Natural Selection

The Tree of Life
• All living things share a common
• We can draw a Tree of Life to
show how every species is related.
• Evolution is the process by which
one species gives rise to another
and the Tree of Life grows
The central idea of evolution is that life has a
history. The living things on Earth have
changed over time, and that the different
species on Earth today share common
Over a great period of time evolution can result
Speciation - The formation of a new species
How does speciation
• Millions of
organisms that lived
in the past have
gone extinct. We
know of their
existence from
fossils or rarely from
frozen specimens.
Related Species
• There are many
organisms that are
different species, but
are obviously very
closely related. It is very
apparent that the
American cougar and
the African lion are
related. They both
share a common
ancestor from the past.
Related Species
• All of the many cat
species are related.
They have very
similar anatomy and
DNA. Cats share a
common ancestor
that lived in the past.
Not all cat species
that evolved are
alive today.
Related Species
• The saber- toothed
cat became extinct
about 10,000 years
ago It has the same
basic skeleton as
other cats. The most
feature of this cat is
its long canine teeth.
Evolution as Theory and Fact
• Confusion sometimes arises as to
whether Evolution is a theory or a fact.
Actually it is both!
• The theory of Evolution deals with how
Evolution happens. Our understanding
of this process is always changing.
• Evolution is also a fact as there is a
huge amount of indisputable evidence
for its occurrence.
Rodin’s “The Thinker”
Darwin’s Voyage
• From 1831-1836, a
young naturalist called
Charles Darwin toured
the world in HMS
Voyage of the Beagle
• He was dazzled by the
amazing diversity of
life and started to
wonder how it might
have originated
Survival of the Fittest
• In his Origin of Species,
published in 1859, Darwin
proposed how one species
might give rise to another.
Natural Selection
explains adaption
• Where food was limited,
competition meant that only
the fittest would survive.
• This would lead to the natural selection
of the best adapted individuals and
eventually the evolution of a new species.
Darwin in 1860
Evolution is possible because
• Genetic Variation
Why Genetic Variation?
If a population was 100% the same…
the processes that cause evolution
would have NO effect because the
genetic make-up would never
Evolution only occurs when there is a
change in gene frequency within a
population over time.
1st generation of Beetle
Many generations later
of the same population
What causes these
There are 5 main mechanisms for
evolutionary changes:
5 Agents of evolutionary change
Gene Flow
Genetic Drift
Non-random mating
Natural Selection
•A change in a DNA sequence, usually
occurring because of errors in
replication or repair. Mutation is the
ultimate source of genetic variation.
Fruit Fly Example
Types of mutation
• However, occasional
mutations or copying errors
can and do occur when
DNA is replicated.
• Mutations may be caused
by radiation, viruses, or
Mutant fruitfly
• Mutations are rare and often have
damaging effects. Consequently organisms
have special enzymes whose job it is to
repair faulty DNA.
Sometimes mutations are
Only if they help
the organism
• Mutations produce the genetic variation
needed for evolution.
Gene Flow
Either by immigration or emigration
(individuals move to another population
and shares their genes there).
• Gene flow moves alleles from one
population to another.
Genetic Drift
The change of
allele frequencies
in a population
over time due to
random events
Genetic Drift
• A random change in allele
• These individuals may
carry alleles in different
relative frequencies than
did the larger population
from which they came
• If so, the population that
they found will be
genetically different from
the parent population
• This cause is not natural
selection, but chance
Genetic Drift
For example:
Green beetles reduce in numbers because
an external factor has caused them to
produce less offspring causing a more
dominant brown population.
• Genetic drift changes allele frequencies due
to chance alone.
Natural Selection
A process of evolution in which traits that
result in better fitness of an individual
survives to the next generation.
• Natural selection selects for traits
advantageous for survival.
Non-Random Mating (Sexual Selection)
Peahens choose their mates by the size and shape
of his tail. This makes sense in evolutionary terms -the largest tail would indicate a healthy bird and a
better chance for healthy offspring.
The process by
which organisms
choose their mates
based on
advantageous traits
that will be carried to
the next generation.
Reproductionbrings together a
new combination of
• Introduces new
combinations of
genes every
Sexual reproduction = Genetic Variation
• In nature, populations are expected to
evolve because a population can’t maintain
equilibrium forever.
– Population numbers will
always be threatened by
different factors
– Individual organisms will
continue to migrate
– New genetic mutations occur
– Species select mates; random
mating rarely occurs
– The environment has
changed on Earth and
continues to change
Evolution at work
Although evolution cannot be seen on the
small scale of time, the works of evolution
are occurring all the time in populations.
These processes are what contribute to the
evolutionary changes between all species.
"One obstacle to understanding evolution
is the common misconception that
individual organisms evolve. It is the
population, not its individuals, that
evolves, as some heritable variation
becomes more common at the expense
of others."