile-resistant MRSA (Staphylococcus aureus)
It has become adapted to new conditions in which there are many antibiotics in use
and has developed new strategies to resist them.
Gradualism vs Punctuated Equilibrium
Polymorphism means ‘many forms’ and describes a situation where an
allele has two different forms, which may be selected for in different
If the environments are stable then the relative frequencies of the alleles
do not change and is known as stable polymorphism.
If the environment changes, the frequencies of the different forms start to
change and this is known as transient polymorphism.
Transient polymorphism
In transient polymorphism, one form is gradually being replaced by another.
It represents a temporary situation as a by-product of natural selection.
For example, during the course of industrial melanism, it was held that the
melanic form of peppered moth gradually predominated the non-melanic
form in the trees of Manchester, England due to selective pecking of the
latter (lighter form) by the birds.
Balanced Polymorphism
When natural selection stabilizes two or more alleles in a population so that
their frequencies do not change, a balanced polymorphism occurs.
Sickle-cell anemia provides an example that occurs in certain parts of the
Sickle-cell is caused by a mutation in one of the genes for the oxygencarrying protein hemoglobin
Two alleles of the gene occur HbA and HbS and produce 3 phenotypes.
Most people have the genotype HbAHbA and have normal hemoglobin.
These individuals do not develop sick-cell but they a very susceptible
to malaria.
People who are heterozygous HbAHbS have sick-cell trait. They have
some sick-shaped cells and some normal cells, and are resistant to
Homozygous individuals HbSHbS have sickle-shaped blood cells and
are resistant to malaria but develop sickle-cell anemia, which can be
A balanced polymorphism exists in parts of Africa where malaria is endemic.
Both forms of the allele remain in the population.
Heterozygotes are selected for due to their resistance to malaria but both
homozygotes are selected against because they are vulnerable to either malaria or
sickle-cell anemia.
An unusually high proportion of the population are heterozygotes because of the
resistance to malaria.
Another example of balanced polymorphis is the cichlid fish Perssodus
microlepis, which is found in Lake Tanganyika in Africa.
These small predatory fish dart out from hiding places to bite off the
scales of larger prey fish.
These cichlids have developed a mouth that is angled to the side so they
can attack from behind without being seen.
Some have a mouth angled to the left, so they can attack the right flan of their
prey, but this means their prey will be watching its right flank when it gets into
Perissodus territory.
Others have a a mouth angled to the right, and attack from the other side.
If numbers increased of either phenotype the prey would spend more time
watching that side giving the other an advantage.

similar documents