NATURAL
SELECTION
The great biological paradigm
© 2008 Paul Billiet ODWS
Observation 1:
Exponential growth
Populations tend to produce more
offspring than the environment can
support
© 2008 Paul Billiet ODWS
Thomas Malthus
Essay on the Principle of Population
(1798)
 Populations in nature cannot
continually increase sooner or later
food supply is insufficient and famine
stops further growth
 Both Charles Darwin and Alfred
Russel Wallace had read Malthus and
understood the idea of exponential
population growth
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© 2008 Paul Billiet ODWS
Population Growth
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1 pair of cockroaches could produce 164 000
million in 7 months
Cockroach population
© 2008 Paul Billiet ODWS
Observation 2: Zero growth
The numbers of individuals in a
population remain stable
In terms of population growth the
population at its carrying capacity has
zero growth
© 2008 Paul Billiet ODWS
Population growth
K
3
2
Numbers
1
© 2008 Paul Billiet ODWS
Time
Deduction 1 Competition
There must be a struggle for survival
Some of the offspring produced in a
generation do not survive.
Darwin identified competition as a
major factor limiting population sizes
© 2008 Paul Billiet ODWS
Observation 3: Variation
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Some of these variations are inherited by
the offspring
The mechanism of the inheritance of genes
was being worked out at this time remained
undiscovered by biologists until 1900
Darwin was however aware that sexual
reproduction mixes variations to produce
new combinations (recombinants)
© 2008 Paul Billiet ODWS
Neo-Darwinism
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The great synthesis of the 20th century
Mendel’s work was able to explain some of
the patterns of inheritance through the
mixing that occurs during meiosis and
fertilisation
Darwin could not explain the origin of new
variants
This had to wait until the 1920s and 1930s
when work began on mutations after the
discovery of radiation
© 2008 Paul Billiet ODWS
Deduction 2: Survival of
the fittest
There will be a struggle for survival
between the members of the
population
 Individuals with advantageous
variations will breed and produce
more offspring
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© 2008 Paul Billiet ODWS
Natural selection in action
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As generations pass by, the proportions of
the alleles for the different variants will
change in favour of those that provide the
best adaptations
Natural selection has been observed at
work in populations of species over the past
century
Examples include:
pesticide resistance in insects,
antibiotic resistance in bacteria,
industrial melanism in moths,
tolerance to heavy metals in plants
© 2008 Paul Billiet ODWS
The Origin of Species by
Natural Selection
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Darwin and Wallace argued that if natural
selection proceeded for a long enough
period of time it could bring about the
evolution of new species
Darwin himself favoured a long period of
slow changes
Recently this has been refined to include
the possibility of rapid changes over a short
period of time (punctuated equilibrium)
© 2008 Paul Billiet ODWS
Natural selection is not the
only way
Whether fast or slow, observing the
evolution of a new species is unlikely
in the lifetime of a scientist
 That species evolve is a fact but that
they evolve by natural selection
remains a theory
 Other mechanisms exist that can also
lead to the evolution of species
(e.g. genetic drift)
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© 2008 Paul Billiet ODWS
Convergent Evolution
Thylacine Thylacinus cynocephalus
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Species exposed to the
same selective pressure
in different parts of the
world tend to develop the
same adaptations
Even though they may be
completely unrelated
e.g. the placental wolf and
the marsupial thylacine or
Tasmanian wolf
© 2008 Paul Billiet ODWS
Wolf Canis lupus
Divergent Evolution
Populations of a species that are
separated and evolve under different
selective pressures develop different
adaptations as they diverge
 They are usually geographically
separated so that there is a barrier to
the mixing of genes
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© 2008 Paul Billiet ODWS
Adaptive radiation
Species that populate a new area where
there are vacant niches will diverge and
specialise as they fill the vacant niches
(e.g. Darwin’s Finches on the
Galapagos)
© 2008 Paul Billiet ODWS