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12DA Biology
Variation and Selection
UNIT 8
Unit 8 REVISION NOTES: Variation and Selection
Variation between individuals in a species is caused by differences in their genetics or by
their environment.
Genetic Variation
Each gamete produced by a parent contains a different mix of chromosomes (Independent
Assortment)
Environmental Variation
This is caused by changes in their surroundings. For example plants in good light will
grow much taller than plants in poor light.
Some variation is due to the combination of genetic variation and environmental variation.
E.g. Height – need good diet,
Skin colour – exposure to sun becomes darker.
Continuous Variation
This is where there is a gradual change across the population. E.g. Height, mass
Discontinuous Variation
This is where there is a distinct difference in the population, the population can be placed
into specific groups.
E.g. roll tongue, ear lobes, blood group.
Variation, Selection and Evolution
If there is a lot of variation between individuals in a species, it is likely that some will be
better adapted to survive.
Animal Example:
A giraffe with a longer neck will be able to obtain more food than one with a shorter neck.
The shorter neck will die, the longer neck will survive to reproduce!
Plant Example:
12DA Biology
Variation and Selection
UNIT 8
A larger seedling will have longer roots, larger leaves, it will be easier for it to obtain water
and sunlight, therefore it will grow larger and survive. The smaller seedling will die!
Antibiotic Example:
 Bacteria treated with antibiotics
 Most are destroyed
 Those that survive contain the gene which is not affected by antibiotic
 Multiply
 Treated with antibiotics again
 Does not kill any, because they contain the gene which gives RESISTANCE
to the antibiotic.
Charles Darwin’s Theory of Evolution:
 Variation between individuals
 Competition for resources
 Fittest (strongest) survive!
If it does not ADAPT?
The organism will become extinct!
e.g. Woolly Mammoth became extinct because of climate change.
Dodo became extinct because of Man hunting them.