Evidence of Evolution
Evidence of evolution suggests that the mechanisms of inheritance, accompanied by
selection, allow change over many generations.
Explain how Darwin/Wallace’s theory of evolution by natural selection and isolation
accounts for divergent and convergent evolution.
Background:
It was observations of variations between species and individuals by scientists in the 19th
century that led to the proposal by Darwin and Wallace of the theory of natural selection.
Evolution - this is the way in which living things change genetically to produce new
more complex forms of life over long periods of time.
So, evolution refers to lasting change to a population over generations.
In his book the ‘Origin of Species’ Darwin argued that evolution was an inevitable
consequence of two indisputable facts of nature – individual variation and the struggle for
existence.
The Theory of Evolution by means of Natural Selection:
The theory of natural selection proposes that it is the environment that selects favourable
variations and eliminated harmful ones. After many generations of selection, the
characteristics of a population may change and the population becomes adapted to the
environment.
The theory of evolution by natural selection had 4 main points:
1. In any population there are variations.
2. In any one generation there are offspring that do not reach maturity and
reproduce. The characteristics of these organisms are removed from the
population.
3. Those organisms that survive and reproduce are well adapted to that environment;
they have favourable variations (survival of the fittest)
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4. Favourable variations are passed on to offspring; they become more common in
the population.
The main contributions by Darwin and Wallace to the theory of evolution were a.) the
idea that species change, and b.) the mechanism of natural selection to explain how
change takes place.
Practical 1 – An investigation to model natural selection
Aim: To plan a first hand a investigation to model natural selection.
You need an investigation design that will allow valid and reliable data to be
collected.
A simple but useful investigation is called ‘Stick bird’.
Within a hypothetical population of ‘worms’ (toothpicks) that inhabit a
predominantly green coloured environment (green grass), there are two colour
variations; cream and green. The worms are food for a predator known as a ‘stick
bird’ (students).
Stick Bird – scenario
Toothpicks are mixed and scattered randomly over a measured grassed area.
Stick birds (students) are later brought to that area and remain outside a ‘fence’.
They are told to prey on the worms in the field (collect as many toothpicks as
they can in) a given time. After the given time is up, the ‘stick birds’ are driven
from the field by the farmer (teacher!). They escape back to the class room with
their prey.
Tally and compare the numbers of green and cream toothpicks recovered.
Calculate percentages of each colour recovered.
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 Outline the impact on the evolution of plants and animals of:
o Changes in physical conditions in the environment.
o Changes in chemical conditions in the environment.
o Competition for resources.
Evolution is the process of change that occurs in living organisms over many generations.
It is the result of natural selection by the environment of favourable variations in the
organisms.

Changes in the environment of living organisms may lead to the evolution of a
plant or animal species.

These environmental changes in condition may be physical, such as temperature
change, or chemical such as changes in salinity of water.

Change may also be triggered by competition – e.g. competition for resources
such as food and water, or competition to reproduce.
Physical Changes to the Environment:
These include natural conditions such as temperature and availability of water.
For example: the Australian land mass has become drier over time. This has lead to
changes in the species of kangaroo that are present today. Approximately 25 million
years ago, Australia was considerably wetter than today and was covered with large
areas of rainforest. During this period kangaroos were small and omnivorous, with
unspecialized teeth, eating a variety of foods from the forest floor. Food was
nutritious and abundant; there was no need for specialized grinding teeth.
As Australia became more arid and grass became the dominant vegetation in some
areas, environment selective pressure resulted in larger kangaroos favouring teeth
suitable for grass. These teeth, high crested molars, efficiently grind low-nutrition
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grass into more easily digestible paste. Slicing pre-molars are of little use and so
became much reduced from ancestral kangaroos.
Chemical Changes to an Environment:
Chemicals that can affect the evolution of species include salts and elements such as
iron.
Many parts of Australia have soils that have high salinity. There are a range of salt
tolerant plants that have evolved to inhabit those areas. The animals that feed off
those plants have also evolved to inhabit those areas.
Example: Chemicals such as dieldrin and organophosphates have been used
extensively to control the sheep blowfly, Lucilia cuprina. (A major problem to sheep
industry – it stresses, weakens and can be lethal to sheep). Genetic resistance has
occurred within the fly population in response to those chemicals.
A possible case study of changes in a species!
Competition for Resources:
This occurs within a species and between species. If a new species is introduced to an
area then the competition may lead to different species using different resources.
Resources may include food, space or mates. If populations that live in the same area
could specialize on slightly different resources or breed at different times they would
avoid direct competition.
Some species of fruit fly have evolved into different species with each confined to a
different type of fruit tree. This is possible if there are different flowering and fruiting
times on each tree type suited for different breeding cycles in the fruit flies.
Eventually two distinct species can result.
Possible case study of change in species!
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Case studies: Effects of environmental change on evolution of living things.
o Analyse information from secondary sources to prepare a case study to
show how an environmental change can lead to a changes in a species.
Aim: To prepare a case study to show how an environmental change can lead to
changes in a species.
A case study here would need to describe the Australian example (a report) and
would need to explain how environmental pressures can change species over time (an
explanation).
o Access a range of resources including popular scientific journals, CD –
ROM encyclopedias and the internet.
o You will need to extract information, and summarise and collate your
data.
o Analyse the information to make and justify generalisations that have
led to changes in species.
Evolution Down Under – Changes in a species!
Here are some possible case studies of changes in species for you to consider!
Choose 1 case study from those listed below or one of your choice.
a.) Changes in physical conditions in the environment – teeth of kangaroos have
evolved in response to changes in physical conditions in Australia over the last 25
million years.
b.) Changes in chemical conditions – use of chemicals to control sheep blowfly,
Lucilia cuprina, genetic resistance has occurred within the fly population in
response to these chemicals.
c.) Competition for resources – some species of fruit fly have evolved into different
species with each confined to a different type of fruit tree.
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The areas of study supporting the theory of evolution are listed and discussed below.
A.) Palaeontology
Background:
Palaeontology is the study of fossils.
Fossils are the preserved remains or traces of past life on Earth.
Fossils are mainly found in sedimentary rocks.
Fossils can include preserved body parts, moulds or casts of decayed organisms or
imprints left in the mud such as footprints.
Fossils can provide evidence for evolution. The fossil record provides a time line of
evolution of life engraved in the order in which the fossils appear in the rock layers.
Some parts of the fossil record show a gradual change in life forms over millions of
years.

Fossils discovered to date show changes when compared to modern
organisms.

Some fossilized organisms do longer occur as living organisms (they are
extinct)

In other cases there is no fossil record of modern species – this could be due to
the modern species being recently formed and therefore having no fossil
record.

The more modern fossils show an increase in complexity.

Very often the fossil evidence can be linked to environmental change, i.e.
organisms had new environments in which they had to adapt.

Comparisons of fossils with present day life forms: The fossil record of the
modern horse is very well documented. It covers a time span of about 60
million years and involves many hundreds of species, most of which are now
extinct.
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
There are many trends to be seen in the evolution of modern horse – the two
main changes seen were; 1.) the increase in size and 2.) the reduction in the
number of toes.
Of particular interest are the transitional fossils that have characteristics
belonging to ancestral and descendant groups.
Transitional Forms
These are examples of organisms that indicate the development of one group of
organisms form another or from a common ancestor.
They help to give us an understanding of how evolutionary change may have come
about.
A study of the fossil record suggests that the modern groups of vertebrates appeared
in the following order:
Jawless fish – 500 million years ago
Bony fish – 400 million years ago.
Amphibians – 360 millions years ago
Reptiles – 300 million years ago
Birds – 190 million years ago
Mammals – 150 million years ago
The fossil record also supports the theory that they developed from each other as
many intermediate types have been found which show the transition from one major
group to another.
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Example1: The most famous transitional form is Archaeopteryx.
A small dinosaur with feathers appeared in the late Jurassic period. This is a fossil
first thought to be a therapsid reptile (mammal like reptiles).Its reptilian features
include teeth and a reptile like skeleton. Archaeopteryx also had feathers and a
wishbone sternum used to attach flight muscles. This provides evidence of an
evolutionary pathway from dinosaurs to birds.
Figure 2: Archaeopteryx
Diagram 3: The Features of Archaeopteryx suggest that birds developed from
reptiles
Table 1: Features Shared by Archaeopteryx with reptiles and birds
Reptilian Features
Bird Features
Teeth
Feathers
Long tail
Wishbone
Claws
No keel
Solid bones
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Example 2: `Crossopterygian
Fish that could absorb oxygen from the air appeared about 400 million years ago at the
end of the Devonian age. It is thought that amphibians developed from fish along this line
of descent The crossopterygian fish had bones in its fins which suggests it could ‘drag
itself’ on land.
Diagram 3: Crossopterygian
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Table 2: Features that the crossopterygian shared with fish and amphibians.
Fish
Amphibian
Scales
Lobe-fins (useful on land)
Fins
Lungs
Gills
Example 4: Plants
The fossil record suggests that modern groups of true land plants appeared in the
following order.
Seed ferns, lycopods, horsetails, ferns
400 million yrs ago
Gymnosperms
300 millions yrs ago
Angiosperms
185 million yrs ago
The origin of angiosperms is not well documented; however seed ferns appear to be
ancestors of present day gymnosperms.
Seed ferns are extinct but their fossil record shows them to have a fern like appearance
(firs classified as ferns), however naked seeds have been seen attached to the leaves. All
the evidence suggests that their life history was similar to that of gymnosperms.
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B.) Comparative embryology
Comparative embryology is the study of embryos of different organisms looking for
similarities and differences between them. There is an obvious similarity between
embryos of fish, amphibians, reptiles, birds and mammals.
A comparison of embryos of vertebrates shows that they all have gill slits
(pharyngeal(throat) pouches) at some stage of their development, even though they do
not remain later in life, except in fish. (Amphibian larvae also) This similarity
suggests a common ancestry.
Why do human embryos possess gill pouches? Perhaps, because life on Earth began
in an aquatic environment, we inherited them from an aquatic ancestor.
Embryology of vertebrates
Embryology (also known as ontogeny) is the study of the development of embryos.
19th century biologists studying the development of embryos made observations on
variation amongst different species and their relationships (phylogeny).
It was noted that structures present early in embryonic development are widely
distributed among different animal groups, while more specialised features, which
distinguish the groups, appear later in embryonic development.
For example, a human embryo at an early stage has characteristics of all animals in the
vertebrate group. The embryo develops mammalian characteristics followed by
characteristics relating to the primate order, the human species and finally the traits
(characteristics) that make the individual.
Charles Darwin also recognised a connection between embryology and evolution. He
proposed that primitive features in organisms tend to be generalised while derived
features tend to be specialised. Darwin realised that you could infer evolutionary change
by observing changes in development.
The following 19th century drawings (not to scale) illustrate comparable stages of
development in a series of vertebrate embryos.
The embryos in each comparative stage are remarkably similar and provide evidence of a
common ancestry. The following diagrams are attributed to Haeckel (1866).
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(Haeckel, 1866)
In the first series of diagrams, during early development, each of the embryos
demonstrates the basic traits of the primitive vertebrates, eg gill slits and a vertebral
column.
(Haeckel, 1866)
In the second series of diagrams the embryos have developed further. The fish and
amphibian are noticeably different from the other vertebrates which are all similar in
appearance and which have developed limb buds.
(Haeckel, 1866)
In the third series, during late development, the reptile and bird embryos have
characteristic features of their classes. With the exception of the lack of a tail in the
human embryo, the four mammal embryos all look similar.
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C.) Comparative anatomy
Comparative anatomy is the study of the differences and similarities in structure
between different organisms.
The structures that they have in common are evidence of similar inherited
characteristics from common ancestors.
Modern day vertebrates are grouped into classes as they possess quite distinct
features. However, there are other underlying similarities that suggest they are more
closely related than appearances might suggest.
The structures that they have in common are evidence of similar genes inherited from
a common ancestor.
Example 1: The pentadactyl (5 –fingered) limb
All land vertebrates show a similar basic pattern in the bones of their arms and
legs. It is believed that they inherited this from their aquatic ancestors, the lobed
finned fish.
Homologous Structures – these are organs with similar structures but different
functions. The fore limbs of vertebrates provide a good example of this. In each case
the limbs have the same basic structure (anatomy) but are adapted for different
functions.
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Comparative anatomy also shows the presence of vestigial organs. These are
structures which are the remains of something that once functioned in an ancestor.
Their existence indicates that organisms change (evolve) so that organs sometimes
loose their function but are retained as they cause no harm.
Humans are thought to have more than one hundred vestigial organs: some examples
include the appendix, the coccyx (tail vertebrae) and muscles to move the ears.
Other vestigial structures include: hind legs in whales, wing bones in flightless birds,
vestigial eyes in animals that live underground or in dark caves.
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Example 2: Xylem
Structural similarities between some plant groups suggests that they shared a common
ancestor.
Ferns, conifers and flowering plants all have vascular tissue, including conducting
vessels (xylem) which transports water throughout the plant. In a leaf they are found
in the veins.
D.) Biochemistry

Studies of animals have found that many possess many similar molecules.

The degree of similarity reflects genetic closeness.

Some molecules studied include: haemoglobin, RNA and hormones.

When scientists are studying these proteins they look at the amino acid
sequence this is a clue to relationships between animals.

From these studies it has be shown that humans are most closely related to the
chimpanzee than the gorilla, gibbon, monkeys or other mammals.

Other studies involve the compatibility of blood when it is mixed.

Closely related animals have a small antigen-antibody reaction. These studies
help clarify the groups into which organisms should be classified.

DNA sequencing methods have allowed the DNA or organism to be compared
directly. The nucleotide sequences making up the gene or chromosome can be
analysed and compared.

The greater the similarity found the closer the relationship between the
organisms in question.
DNA Hybridisation
DNA hybridization can be used to identify similarities in DNA structure.
In a process called chemical hybridization the DNA molecules from different
species can be compared.
The following is carried out during this process:
1. Two strands of DNA are separated using heat.
2. The single strands formed are mixed with single strands from another species.
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3. The two different strands will join to form a hybrid molecule. However, not
all the base pairs will match. This is because there are differences in the
sequences of bases for each species.
4. The degree of pairing depends on the similarity of the two different kinds of
organisms. A high degree of pairing will occur if the two sequences are very
similar. However a low degree of pairing will occur if they are very different.
Figure 9: DNA Hybridisation
Species that have diverged recently from a common ancestor would be
expected to show a high degree of hybridization because their DNA sequence
would be very similar.
However, species which have diverged from a common ancestor a very long
time ago would show less hybridization.
Example:
Hybridisation studies of primates, supported by studies of haemoglobin have
shown that humans are more closely related to chimpanzees than to gorillas.
Orangutans are a ‘sister species’ to all three other primates – they were the first to
diverge from a common ancestor.
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The current view of the evolution of primates is as follows:
According to anthropologists this means that the family of apes includes humans:
we are therefore not only descended from apes - we are apes!
DNA Sequencing

Allows scientist to compare genes, chromosomes even entire genomes of
different organisms.

It was Frederick Sanger who developed the major method of sequencing
in 1974. he won a Nobel prize for his work in 1980.

Using the sanger sequencing method, sections of DNA 500 to 800 bases
long can be ‘read’. Today advances in technology have been able to speed
up Sangers method. DNA databases are now established and work
continues to map genomes of living organisms.

The human genome project started in 2000 and finished in 2003.
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