IB Evolution
EVOLUTION OCCURS WHEN HERITABLE
CHARACTERISTICS OF A SPECIES CHANGE
Clear Distinction
 Acquired characteristics – develop during your
lifetime

ex. Tan skin in the summer, dying your hair
 Heritable Characteristics – Passed down from parent
to offspring

Ex. Eye color, dimples
 Evolution ONLY concerns heritable characteristics.
Evidence from fossils
 Fossils have been found in rock strata
 Fossils have been dated using radioisotope dating
 Sequence of fossils in which they appear matches the
sequence of when they evolved.

Bacteria – simple algae- fungi/worms – land vertebrates –
boney fish – amphibians – reptiles – birds - mammals
Selective Breeding
 Selective breeding of domesticated animals shows
that artificial selection can cause evolution

Livestock, dogs, egg-laying hens
It is clear that domesticated breeds have not always existed in
their current form. Humans have selected what they wanted –
Artificial Selection
Homologous structures
 Adaptive Radiation – Similar structure, different
functions.

Ex. Arms of a human, flipper of a whale

Homologous structures – may look superficially different
and perform a different function, but have the same structure
Pentadactyl limbs – composed of humerus, radius/ulna,
carpals/tarsals, metacarpals/metatarsals

Analogous structures – Same function, different structure.
Ex. Wings of a bird, wings of a butterfly
Convergent Evolution – different origins and have become
similar because they perform the same or similar function
Vestigial Organs
 Reduced structures that serve no function
 Teeth found in embryo whales – adults toothless
 Appendix, wisdom teeth
 Small pelvis bone found in some snakes
Speciation
 Populations of species can gradually diverge into
separate species by evolution.
 If two populations of a species become separated so that
they do not interbreed, and natural selection then acts
differently on the two populations, they will evolve in
different ways.
 After a time, they will be recognizably different
 If the populations merge, and have a chance to
interbreed, but don’t, they have evolved into separate
species
Speciation may occur where different parts of a
population of a species become isolated from
one another.
Example: a species of butterfly is widely
distributed across an area of grassland.
Climate changes
cause the area to
become wetter…
Because the
butterflies are not
strong fliers…
…and
a river forms
…they cannot cross the
river
The two groups of butterflies are now isolated from
each other
One side of the
river remains as
grassland
The other side
develops into
woodland
The selective pressures
are different on either
side of the river, so…
…the two populations
evolve different wing
patterns
NOTE: At this stage the
two populations may
not be different enough
to be considered as two
separate species…
…they may be considered
as subspecies or varieties.
The climate changes
again and becomes
drier
The river barrier is lost
The land becomes mainly
grassland
The two populations of
butterfly can now mix
together, but…
…successful mating requires that they
recognise each other by wing pattern, so…
…they do not interbreed.
The two populations are now definitely different species.
Evidence from patterns of variation
 Gradual Divergence – populations that are
recognizably different, but not to the extent that they
are clearly separate species
-Ex. Peppered Moth
Variation
 Natural Selection can only occur if there is variation
amongst members of the same species

If all individuals were identical, there would be no way of some
individuals being favored more than others, and no evolution
Sources of Evolution
 Mutations – new alleles are produced by gene
mutation which enlarges the gene pool of a
population
 Meiosis – produced new combinations of alleles in
crossing over and the independent orientation of
bivalents
 Sexual reproduction – gametes come from
different parents, so the offspring has a combination
of alleles from two different individuals

In species that do not carry out sexual reproduction, the only
source of variation is mutations.
Adaptations
 Characteristics that make an individual suited to its
environment and way of life.
 The term ‘Adaptation’ implies that characteristics
develop over time and thus that species evolve.
 Adaptations develop by natural selection, not with the
direct purpose of making an individual suited to its
environment.
 Adaptations do NOT develop over a lifetime of one
individual. IF they do, they are acquired characteristics
and are not inheritable.
Overproduction of offspring
 Living organisms vary in the number of offspring
they produce.

Slow breeding 1 offspring every 3 years

Southern ground hornbill
 Fast breeding rate – apart from bacteria, the fastest
of all is the fungus Calvatia gigantea

Produces 7 trillion spores
 No matter what the breeding rate, there is an overall
trend in living organisms for more offspring to be
produced than the environment can support
 This will lead to a struggle of existence within a
population
 This will lead to survival of the fittest
 This is evolution