Download Lecture 5-Variation

Variation;
If there is no variation……..
………………there is no biological evolution
What is Variation?
• Any difference between cells, individual organisms, or groups of
organisms of any species.
• Caused either by - changes in the genetic structure, or by
- by the effect of environmental factors.
• May manifest in physical appearance, metabolism, fertility, mode of
reproduction, behaviour, learning and mental ability, and other
obvious or measurable characters.
Aspects related to Variation
We are going to learn;
1. Origin
2. Maintenance
3. Amplification, &
4. Randomness of variation.
Sisters and brothers of same parents: Similar or different?
1. Origin of Variation
Mutations
Genetic
variations
Crossing over
Chromosomal
mutations
Recombination
Variation
Gene
mutations
• Base substitution
• Deletion
• Insertion/Addition
•
Changes in •
Chromosome •
structure
•
Duplication
Deletion
Inversion
Translocation
Changes in • Heteroploidy
Chromosome • Polyploidy
number
Environmental
variations.
Environmental variations
Sun tanning
Mark...!!! Gone….
Ihi.. Ihi hee
My mark is gone
Are the environmental variations heritable?
No…
They will not alter the genome.
So they are not heritable.
Gene mutations (= Point mutations)
An example for a gene mutation
• Sickle cell anemia
141 AA in each β chain
with glutamic acid at
position 6
Gene for Hemoglobin = Hbs
When Hbs Hbs
Normal Hb are produced
They will have normal RBC.
Mutant gene of Hbs = Hbs
Due to this mutation
Valine comes in place of
Glutamic acid in the
β chain.
When Hbs Hbs
141 AA in
each α chain
Abnormal Hb are produced
They will have abnormal RBC.
Chromosomal Mutations – structural changes
Chromosomal Mutations – number changes – heteroploidy – due to disjunction in meiosis
Down’s syndrome 45 + XY
Down’s syndrome 45 + XX
A Mongolian child
Klinefelter syndrome 44 + XXY
Turner’s syndrome 44 + X0
• polyploidy
Weight of mutations in evolution
• Mutations alter the genome, so that they are heritable (pass from one
generation to the next.
• Recessive,
• Appear as a phenotype when become homozygous,
eg. Hemophilia Normal gene XH
Mutant gene Xh
When XHXH - Normal
When XHXh carrier
When XhXh or Xh (males) hemophiliac - die
• Lethal,
• Will not make a big impact in the process of evolution.
Clock analogue: if a cog of a wheel is broken
(mutation), the clock will not work.
Rarely, though, a good thing also could happen. But it
is very rare.
Crossing over
Nothing happens to the genome.
Genes are only mixed so that the characters
are also mixed
Recombination
In humans, the total
number of possible
recombinations are 246
Due to recombination,
genes/chromosomes
are mixed.
As a result, a large
number of
phenotypes/variations
are produced
Do not worry.. This never happens
Factors causing genetic variations
• Mutagenic chemicals. i.e. Cholcisin, Sodium Azide
• Electromagnetic radiations i.e. x-rays, uv rays, gamma rays etc.
• Age i.e. Non-disjunction during meiosis- produce aberrant gametes.
• Random and spontaneous errors during DNA replication,
• Recombination
Importance of genetic variations in evolution
• Mutations are usually lethal so that they are naturally removed from
a population.
• Recombination (and crossing over) alone will generate a large number
of variations
• They only mix characters. A large number variants with slight changes
are produced
• Better variations are sifted by the environment and left to survive.
• This will continue so that better variants are always selected to
survive.
2. Maintenance of variation
• Variations are maintained within a population due to reproduction
3. Amplification of the variation/better mutation.
Generation
Produces only 2 progeny
Produces 3 progeny
Produces 4 progeny
1
1
1
1
2
2
3
4
3
4
9
16
4
8
27
64
5
16
81
256
6
32
243
1024
7
64
729
4096
8
128
2187
16384
9
256
6561
65536
10
528
19683
262144
The variation will be amplified in the individual that produces the highest number of progeny =
in the individual that has the highest reproductive potential
4. Randomness of variation
Do the variations evolve in
response to changes in the
environment?
Similar Q:
Whales developed
flippers in order to
swim.
Or
Do the variations that
already exist are selected
by the environment?
Or
Whales could swim
because they already
had flippers.
Lederberg’s replica
plating experiment
Conclusion of
Lederberg’s
Duplicate plating
experiment.
The streptomycinresistant bacteria
were there in the
population before
they encountered
streptomycin.
They did not
evolve resistance
in response to
exposure to the
antibiotic.
End of lecture