The processes that have transformed life on
Earth from its earliest forms to the vast
diversity that characterises it today.
“As many more individuals of each species are
born than can possible survive, and as,
consequently, there is a frequently recurring
struggle for existence, it follows that any being, if
it vary however slightly in any manner profitable
to itself, under the complex and sometimes
varying conditions of life, will have a better
chance of surviving and thus be naturally
selected. From the strong principle of inheritance
any selected variety will tend to propagate its
new and modified form.” Charles Darwin
• Wrote in 1859 “On the Origin of Species
by Means of Natural Selection”
• Two main points:
– Species were not created in their
present form, but evolved from
– Proposed a mechanism for
evolution - Natural Selection.
Darwin’s Five Points
1. Population has variations.
2. Some variations are favourable.
3. More offspring are produced than survive.
4. Those that survive have favourable traits.
5. A population will change over time.
Individuals with favourable traits are more
likely to leave more offspring better suited for
• Natural selection happens because there is
variation i.e. natural differences in
• Variation is caused by differences in genes.
• When one species splits into two or more
• Speciation is why we see biodiversity on Earth.
• Steps of speciation
• Variation – within a populations is required
before speciation can occur.
• Isolation – required for a new species to occur
e.g. Grand Canyon ground squirrel.
• Selection – natural selection affects genotype.
Changes prevent groups from breeding in the
• The study of fossils — any form of preserved
remains thought to be derived from a living
• Fossils are an important source of evidence
Geophysical evidence suggests that
geographical regions and climatic conditions
have varied throughout the earth’s history,
and these changes would have favoured a
mechanism for evolutionary change.
Ecological considerations also support this.
Plants appeared on land before animals, and
insects before insect-pollinated plants.
Evidence from fossils
• The lowest rock layers are usually the oldest
• The oldest rock layers contain the oldest fossils.
• The rock layers that formed later contain more
complex kinds of organisms.
• The variety of fossils increases in the upper, more
recent layers of rock.
• No fossil records exist of modern, living plants and
Relative age - Relative means that we can
determine if something is younger than or older
than something else. Relative time does not tell
us how old something is, all we know is the
sequence of events. For example: the
sandstone in this area is older than the
Absolute age - Absolute age means that we can
more or less precisely assign a number (in years,
minutes or seconds) to the amount of time that
has passed. We can say how old something
is. For example: The sandstone is 300 million
Early fossil examiners were able to correlate or
match layers of sedimentary rock merely by
looking at the fossils they contained (fossils in a
particular rock were different from rocks above
Some plant and animals only lived a short
time in geological history, yet had a wide
spread distribution. Known as index fossils –
used to correlate layers of rock and therefore
determine the age of the layer.
Biogeography – patterns of distribution
• Distribution gives clues to the evolutionary
history of organisms and of the Earth itself.
• A major cause of speciation in most groups is
thought to be geographic fragmentation of
ancestral species e.g. continental drift and
consequent isolation of populations.
• Comparative anatomy is the study of similarities
and differences in the anatomy of organisms.
• Two major concepts of comparative anatomy
Homologous and Analogous Structures
• Homologous structures are those that are
similar in structure and development but
which may have different functions.
• The wing of a bird, the flipper of a whale and
the leg of a dog all possess the structural plan
of the pentadactyl (5-digit) limb although their
• Homology, therefore, implies common
ancestral origins and suggests that
differences in structure have evolved in
response to different environmental
• This is called divergent evolution.
• Analogous structures have a similar function but
no structural relationship. The wing of an insect
and the wing of a bird serve the same function
— flight—but are structurally dissimilar.
• This suggests that these two groups have
different ancestral origins.
These structures are regarded as examples of
convergent evolution, whereby structures
with different origins have become adapted to
a common function.
• Parallel evolution occurs when related
species evolve similar features
• For example, within eucalyptus, a number of
species have evolved a white, waxy coating
on their leaves, which protects them from
frost damage at high altitudes or from water
loss in dry conditions.
An organ that was once useful in an animal’s
evolutionary past, but now has no apparent
nor predictable function e.g. rudiments of
pelvis and hind limbs in snakes, wings on many
Limitations of fossils
• There are many gaps in the fossil record. This
• dead organisms decompose rapidly.
• dead organisms are eaten by scavengers.
• soft-bodied organisms do not fossilise easily.
• only a small fraction of organisms die in
conditions favourable to fossilisation.
• only a fraction of the fossils have been