Transcript NATURAL SELECTION

NATURAL
SELECTION
The mechanism for biological
evolution.
Observation 1:
Exponential growth
Populations tend to produce more
offspring than the environment can
support.
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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Population Growth
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1 pair of
cockroaches
could produce
164,000,000,000
in 7 months, if
there were no
limiting factors.
Human Population Growth
We see the
same pattern in
human
population
growth.
 We haven’t
bumped into our
limits yet.

Observation 2: Zero growth
In a balanced ecosystem, the
numbers of individuals in a population
remain stable.
 In terms of population growth the
population at its carrying capacity
has zero growth
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Population growth
K
3
2
Numbers
1
Time
Deduction 1: Competition
There must be a struggle for survival
 Some of the offspring produced in a
generation do not survive.
 What determines who the survivors
are?
 Darwin identified competition as a
major factor limiting population sizes
Observation 3: Variation
and Adaptation
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The competition for survival tends to be won
by individuals that have genetic variations
that help them survive.
The mechanism of the inheritance of traits
was being worked out at this time and
remained undiscovered by biologists until
1900
Darwin was however aware that variations
were somehow inherited and passed on
from parents to offspring.
Deduction 2: Survival of
the fittest
There will be a struggle for survival
between the members of the
population
 Individuals with advantageous
variations (adaptations) will breed and
produce more offspring
 Over time, the population will become
more like the individuals with an
adaptive advantage.
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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
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)
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 natural selection is not the
only mechanism.
Genetic Drift
In every generation, individuals may
survive out of sheer luck.
 The random survival of individuals
will change the makeup of the next
generation in random ways.
 Random survival does not lead to
adaptation.

Mutation
Mutations are changes in the genetic
code that happen by chance in the
cell.
 Only mutations that happen in sex
cells affect evolution.
 Mutations are not likely to be
adaptive, but sometimes they are.
 Without mutation, there would be no
evolution by natural selection.
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