Download Variation

Variation
Mutation
-
a “spontaneous change in a gene or a chromosome”
-
mutations are natural phenomena
most are harmful or lethal or lead to the death of the organism
only a few are beneficial
they are unpredictable as they are random and spontaneous
they are also rare
A mutation is the process of changing
A mutant is the new form after the mutation has occurred.
The mutation rate can be increased by:
(a)
Radiation
X-rays, γ-rays (gamma), β-rays (beta) (e.g. leukaemia)
(b)
Chemicals mustard gas, nitrous oxides / acids
(c)
Antibiotics act as selective agents
(d)
For insects, mutation rate is doubled by a 10ºC rise in temperature.
Some mutation enables the species to adapt to the new environments.
The presence of mutant characteristics can be related to a change in the structure of the
chromosome (or gene):
(a)
Chromosome Aberration
in which a whole or part of a chromosome becomes rearranged
Example:
Down’s syndrome (a rare condition)
there is an extra chromosome on chromosome 21
person becomes both mentally and physically retarded
more likely to occur with females over 35 conceiving
(b)
Chromosome Multiplication
in which the number of chromosomes is increased by a “multiple” of
the haploid number
this covers all forms of polyploidy
Example: in 1870, a new species of rice grass was discovered on the mud flats of
Southampton water
(c)
Gene Mutation
a change in the chromosome at a molecular level
cannot be observed under a microscope
e.g. haemophilia – mutant appears unaltered, sickle-celled anaemia
HbAHbA
HbAHbS
HbSHbS
- normal blood
(but can suffer from malaria)
- some sickle cells, but survive OK (resistant to malaria)
- die young (e.g. 6 months) because person cannot bind oxygen and the
sickle-shaped red blood cells get trapped in the capillaries.
N.B. It is advantageous to be HbAHbS in areas where malaria is a killing disease.
The HbS allele gives resistance to malaria.
Selection
(a)
Artificial selection
“man selects individuals who’s genes will contribute to the next generation”
Reasons:
for increased economic yield (e.g. milk production / grain per acre)
appearance (e.g. colour)
food value (e.g. protein content / low fat content)
resistance to disease
This has been done for years – it is “applied genetics”. For example, a farmer may
use a cow, which produced bad milk for meat, but a cow that produces good milk for
reproduction purposes.
Problems:
(a)
breeds die
(b) organism becomes similar as they share the same genes
(c)
if a new disease emerges and all are susceptible, all die out
(d) loss of variation reduces the chances of long-term survival
Remedy:
(a)
seed banks (containing genes of past species)
(b)
rare-breed societies
(b)
Natural selection
Thomas Malthus (1798) studied population growth. He predicted that the number of
people in the world would eventually be regulated by famine, disease and war.
1858: Darwin and Wallace – read Malthus’s essay and with their own observations
produced the “theory of evolution by natural selection”
1st observation
Each generation of a species has more offspring than parents
2nd observation
Despite the high reproduction rate, the total number of most
species remain about the same once established in a particular
environment.
Conclusion 1
There must be something controlling the numbers of a
population. (i.e. the struggle for survival) – especially for the
young so that they die before the reproducing age – only
strongest pass on genes.
3rd observation
individual organisms in a species differed from one another in
some ways – i.e. there was variation
4th observation
although the offspring are different in some ways, they still
inherit many parental characteristics. (Darwin found this without
understanding genetics)
Conclusion 2
certain variations help an organism to survive the struggle for
existence – others do not.
Darwin’s theory
The survival of the fittest by natural selection
-
nature selects those organisms that are best suited for survival.
-
these reproduce and pass on the genes that enabled them to survive to the next
generation
Darwin did notice unique variations – not passed on by either parent
This is now known as mutation.
Conclusion
variation, natural selection and inheritance limits unfavourable characteristics and leads to
the accumulation of more favourable characteristics in a species.
Example:
Evolution of giraffe
Lamark (15-20 years before Darwin)
Darwin
-
food became short on ground
giraffes turned to trees for food
in the process, they stretched their necks
passed on this characteristic to their offspring
there was a variation of the neck length in the giraffe population
the food was short on the ground (selection pressure)
those with longer necks reached the leaves on the trees and survived
they reproduced and passed on their genes – for longer necks – to offspring
those with shorter necks starved / died – did not reproduce
Natural selection depends on the pressure from the environment. These pressures are known
as selection pressures and they select the better-adapted variations.
Example – sickle-celled anaemia
condition where some red blood cells that are abnormal are in the blood
the oxygen is not supplied properly to the cells
but, sickle cells provide resistance to malaria
Mutant gene = ‘h’
Homozygous (HH): advantage in countries without malaria, but disadvantage where
malaria is present – may die of malaria
Heterozygous (Hh): some sickle cells, but survive anyway. Resistant to malaria
Homozygous (hh): sickle cells only. Poor carriage of O2. Die young.
Selection pressure here is the disease malaria.