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Variation and Natural Selection
Chapter – 19
Variation: Difference between individuals of the same species. There are two
kinds of variation continuous and discontinuous variation.
Variation
Continuous
Different characteristics within a population
Height, Body mass, Intelligence
Discontinuous
Distinct features
Blood group, toungue roll, ear lobe
Continuous Variation
Properties
Discontinuous Variation
 No distinct categories
 Distinct categories
 No limit on the value
 No in-between
 Tends to be quantitative
categories
 Tends to be
qualitative
Examples
 Height
 Tounge rolling
 Weight
 Finger prints
 Heart rate
 Eye colour
 Finger length
 Blood group
 Leaf length
Representation
 Line graph
 Bar Chart
Controlled by
 Gene and environment
 Genes to some
 Range of phenotype
extend
between two extremes
 Limited number of
 Eg. Height in humans
phenotypes with no
intermediates
 Eg. A, B, AB and O
blood groups in
humans
Genetic Variation:
Variation in the individual is due to their difference in their genotype. Eg. Eye
colour, hair colour etc.
Causes of genetic variation:Mutation: A change in gene or chromosome
Gene mutation: A change in the base sequence of DNA
Factors that make mutation more likely:
Ionising radiation:
Radiation can damage bases in DNA molecules.
Chemical mutagens:
 Heavy metals like lead and mercury interfere with the process in which
DNA is copied
 Tar from cigarette smoke.
Environmental variation:
Variation due to the difference between the environments of individuals
Eg. Flower colour in hydrangeas, the plant produce blue colour flower in acidic
soil and pink flowers in alkaline soil.
Adaptive features:An inherited feature that helps an organism to survive and reproduce in its
environment
Or
The inherited functional features of an organism that increase its fitness.
Fitness:The probability of an organism surviving and reproducing in the environment in
which it is found
Xerophytes or desert plants:
Plants that are adapted to live in places where water is in short supply are called
xerophytes.
Adaptive features of desert plants or xerophytes:
 Waxy cuticle
 Hairy leaves
 Fewer stomata and stomata on the lower epidermal layer
 Small surface area
 Deep and spreading roots
Hydrophytes:
 Plants that live in very wet places or water is called hydrophytes.
 These plants have no problem of water shortage
 They don’t need adaptations to conserve water
Define Natural selection:
The organisms are well adapted to their environment. They have greater chance to
breed and pass on their genes to next generation.
Components of natural selection:
Variation, over population, competition, differential survival, adaption, evolution
Natural selection allows species to respond to the changes in the environment
Industrial melanism:
There were two types of moth in Britain and Ireland
Light peppered moth (speckled) and black peppered
The light peppered moth camouflaged very well against light lichens on trees
In 19th century pollution killed off some of the lichens on the tree and soot
deposited on the bark caused the bark to appear black
Now the dark moths had a better camouflage
Light coloured moths were no longer camouflaged and were eaten by predators
The dark moths had a greater chance of reproducing and passing on the alleles that
made them dark
This lead to a gradual increase in the proportion of dark moths
Selection pressure:
The factor which confers an advantage on the dark moths and a disadvantage on
the pale moths in polluted areas were predation by birds. This is called selection
pressure.
Artificial selection: Humans choose a feature of an organism to improve plants and animals
 These plants and animals are bred to produce next generation
 The good offspring is kept for breeding
 This selective breeding is continued for many generations.
Antibiotic resistance in bacteria
 Bacteria can become resistant to certain antibiotics
 In a large population of bacteria, there are some that are not affected by
antibiotic
 They survive and reproduce
 MRSA is (methicillin-resistant staphylococcus aureus) very dangerous
because it is resistant to most antibiotics
 Development of resistance is due to mutations occur in the gene of
individual bacterial cells
 The mutations protect the bacterial cell from the effect of antibiotics
The problem of antibiotic resistance can be limited by:
 Using antibiotics less often
 Do not use antibiotics for viral or fungal infection
 Complete the course of antibiotics prescribed
 Develop new antibiotics
 Don’t use the same antibiotics for too long
 Use combination of antibiotics.
 Isolate the antibiotic resistant patients to maintain a good hygienic condition
and to prevent spreading of infection.
Sickle cell anemia:
 Sickle cell anemia is a genetic disorder
 There is a mutation in the gene that codes for the production of
haemoglobin.
 The normal allele is HbA codes for normal haemoglobin
 The mutant allele is HbS codes for faulty type of haemoglobin
 This faulty haemoglobin has a tendency to produce fibres inside red blood
cell when oxygen concentration is low
The problem that occurs as these cells circulate in blood system: RBC get stuck in capillaries and the capillaries get blocked
 Reduced supply of oxygen to tissues
 Reduced removal of carbon dioxide or waste
 The person may feel tired and short of breath
 Cells cannot respire much to release energy for body activities
 Causes sickle cell crisis that may damage many different organs including
the kidney, liver, heart etc.
 They can cause pain that can last for a few hours to week
 Increased chances of thrombosis
 A person with HbA HbA genotype has normal haemoglobin (Homozygous)
 A person with genotype HbA HbS has a mixture of normal and sickle cell
haemoglobin (Heterozygous)
 A person with genotype HbS HbS has all sickle cell haemoglobin.
Why sickle cell anemia is common in people who live in are where malaria
occurs:
 Malaria is a serious disease caused by a single celled parasite that is injected
in blood by an infected mosquito bites
 Malaria is very common in many parts where the sickle cell allele is present
in the population
 People homozygous for the sickle cell allele die early for sickle cell disease
 People homozygous for the normal allele die early from malaria
 People heterozygous were resistant to malaria with all normal haemoglobin