1. SELECTION AND
EVOLUTION
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2. Important terms
• Natural selection
• Evolution
• Variation
• Overproduce
• Allele frequencies
• Isolating mechanisms
• Artificial selection
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3. ALBIO9700/2006JK

4. 1856
Charles Darwin Alfred Wallace
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5. Evolution
• Changes within lines of descent over time
• Successive changes in allele frequencies in
population as brought about by occurrences
such as mutation, genetic drift, gene flow,
and selection pressure
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6. NATURAL SELECTION
• The way in which individuals with particular
characteristics have a greater chance of
survival than individuals without those
characteristics, which are therefore more
likely to breed and pass on the genes for
these characteristics to their offspring
• The strongest survive and propagate and
therefore increase the strength of the
species
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7. Observation 1
• Organisms produce more offspring
than are needed to replace the
parents
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8. "The elephant is reckoned the slowest breeder of all known
animals, and I have taken some pains to estimate its
probable minimum rate of natural increase; it will be
safest to assume that it begins breeding when 30 years old
and goes on breeding until 90 years old; if this be so, after
a period from 740 to 750 years there would be nearly 19
million elephants descended from this first pair." –
Charles Darwin
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9. Observation 2
• Natural populations tend to remain
stable in size over long periods
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10. Observation 1 + Observation 2
Deduction 1
• There is a competition for survival
(a ‘struggle for existence’)
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11. Observation 3
• There is variation among the
individuals of a given species
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12. Deduction 1 + Observation 3
Deduction 2
• The best adapted variants will be selected
for by the natural conditions operating at
the time.
• Natural selection occurs
• The ‘best’ variants have a selective
advantage; ‘survival of the fittest’ occurs
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13. ALBIO9700/2006JK

14. Examples of Natural Selection
Darwin identified 13 species of finches
in the Galápagos Islands differing in
beak size and shape
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15. Examples of Natural Selection
(cont)
Natural selection was discovered among
"peppered" moths living near English
industrial cities ALBIO9700/2006JK

16. Overproduction
• If left unchecked by environmental factors ,
numbers in a population may increase
exponentially
• Environmental factors
i) biotic – living organisms (predation,
competition for food, pathogenic infection)
ii) abiotic – non-living components (water supply,
soil nutrient levels)
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17. • Natural selection keeps things the way they are
– stabilising selection
• Unless a new environmental factor or new allele
is introduced – directional selection
• Evolution occurs because natural selection gives
some alleles a better chance of survival than
others. Therefore, changes in allele frequency in
a population are the basis of evolution .
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18. Processes that affect allele
frequencies in populations?
• Malaria and sickle cell anaemia
• Antibiotic resistance
• Industrial melanism
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