7.3 How do we know that evolution has occurred?As stated earlier, a scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. evidence is needed to support a theory and initially this evidence may be tentative and limited. scientists thus need to seek out further evidence from a range of sources, which must be critically examined to ensure that it does support the theory. so what evidence do we have to support the theory of evolution by natural selection?

Fossils as evidenceIn Chapter 6 of Big Ideas Science Book 2 you learnt about fossils and how they are formed. Fossils are the remains or traces of organisms from a past geologic age embedded in rocks by natural processes. They are extremely important for understanding the evolutionary history of life on Earth as they provide direct evidence of evolution and detailed information on the ancestry of organisms. Palaeontologists study fossil records and determine their relations to different geologic time periods.

For fossilisation to take place, the traces and remains of organisms must be quickly buried so that weathering and decomposition do not occur. Skeletal structures or other hard parts of the organisms are the most commonly occurring form of fossilised remains. There are also some trace ‘fossils’ showing moulds, cast or imprints of some previous organisms.

It is possible to find out how a particular group of organisms evolved by arranging its fossil records in a chronological sequence. Such a sequence can be determined because fossils are mainly found in sedimentary

rock. Sedimentary rock is formed by layers of silt or mud on top of each other. Thus, the resulting rock contains a series of horizontal layers, or strata. Each layer contains fossils that are typical for a specific time period during which they were made. The lowest strata contain the oldest rock and the earliest fossils, while the highest strata contain the youngest rock and more recent fossils. This is called comparative dating. You learnt about the different methods of finding the age of fossils in Chapter 6 of Big Ideas Science Book 2.

Transitional fossils as evidenceFossils or organisms that show the intermediate states between an ancestral form and that of its descendants are referred to as transitional fossils or missing links. They can be recognised as they retain certain primitive traits. There are numerous examples of transitional fossils in the fossil record, providing an abundance of evidence for change over time. When Darwin first published his theory, he stressed that the lack of transitional fossils was the most formidable obstacle to his theory

because at that time very little was known about the fossil record. Since then, numerous excellent examples have been found, starting with the discovery of Archaeopteryx in the Solnhofen area of Germany just two years after Darwin’s work was published.

Archaeopteryx is the earliest and most primitive bird known. Archaeopteryx

Fig 7.26 The fossil remains of�� Archaeopteryx, a transitional fossil, from southern Germany.

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Comparative morphology as evidenceEvolutionary theory predicts that related organisms will share similarities that are derived from common ancestors. Similarities in morphology, or form, can be revealed by comparing the anatomies of different

organisms, by looking at cellular similarities and differences, by studying embryological development, and by studying vestigial (i.e. functionless) structures within individual organisms.

Comparative anatomyStructures that are found across

organisms and have a similar plan but

different function are known as

homologous structures. The pattern of

limb bones in all groups of tetrapods (i.e.

four-legged vertebrates) is an example

of an homologous structure. Such a

structure is called the pentadactyl limb.

This structure can even be traced back

to the fins of certain fossil fishes from

which the first amphibians are thought

to have evolved. The pentadactyl limb

has a single proximal bone (humerus),

two distal bones (radius and ulna), a

series of carpals (wrist bones), followed

by a series of metacarpals (palm bones)

and five phalanges (digits).

Throughout the tetrapods, the

fundamental structures of pentadactyl

limbs are the same, indicating that they

originated from a common ancestor.

But, in the course of evolution, these

fundamental structures have been

modified to serve different functions

as a result of adaptation to different

environments and modes of life.

For example:

in the monkey, the forelimbs are •

elongated to form a grasping hand for

climbing and swinging among trees

in the horse, the forelimbs are •

adapted for support and running

by elongation of the third digit,

bearing a hoof

in the whale, the forelimbs become •

flippers for steering and maintaining

equilibrium during swimming

in the bat, the forelimbs have turned •

into wings for flying, while the

hook-like first digit remains free for

hanging from trees.

This is the result of divergent evolution,

where a common ancestor has resulted

in two or more different species.

Fig 7.27 Unique in the animal kingdom, the coelacanth is a 400 million year old living fossil fish! The coelacanth ��pre-dates dinosaurs by millions of years and was thought to have become extinct with them. In 1938, the coelacanth was discovered living in caves off the continental shelf. This environment has changed little over the last 400 million years and, as a result, neither has the coelacanth.

What do you know about fossils as evidence?

1 how does comparative dating work?

2 in your own words, describe a transitional fossil.

3 living fossils have remained relatively unchanged for often millions of years, while around them other species have adapted or become extinct. how has this been possible?

displays a number of features common to both birds and dinosaurs and has often been considered a link between them. Many scientists have argued that birds evolved from theropod dinosaurs and Archaeopteryx was a critical piece of evidence for this argument. It shows a number of bird-like features, such as a wishbone and flight feathers, and a number of dinosaur features, such as bone-filled regions between the teeth, and long V-shaped lines in the tail. Feathers, which were once thought to be found only in birds, are now widely known to occur in a number of fossil dinosaurs.

Living fossils as evidenceAccording to fossil records, some modern species of plants and animals are found to be almost identical to species that lived in ancient geological ages. Living fossils are existing species of ancient lineages that have remained unchanged in form for a very long time.

Examples of living fossils include the coelacanth fish (Fig 7.27), the Ginkgo trees and Metasequoia conifers of China, and the Wollemi pine, which was only discovered in New South Wales in 1994.

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Functionless structures inherited from ancestorsVestigial structures, which are functionless structures found in organisms, have perplexed naturalists throughout history and were noted long before Darwin first proposed the concept of common descent. We now understand that individual organisms contain, within their bodies, abundant evidence of their histories. Some of these structures are clearly designed to perform a certain task that they no longer do. Without the theory of evolution the tiny wings of a cassowary are a puzzling structure. So, too, are the hind limb buds of many snake species, which still carry vestigial pelvises hidden beneath their skin.

Vestigial structures are now interpreted as evidence of an ancestral heritage in which these structures once performed the tasks they were designed to undertake. The wings of a cassowary are a reminder that a distant relative of this organism once used its wings to fly. Similarly, snakes evolved from a four-legged ancestor.

Humans, too, carry the evolutionary baggage of our ancestry. The ancestors Fig 7.29 The rear legs on a snake are an example of a vestigial structure.��

of humans are known to have been

herbivorous, and molar teeth are

required for chewing and grinding

plant material. Over 90% of all adult

humans develop third molars

(otherwise known as wisdom teeth).

Usually these teeth never erupt from

the gums and, in one-third of all

individuals, they are malformed and

Fig 7.28 The forelimbs of different mammals show the same basic structure with five fingers that are modified for ��different uses.

Monkey (grasping)

Pig (walking)

Horse (running)

1

1

1

1

1

2

2

2

3

3

3

4

4

4

5

5

5

2

2

23

3

3

34

4

4

5

5

5

Digits

Whale (swimming)

Anteater (tearing)

Bat (flying)

Forelimb

upper arm humerus

forearm radius + ulna

wrist carpal

hand/foot metacarpals+ phalanges

impacted. These useless teeth can

cause significant pain, and an

increased risk of injury, and may

result in illness and even death.

Another vestigial structure from our

herbivorous ancestry is the appendix.

While this intestinal structure may

retain a function of some sort,

perhaps in the development of the

immune system, it is a rudimentary

version of the much larger caecum

that is essential for the digestion of

plants found in other mammals.

Convergent evolutionUnder similar environmental conditions, fundamentally different structures in different groups of organisms may undergo modifications to serve similar functions. This phenomenon is called convergent evolution and results in unrelated organisms showing superficial resemblances. Similar structures in organisms that appear to have no close ancestral links but show adaptations to perform the same functions are described as analogous structures. Examples include the wings of bats, birds and insects, and the tail fin of fish, whale and lobsters.

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Human Pig Reptile Bird

Other evidence for evolution

Embryological similarity suggests common ancestryWhat is interesting about the development of vertebrates is that their embryos may show many interesting features that are not seen in the fully developed animal. As the embryo develops, it goes through a variety of stages. Many of these stages show homologous structures with different species.

One example is bird limbs. Birds are tetrapods and, as previously mentioned, tetrapods all have five-digit limbs. However, an adult bird appears to have a three-digit limb in its wings. This might appear to be a problem until you examine the embryos of birds and you find that this limb develops from a five-digit precursor. Another example is teeth in some toothless whales, which develop teeth as embryos, but these are absorbed later in embryonic development. Why should a toothless whale develop teeth that are later absorbed? Why should organisms that are so different as adults have so many similarities as embryos?

If life forms developed independently, one would think that their embryonic

development would be distinct and reflect what the organism will look like when it is fully developed. Why should a bird’s three-digit limb develop from a five-digit limb? It makes no sense if organisms developed independently.

The evolutionary answer is that evolution is conservative; that is, it makes use of what has gone before. From the point of view of a natural process with limited resources, developing something new is much more difficult than modifying what already exists. The embryological similarities can be explained by understanding that these organisms all had a common ancestry. Whales develop teeth embryonically because they evolved from ancestors that had teeth. Birds develop their three-digit

What do you know about comparative morphology as evidence?

1 Define the term ‘homologous structures’ in your own words.

2 Give an example of an homologous structure.

3 how does the presence of vestigial structures support the theory of evolution?

Fig 7.30 (a) The wing of a bird and (b) the wing of a butterfly are analogous structures because the bird lineage and the insect lineage evolved wings independently.��

a b

Fig 7.31 Comparative embryology shows homologous structures in vertebrates.��

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Common ancestor

Humans

Chimpanzees

Gorillas

9 million years ago

7 million years ago

What do you know about other evidence for evolution?

1 which two biomolecules are of most interest to biologists studying evolution?

2 how do the 20 amino acids used in protein synthesis provide evidence for evolution?

3 explain in your own words how DnA sequencing supports the concept of evolution from a common ancestor.

Fig 7.32 Gene sequencing has shown that humans, gorillas and chimpanzees all evolved from a common ��ancestor.

limbs as embryos from five-digit limbs because they evolved from ancestors with five-digit limbs.

Comparing moleculesAdvances in the understanding of the biochemical processes of life have provided a wealth of evidence in support of evolution. Biochemical homologies provide some of the strongest evidence for evolution because of the detailed level of information they provide. Most biochemical evidence for evolution comes from comparative examination of genetics or proteins.

Comparing amino acids in proteinsAs you learnt in Chapters 5 and 6, proteins are coded by genes and are made up of a string of amino acids. Proteins range in size from about 50 amino acids to thousands and are among the most important chemicals in life. The characteristics of a protein are determined by the sequence of amino acids from which it is constructed.

All livings things use the same 20 amino acids to make proteins, even though about 250 amino acids occur naturally. If life evolved from a common ancestor with only these 20 amino

acids, we might expect that these same 20 amino acids are always used.

Protein sequences also show remarkable similarities. As you learnt in Chapter 6, a number of different codons can code for the same amino acid, and small changes in some of the amino acids that make up a protein do not appear to have much, if any effect, on the functioning of the protein. So, we can have a set of proteins that do essentially the same thing but are not identical.

Haemoglobin provides such an example. Several types of haemoglobin molecules are found among different vertebrates and invertebrates. These haemoglobin molecules are all very similar in structure and all serve the function of binding oxygen in the blood, yet they differ in their amino acid sequences. Given that there are so many different sequences of amino acids that could make a functional haemoglobin molecule, why should these variations of haemoglobin exist among creatures that are vastly different? Evolution provides a meaningful answer.

Comparing gene sequencesThe best evidence for descent from a common ancestor comes from a study of gene sequences. Comparative sequence analysis examines the relationship

between the DNA sequences of different species, producing several lines of evidence that confirm Darwin’s original hypothesis of common descent.

If the hypothesis of common descent is true, then species that share a common ancestor will have inherited that ancestor’s DNA sequence. Also, they will have inherited mutations unique to that ancestor. More closely related species will have a greater proportion of identical sequences and will show shared substitutions compared to more distantly related species.

The most detailed DNA sequence reconstructions have been carried out using the genomes of mitochondria, which are shared by all eukaryotic organisms, are short and easy to sequence. The broadest DNA sequence reconstructions have been performed using either the sequences of a few very ancient proteins or the ribosomal RNA sequence.

Early DNA sequencing work on the genome of humans and great apes has shown that humans share a common ancestor with gorillas and chimpanzees. In fact, the chimpanzee is our closest living relative, with 98% similarity in genome. DNA sequencing of the beta-haemoglobin gene has also confirmed this.

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How do we know that evolution has occurred?7.3

Remember1 what is the professional title for a person who studies

the fossil record and geological time periods?

2 Archaeopteryx (below) was part bird and part lizard. what term is applied to fossils that show the evolutionary progression between two very different forms?

3 what is a vestigial structure?

Understand4 The layering of sedimentary rocks is useful in

comparative dating. what is the basic principle of comparative dating?

5 Distinguish between the term ‘transitional fossil’ and a ‘living fossil’.

6 explain precisely how fossils provide evidence for evolution.

Apply7 suggest why a vestigial structure, once it has

been reduced to a certain size, may not disappear altogether.

Analyse 8 how might you investigate the hypothesis that birds

are more closely related to reptiles than to mammals?

Evaluate 9 how does the study of DnA sequences help in our

understanding of evolution?

10 imagine you could go back to the time when Darwin was formulating his theory on evolution. if you could only pick one of the pieces of evidence to convince him that his theory was right, which would it be and why?

11 To what extent is the theory of evolution testable?

Create12 create a poster to show the different ways of

gathering evidence for evolution.

Big ideas

>>CONNECTING IDEAS<<

13 set up a classroom debating situation with one team charged with the case of supporting Darwin’s theory of evolution and the other team with rejecting it.

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>>DIGGING DEEPER<<Research

choose one of the following topics for a research project. A few guiding questions have been provided but you should add more questions that you wish to investigate. Present your research in a format of your own choosing.

Review

Key words

modern-day evidence for evolutionThere is evidence of current populations evolving by natural selection all around us. Research one of the following topics and see if you can find evidence of evolution by natural selection occurring today.

• Cancontrolledbreedingmodify organisms?

• Whenfewerpredatorsarepresent, how does brighter colouration evolve?

• Howdoesnaturalselectionlead to pesticide resistance?

Climate change and natural selectionhow do you think climate change will affect species on the earth? which

species do you think will be most affected? why is this? what could these species do to avoid becoming extinct due to changing habitats? how could they do this? would all species be able to avoid the effects of climate change? Do you think new species might evolve as a result of climate change?

Darwin and the galapagos islandsmuch of Darwin’s theory developed while he was visiting the Galapagos islands. which new species did he find there? what was so unique about these species? how did his findings help him develop his ideas? what was unique about the Galapagos islands to help him develop his theories?

Reflect

mewhat new science skills have you learnt in this chapter?•

what was the most surprising thing you found out about evolution?•

what was the most difficult aspect of this topic?•

how has your understanding of how scientific theories are developed improved?•

my worldwhy is it important to understand evolution and natural selection?•

what is being done in the world to better understand the process of evolution?•

which technologies have helped us to understand the process of evolution?•

my futurehow do you think climate change might affect the natural selection of species?•

what career paths could a study of evolution lead to?•

allopatric speciation

analogous structure

behavioural isolation

common descent

comparative dating

continental drift

convergent evolution

directional selection

divergent evolution

evolution

extinction

fitness

fossil

gene flow

gene pool

homologous structure

intersexual selection

intrasexual selection

isolating mechanism

living fossil

mechanical isolation

missing link

natural selection

niche

parapatric speciation

pentadactyl limb

physical isolation

plate tectonics

population

scientific theory

sexual dimorphism

sexual selection

survival of the fittest

sympatric speciation

temporal isolation

transitional fossil

transmutation

vestigial structure

Test yourself

log onto www.oxfordbigideas.com to do the student self-test and revision activities.