This document discusses key concepts of natural selection and evolution, including that organisms overproduce offspring but population sizes remain stable, indicating competition for survival. There is genetic variation within species on which natural selection acts, favoring traits that increase reproductive success. Examples are provided of natural selection in finches and moths. Selection changes allele frequencies over generations, driving evolutionary change.

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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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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