IB BIOLOGY UNIT 5: EVOLUTION
5.1.1 EVOLUTION
● Evolution can be defined as the cumulative change in the heritable characteristics of a
population over time.
● "heritable" means that the changes must be passed on genetically from one generation
to the next, which implies that evolution does not happen overnight.
● "cumulative" means that usually one change is not enough to have a major impact on a
species.
● "population" means that the changes do not affect just one individual.
● The fossil record, selective breeding, and homologous structures are three of the most widely
recognised pieces of evidence for evolution by natural selection.
● Besides these, biogeography, vestigial structures, and comparative embryology can be used
to provide further evidence for evolution by natural selection.
5.1.2 FOSSIL RECORD
● Fossils are the petrified remains or traces of animals and plants, and the fossil record is
the accumulation of evidence from these remains and traces, such as skeletons and
footprints.
● The fossil record shows that over time changes — evolution — have occurred in the features
of living organisms.
● In order for fossilisation to occur, the following conditions are required:
● Hard body parts such as bones, teeth, and shells;
● Preservation of remains;
● High pressure to promote mineralisation of remains to turn hard body parts into
fossilised rocks;
● Anoxic (low oxygen) conditions to protect against oxygen damage and prevent
decomposition by saprotrophs.
● The stages of fossilisation generally occur as follows:
1. Death and Decay – Soft body parts are decomposed or scavenged, leaving only the
hard body remains.
2. Deposition – The hard remains are rapidly covered with silt and sand, and over time
more layers continue to build.
3. Permineralisation – Pressure from the covering layers of dirt/rock cause the hard
organic material to be replaced by minerals.
4. Exposure – Movement of earth plates may displace the fossil and return it to the
surface for discovery.
,● Using the fossil records, it has been found that:
● The life that existed more than 500 million years ago was vastly different in appearance
from life today.
● Although Earth has had extensive oceans for most of its existence, fish fossils have
only been found in rocks 500 million years old or younger.
● Although most of the top predators today are mammals such as bears, none of them
existed at the time of the dinosaurs or before.
● Apart from organisms such as certain types of sharks and cockroaches, the majority of
living organisms today have no similar form in the fossil record.
5.1.3 TRANSITIONAL FOSSILS
● While fossils may provide clues as to evolutionary relationships, it is important to realise that
the fossil record is incomplete.
● Fossilisation requires an unusual set of specific circumstances in order to occur, meaning very
few organisms become fossils.
● Only the hard parts of an organism are typically preserved, meaning usually only fragments
of remains are discovered.
● With limited fossil data, it can be difficult to discern the evolutionary patterns that result from
ancestral forms.
● Transitional fossils demonstrate the intermediary forms that occurred over the
evolutionary pathway taken by a single genus.
● They establish the links between species by exhibiting traits common to both an ancestor and
its predicted descendents.
● An example of a transitional fossil is archaeopteryx, which links the evolution of dinosaurs with
its jaws and claws and to birds by its feathers.
● As new fossils are discovered, new evolutionary patterns are emerging and old
assumptions are challenged.
5.1.4 RADIOACTIVE DATING
● Fossils can be dated by determining the age of the rock layer in which the fossil is found.
● Different kinds of organisms are found in rocks of particular ages in a consistent order,
indicating a sequence of development:
● Prokaryotes appear in the fossil record before eukaryotes.
● Ferns appear in the fossil record before flowering plants.
● Invertebrates appear in the fossil record before vertebrate species.
, ● This chronological sequence of complexity by which characteristics appear to develop
is known as the law of fossil succession.
● This ordered succession of fossils suggests that newer species likely evolved as a
result of changes to ancestral species.
● One of the most commonly used methods for determining the age of fossils is via
radioactive dating.
● This method involves comparing the ratio of radioactive isotopes in the fossil to that
found in the atmosphere today.
● Radioisotopes are alternative forms of an element that have the same number of protons but a
different number of neutrons. These isotopes are unstable and decay at a constant rate to
form a more stable daughter isotope.
● There are three types of radioactive decay that can occur depending on the radioisotope
involved:
● Alpha Decay – Atom releases two protons and two neutrons to form a new, more
stable element.
● Beta Decay – A neutron decays to produce a proton, electron and anti-neutrino and the
electron is released to form a new element.
● Gamma Decay – Involves the release of electromagnetic radiation — gamma rays —
but does not change the mass of the atom.
● Alpha radiation can be stopped by paper, beta radiation can be stopped by wood, while
gamma radiation is stopped by lead.
● Radioisotopes decay at a constant rate and the time taken for half the original radioisotope
to decay into a stable daughter isotope is known as the half life.
● Different radioisotopes have different half lives and are thus useful for dating different
types of fossilised remains.
5.1.1 EVOLUTION
● Evolution can be defined as the cumulative change in the heritable characteristics of a
population over time.
● "heritable" means that the changes must be passed on genetically from one generation
to the next, which implies that evolution does not happen overnight.
● "cumulative" means that usually one change is not enough to have a major impact on a
species.
● "population" means that the changes do not affect just one individual.
● The fossil record, selective breeding, and homologous structures are three of the most widely
recognised pieces of evidence for evolution by natural selection.
● Besides these, biogeography, vestigial structures, and comparative embryology can be used
to provide further evidence for evolution by natural selection.
5.1.2 FOSSIL RECORD
● Fossils are the petrified remains or traces of animals and plants, and the fossil record is
the accumulation of evidence from these remains and traces, such as skeletons and
footprints.
● The fossil record shows that over time changes — evolution — have occurred in the features
of living organisms.
● In order for fossilisation to occur, the following conditions are required:
● Hard body parts such as bones, teeth, and shells;
● Preservation of remains;
● High pressure to promote mineralisation of remains to turn hard body parts into
fossilised rocks;
● Anoxic (low oxygen) conditions to protect against oxygen damage and prevent
decomposition by saprotrophs.
● The stages of fossilisation generally occur as follows:
1. Death and Decay – Soft body parts are decomposed or scavenged, leaving only the
hard body remains.
2. Deposition – The hard remains are rapidly covered with silt and sand, and over time
more layers continue to build.
3. Permineralisation – Pressure from the covering layers of dirt/rock cause the hard
organic material to be replaced by minerals.
4. Exposure – Movement of earth plates may displace the fossil and return it to the
surface for discovery.
,● Using the fossil records, it has been found that:
● The life that existed more than 500 million years ago was vastly different in appearance
from life today.
● Although Earth has had extensive oceans for most of its existence, fish fossils have
only been found in rocks 500 million years old or younger.
● Although most of the top predators today are mammals such as bears, none of them
existed at the time of the dinosaurs or before.
● Apart from organisms such as certain types of sharks and cockroaches, the majority of
living organisms today have no similar form in the fossil record.
5.1.3 TRANSITIONAL FOSSILS
● While fossils may provide clues as to evolutionary relationships, it is important to realise that
the fossil record is incomplete.
● Fossilisation requires an unusual set of specific circumstances in order to occur, meaning very
few organisms become fossils.
● Only the hard parts of an organism are typically preserved, meaning usually only fragments
of remains are discovered.
● With limited fossil data, it can be difficult to discern the evolutionary patterns that result from
ancestral forms.
● Transitional fossils demonstrate the intermediary forms that occurred over the
evolutionary pathway taken by a single genus.
● They establish the links between species by exhibiting traits common to both an ancestor and
its predicted descendents.
● An example of a transitional fossil is archaeopteryx, which links the evolution of dinosaurs with
its jaws and claws and to birds by its feathers.
● As new fossils are discovered, new evolutionary patterns are emerging and old
assumptions are challenged.
5.1.4 RADIOACTIVE DATING
● Fossils can be dated by determining the age of the rock layer in which the fossil is found.
● Different kinds of organisms are found in rocks of particular ages in a consistent order,
indicating a sequence of development:
● Prokaryotes appear in the fossil record before eukaryotes.
● Ferns appear in the fossil record before flowering plants.
● Invertebrates appear in the fossil record before vertebrate species.
, ● This chronological sequence of complexity by which characteristics appear to develop
is known as the law of fossil succession.
● This ordered succession of fossils suggests that newer species likely evolved as a
result of changes to ancestral species.
● One of the most commonly used methods for determining the age of fossils is via
radioactive dating.
● This method involves comparing the ratio of radioactive isotopes in the fossil to that
found in the atmosphere today.
● Radioisotopes are alternative forms of an element that have the same number of protons but a
different number of neutrons. These isotopes are unstable and decay at a constant rate to
form a more stable daughter isotope.
● There are three types of radioactive decay that can occur depending on the radioisotope
involved:
● Alpha Decay – Atom releases two protons and two neutrons to form a new, more
stable element.
● Beta Decay – A neutron decays to produce a proton, electron and anti-neutrino and the
electron is released to form a new element.
● Gamma Decay – Involves the release of electromagnetic radiation — gamma rays —
but does not change the mass of the atom.
● Alpha radiation can be stopped by paper, beta radiation can be stopped by wood, while
gamma radiation is stopped by lead.
● Radioisotopes decay at a constant rate and the time taken for half the original radioisotope
to decay into a stable daughter isotope is known as the half life.
● Different radioisotopes have different half lives and are thus useful for dating different
types of fossilised remains.
