Transcript SICKLE CELL ANEMIA

Patterns of Natural Selection
and Speciation
Natural Selection
 Genes provide the source of variation. The
environment selects for the best adapted
phenotype. An allele is only common where
it will provide an advantage. (Natural
Selection)
Mutations
1. can be neutral, harmful or beneficial
2. Harmful mutations result in dysfunctional
proteins, they occur frequently but they are
selected against and remain rare.
3. Beneficial mutations allow the cell to produce
a new or improved protein and gives the
individual a selective advantage. They are
rare, but are selected for and become more
common over time.
Example: Sickle Cell Anemia
 Serious blood disorder due to single base pair mutation (point
mutation) leading to a change in one amino acid that makes
up the hemoglobin protein.
 Results in RBCs being sickle shaped, cannot hold oxygen well
 Suffer from fatigue, malaise, jaundice, other minor problems
 Sickle shaped RBCs are more prone to clogging blood vessels
which can be fatal.
The heterozygous advantage
 Heterozygous individuals are only mildly affected by the
disorder since it is codominant, both normal and sickle RBCs
are made.
 Benefit: more resistant to malaria, since the malaria causing
protist cannot infect sickle RBCs and there are not enough
normal RBCs
 Question: Is being heterozygous for sickle cell anemia an
advantage or disadvantage?.....
 Answer: it depends on where you live!
Advantage or Disadvantage?
 Disadvantage in regions where malaria is uncommon
(ie. North America)
 Advantage where malaria is common: Sub-Saharan
Africa
 heterozygous individuals are strongly favoured,
heterozygote advantage; they are more likely to survive
than either homozygous group.
Blue = malaria
Red = sickle cell anemia
Purple = overlap
Question:
Would you expect the sickle cell anemia
allele to be found more frequently in
Sub-Saharan Africa or in North America?
Why?
Types of Natural Selection
Directional selection
 – The environment favours
individuals with an
extreme variation of a
trait.
 Occurs when organism
moves to a new
environment
 ex. gill nets and salmon
fishing in 50s and 60s
Stabilizing selection
 The extremes of a
population are selected
against and the average
is favored.
 Once well adapted to
environment, selection
pressures tend to
prevent them from
changing
 ex. baby weights (3kg)
Disruptive selection
 The environment selects for
extremes and against the
average.
 Example: Peppered moths in
London – those living in rural
areas were almost all light in
colour, while moths in
industrialized areas were all
dark in colour – no medium
coloured moths
Peppered moth simulation
 http://www.techapps.net/interactives/pepperMoths.swf
Sexual selection
 Favours the trait that influences mating success
 Usually based on female choice and/or male vs. male
competition
 Male competition
 Male competes against other males for territory,
or access to females
 Anything that gives him an advantage makes him
more likely to pass on his genes
Sexual Selection
 Female selection (or male selection)
 Leads to sexual dimorphism (physical
differences between males and
females)
 Male must prove he is genetically
good enough
 Plumage, gifts, nesting site or mating
rituals
Reproductive isolation and
Speciation
 Speciation: The evolutionary formation of new species.
 Species: members of groups or populations that
interbreed or have the ability to interbreed with each
other under natural conditions.
 reproductively isolated from other groups
 evolve independently
Modes of Speciation
 Reproductive isolation may lead to speciation.
 The gene pool is isolated, any mutation and
selection that occurs is no longer shared;
 any significant evolutionary changes that
occur in either population (new or old) will
result in the formation of separate species.
Allopatric speciation
 Evolution of populations into separate species as a
result of geographic isolation. Ex. water, canyon,
mountain range, human construction (dams,
highways, canals)
Allopatric speciation
Sympatric speciation
 Evolution of populations within the same
geographic area into separate species, by
exploiting a new niche. (ex. Flies that feed on
hawthorns vs apples)