Evolution and biodlaversity 5
Introduction
Over longyeriods of tim1:.· and
111;111y
gcnerations, the gcnctil: make-up
of species may change as they become adaptl'.'d to new surroundings or
.,ltcred conditions. One result of these changes may be the evolution of
new varieties and species. There is strong evidence for the evolution of
lifr of Earth, both from the fossil record and fi-om organ.isms that are alive
today. Natural selection provides an explanation of how evolution might
1,.,ve occurred. Classification of organisms helps us to understand their
ancestry and our observations of biodiversity, as well as providing forther
evidence for the mechanism of evolution.
5.1 Evidence for evolution
Learning objectives
What is evolution?
Life on Earth is always changing. Just by looking at any group of
individuals of any species - whether humans, cats or sunflowers - you can
see that individuals are not all the same. For example, the people in Figure
5.1 vary in height, hair colour, skin tone and in many other ways. How do
these differences within _a species occur' How do different species arise?
Variation within a species is a result of both genetic and
environmental factors. We say that selection pressures act on individuals
and because of variation, some n1ay be better suited to their environment
than others. These are likely to survive longer and have more offspring.
The characteristics of a species are inherited and passed on to
succeeding generations. The cumulative change in these heritable
characteristics over generations is called evolution. If we go back in time,
then existing species must have evolved by divergence from pre-existing
ones. All life forms can therefore be said to be linked in one vast family
tree with a common orig.in.
What evidence is there for evolution?
The fossil record
Fossils, such as the one shown in Figure 5.2, are the preserved remains of
organisms that lived a long time ago. They are often formed from the hard
parts of organisms, such as shell, bone or wood. Minerals seep into these
tissues and become hardened over time. As the living tissue decays, the
minerals form a replica that remains behind. Soft tissue can sometimes be
preserved in the same way, as can footprints and animal droppings. Most
fossils become damaged over time or are crushed through land or sea _
movement, but some are discovered remarkably well preserved. The earliest
You should understand that:
• Evolution takes place when.
heritable characteristics ch:mge,
over many generations.
• Evidence for evolution comes
from the fossil record.
• Artificial selection can result
in evolution, as the selective
breeding of domesticated
animals shows.
• Evolution of homologous
structures by adaptive radiation
explains similarities in structure
with differences in function.
• Populations of a species can
diverge and this can eventually
lead to the evolution of separate
species.
• The continuous variation
observed across the geographical
range of related populations
supports the idea of gradual
divergence.
Evolution cumulative change in
th e he ri table characteristics of a
populatio n
'(ol 1 11 • ,
i- ,, "
ftli®Jk• I
1
i
Figure 5.1 Most of the variation between
humans is continuous variation, and is
influenced by the environment as well as
genes.
Remains of organisms discovered
preserved in ice, tar and tree
sap have also yielded important
information about the evolution of
species.
so the time scale of the
b"llion years ag0 , .
.
from 0 vcr 3 1
f
species
that died out long ago
fossils found date
t fossils are O
• ·
'
c ii record is i111n1ense. Mos
nVI· ronmental conditions.
,oss
new e
I · h
. did not adapt to
logy. paJaeonto og1sts ave been
because t1icy
1cd PalaeontoZOO years but they have only
f
fossils
is
cal
O
The stu dY
•
· I for over
'
.
.
ecting and classifying foss1 s t 940s. Scientists do this by studying
co ll
·
the
·
h
le to date them smce
. ecimen. Over time t e amount
been ab
. . in a foss1 1 sp
ount of radioactivity '
d" ctive elements decay. The rate
t he am
b use ra 10a
·oactivity decreases eca
. . possible to date a fossil by
o f ra d1
lement, so 1t 1s
. .
.
of decay is fixed for each e .
. . present m 1t. Carbon-14 1s used to
f adioact1V1ty
·a1 h 1
measuring the amount o r
Id for older maten , ot er e ements
study material up to 60 000 years o .
are used.
. .
I te and fossils are very rare, it is
d 1s mcomp e
Although the foss ii recor
d animals might have evolved
dern plants an
. .
possible to show how mo . d h dreds or thousands of millions of
. h t eJOste un
from previous species t a
uggest how modern horses may
I fi ssil sequences s
years ago. For examp e, 0
.
(F. e 5 3) . It is important to recognise
Ii species 1gur ·
have evolved from ear er
.
olved into that species' , based
h 'this species ev
that we can never say t at
h
have many fossils. All that we can
ii
even w en we
on a fcoss sequence I d _ chat they probably share a common
sa is that they appear to be re ate
hi h
fi ·
Y
.
uld ell have existed too, for w c no ossils
ancestor. Other species co
w
have ever been found.
.
h h
h
.
h
changed very little. T e orses oe crab we
A few orgarusms seem to ave
. .
.
.
.mil
fi il specimens a million years old. This would
see today 1s very s1 ar to oss
.
een little selection pressure on these crabs.
seem to suggest th at th ere haS b
.
·
ffi
ils
provi·de
evidence that life on Earth changes and
Observat10ns o oss
that many of the changes occur over millions of years.
Selective breeding
Further evidence for the way evolution might occur comes from
observations of selective breeding. People have altered certain
domesticated species by breeding selected individuals in a process called
artificial selection. Plants or animals with favourable characteristics
are chosen and bred together, to increase the number of offspring in the
next generation that have the favourable characteristics. Those individuals
Figure 5.2 A fossil of Archaeopteryx, which
is seen as an evolutionary link between
reptiles and birds. It looked like a small
dinosaur, but had feathers and could fly.
that do not have the desired features are not allowed to breed. People
have domesticated and bred plants and animals in this way for thousands
of years and, over many generations, this has resulted in the evolution of
numerous breeds and varieties, which differ from each other and from the
original wild ancestors.
Modern varieties of wheat, barley, rice and potatoes produce higher
yields and are more resistant to pests and disease than ever before. Wheat
and rice plants are shorter and stronger than varieties of a hundred years
ago, so that they are less likely to be damaged by wind and rain and are
easier to harvest. The plants of a hundred years ago were also very different
from the original grasses that wheat was bred from 1O000 years ago. Many
plants are also bred for their appearance, and ornamental varieties have
160
~,. . . . .1,
1~,
.
Merychippus
•
Mesoh,ppus
Hyrocotherium
@
@
Figure 5.3 Some of the many species of fossil hor
have
developed single-toed hoov
.
ses, a nd the modern horse, Equus. The fossil sequence shows that, over time, horses
es, Ianger legs and longer faces with larger teeth for grazing.
different petal shapes and colours from the ori·g 1·nal
k
parent stoc .
Animals are chosen and bred by farmers and a · 1 b d
r
•
. .
.
.
.
ruma ree ers 10r spec1a 1
charactenstJCS such as high milk yield in a CO"'
d
al.
.
. .
.
", or goo -qu 1ty woo1 m a
sheep. Individuals with these characteristics are selected to breed, so that
more of the next ge_neration have these useful features than if the parents
had not been artificially selected (Figure 5.4).
Although the driving force for artificial selection is human
intervention, which is quite different from natural evolution, selective or
artificial breeding does show that species can change over generations.
Homologous structures
The existence of homologous structures provides another strand of
evidence for evolution. Homologous structures are anatomical features
showing similarities in shap e, though not necessarily in function , in
different organisms. Their presence suggests that the species possessing
them are closely related and derived fro m a commo n an cestor. A good
example is the vertebrate p entadactyl lim b. This is found in a large range
of animals including bats, w hales and humans , as shown in Figure 5.5. In
each group, limbs have th e sam e general structure and arran gement of
bones but each one is adapted fo r different uses. Bird w in gs and reptile
Figure 5.4 Selective breeding of cows
over many centuries has produced many
breeds including the Guernsey, bred for the
production of large quantities of fat-rich milk.
Other breeds have been produced with flat
backs to facilitate birthing, and longer legs
for easier milking.
tet
a PB! S : ~ C
Bird
Frog
humerus
'--;:,.,_...- - --
- radius
humerus
ulna
carpal -
_,__.,.- ·,-. n
3
figure 5.5 The forelimbs of animals with pentadactyl limbs all have a cle arly visible
humerus, radius, ulna and carpals.
Adaptive radiation is a term
used to explain how organisms
diverge into a range of new forms
from a single conunon ancestor.
It can occur if the environment
changes and new sources of_food
or new habitats become available.
The pentadactyl limb demonstrates
adaptive radiation in the
vertebrates, and Darwin's finches
(Subtopic 5.2) are an example of
how one species adapts to expl01t
new resources.
162
T,lfr••
UY
limbs are also homologous structures. Even though a bird uses its wi ngs
for flying and reptiles use their limbs for walking, they share a co nunon
arrangement of bones.
Variation and divergence
For almost every species, there is genetic variation between individuals in
a population and between populations in different areas furth er di ffe rences
may be visible. For example, some groups of birds may have brighter
plumage than others, some troupes of howler monkeys may be :ibl e to
call more loudly than others and some groups of mice may have slightly
darker fur than others giving them better camouflage. This conti nuous
variation between populations, some very slight and some very obvious,
provides support for the proposal that popularions have the pote11riol to
diverge from one another and evolve into separate species. If this does
happen , there 1s also the possibility that new species could be formed
(Subtopic 5.2).
Ii:\
W
Beyond reasonab_
le doubt?
Evolutionary history 1s a difficult area of science. It is not
possible to go back in time or conduct experiments to establish past
events or what may have caused them. Nevertheless, in some cases,
modern scientific methods can help establish beyond reasonable doubt
what has taken place. Science is used to demonstrate the existence
of phenomena we cannot observe directly. Many crucial scientific
discoveries - such as atoms, electrons, viruses, bacteria, genes and
DNA - have been made and accepted using indirect observation
and the scientific method of'inference'. The theory of evolution has
been considered using multiple lines of research, including studies of
anatomy, biochemistry and palaeontology, which provide empirical
Questions to consider
• Do modern methods establish
the theory of evolution beyond
reasonable doubt?
• Will it ever be possible to 'prove'
that evolution occurred?
• How do the techniques used in
science differ from those that a
historian might use to establish
what has taken place in the past?
data consistent with the theory.
New methodologies that have been used include carbon dating
of fossils, the study of DNA, comparison of exons and intrans
(Subtopic 7 .1) from existing organisms and their fossil ancestors, and
the establishment of endosymbiosis with the discovery of rRNA
sequences using PCR (Subtopics 1.5 and 2.7).All of the data from
these techniques have provided evidence of relationships between
species that are consistent with the relationships suggested by the
theory of evolution.
VTe•J; yourself
1 Define the term 'evolution'.
2 Outline what is meant by a homologous structure.
3 O utline the evidence for natural selection provided by selective
breeding.
- ~ { 163
5.2 Natural selection
Learning objectives
You should understand that:
• Variation between individuals is
required for natural selection to
occur.
• Variation can be caused by
mutation, meiosis and sexual
reproduction.
• Adaptations are features that make
an individual better suited to its
environment and way of life.
• More offspring tend to be
produced than the limited
resources in the environn1ent can
support, which means there is
a struggle for survival between
individuals.
• Better adapted individuals tend
to survive and produce rnore
offspring while less well-adapted
individuals tend to die or
produce fewer offspring.
• When individuals reproduce,
they pass on their heritable
characteristics to their offspring.
• Natural selection increases
the frequency of well-adapted
individuals in a population
and decreases the frequency
of individuals without the
adaptations, and thus leads to
changes in the species.
Characteristics that an organism
acquires during its lifetime, such
as large muscles or special skills
resulting from training, immunity
to a disease or scars on its skin,
cannot be passed on to its offspring.
Only heritable characte1 istics,
which are determined by genes ,
can be passed on to the next
generation via the parents' gametes.
A mechanism for evolution
The theory of evolution by means of natural selection was proposed
by Charles Darwin and Alfred Wallace. Darwin explained his ideas
in a book called On the Origi11 of Species by Means of Nat11ral Selection,
published in 1859.The explanation remains a theory because it can
never be completely proved but there is an abundance of evidence to
support the key ideas, which are based on the following observations and
deductions. Some terms we use now were not used by Darwin, who had
no knowledge of genes or alleles. However, the fundamental basis of his
argument was the same as outlined here.
1 Populations are generally stable despite large numbers of
offspring
Organisms are potentially capable of producing large numbers of offspring
and far more than the environment can support. Trees can produce
thousands of seeds and fish hundreds of eggs. Yet few of these survive to
maturity and we rarely see population explosions in an ecosystem.
2 Better adapted individuals have a competitive advantage
Both plants and animals in a growing population will compete for
resources. These may
·
_ _be food , t ermory
or even t h e opportunity to
find a mate. In add1t1on , predators and disease will take their toll. This
competltlon_will bring about a struggle for survival between the membrn
of a population. Organisms that are well adapted to ti
d. .
·11
be good at competin and ,viii
, .
1e con ItJons w1
. .
.
tend to survive to reproduce, pas, mg on
11entable traits to their offspring, while others die.
3 There is heritable va riatio n within species
Different members of the sam
.
.
. . . d
' e species are all slightly different and this
variation ts uc to the 1nechan.isn1 of sexual re
. of meiosis prod • h
.d
production .The pro, c,s
uces ap 1o1 gametes ·1 d f, h
the gametes an · d' -d
' n urt ermore the genes in
. , m 1v1 ua 1 produces ma b .,
. .
or alleles. When
. , ..
Y e present 111 different forms
an egg is ,eruhsed the z
. .
.
combination of
.
.
'
ygote contains a umqu e
genetic material from .
.
gives an enormous
f
_ ~ts tw o parents. Sexual repro ducuon
source o genetic div .
.
.
.
.d
variation among the · d' .d
ersiry, which gives rise to a \ \"I G
111 1v1 uals of a specie,.
4 Advantageous heritable tra.
generations
its become more frequent over
As a result of variation so •
.
' n1e me1nbcrs of
I .
smted (better adapted) t 1 .
a popu anon may be better
o t ie1r surround·
l
keener eyesight, or have bette .
ings t 1an others. They may have
individuals will out-co
r camou flage to avoid predators.These
mpete others· th e
.ll
.
and pass on the genes f, . h ,· .
'
Y wi survive better, live longer,
o1 t t:ll advanta e
· g o us characteristics to more
1fi4E,1
;
::
offspring. Gradually, as the process is repeated generation after generation,
the proportion of these genes in the population as a whole increases. This
is called natural selection , and it occurs as the 'fittest' (best adapted)
survive to reproduce.
Natural selection and evolution
Once species have evolved to become well adapted to conditions in a
stable environment, natural selection tends to keep things much the same.
However, if the environment changes, a population will only survive if
some individuals have heritable traits that suit them to the new conditions,
and these then become more frequent in the population, because of
natural selection.Three examples of how this can happen in a relatively
short period of time are the beak adaptations of Galapagos finches after a
change in food availability, the response of a moth population to pollution,
and the emergence of new strains of bacteria following the introduction
of antibiotics.
Sexual reproduction promotes
variation
Mutations in genes cause new
variations to arise, but sexual
reproduction also increases
variation in a population by
forming new combinations of
alleles.
• During meiosis, crossing over
at prophase I and random
assortment in metaphase I
produce genetically different
gametes (Subtopic 3.3) .
• Different alleles are also
brought together at fertilisation,
promoting more variation.
In species that reproduce asexually,
variation can arise only by
Darwin's finches
The finches living on the Galapagos Islands (Figure 5.6) , about 900 km
off the coast of Ecuador, were important in shaping Darwin's ideas about
natural selection. Studies of the birds, now known as Darwin's finches,
continue to this day and modern DNA analysis indicates that all 13
species now found on the islands probably evolved from a small flock of
about 30 birds that became established there around 1 million years ago.
When the birds first arrived, the Galapagos Islands were probably
free of predators and initially the resources were sufficient for all the
individuals. As the population grew, the finches started to adapt their
feeding habits to avoid competition and as each group selected different
~ Pinta
The Galapagos Islands
mutation.
• '
()
..
Q
Marchena
km
0
50
1·
ATLANTIC
OCEAN
Equator
Galapagos
Islands
Daphne Major
\.
0
Santa Cruz
Cristou
San
0
PACIFIC
OCEAN
Floreana
0
Espaniol a
Qi
Figure 5.6 The Galapagos Islands.
5 EVOLUTION AND BIODIVERSITY
foods, they developed differently until evemually number 0 ~ separate
·
bl. hed .,.,oday we recogruse 13 different species includ·
species were esta 1s
•"
Ing
the cactus finch which has a long beak that reaches into blossoms, the
ground finch with a short stubby beak adapted for eating seeds buried
under the soil, and the tree finch with a parrot-shaped beak suited for
stripping bark to find insects (Figure 5.7). Darwin's finches provide one of
the best-known examples of adaptive radiation.
One island that has been extensively studied is Daphne Major, a tiny
volcanic island just north of Santa Cruz. Biologists Peter and Rosemary
Grant have collected data on the medium ground finch (Geospizafortis)
on this island for more than 30 years. In 1977, there was a serious drought
on the island and the small seeds that the birds feed on were in short
supply. Some of the birds, which had slightly larger beaks, were able to
open larger seeds and were able to survive but birds with small beaks died.
The following year, the Grants measured the offspring of the survivors
and found that their beaks were about 3% larger, on average, than those
of earlier pre-drought generations they had studied. They concluded that
natural selection had acted on the population and larger-beaked birds had
survived and reproduced more successfully. Females also chose their mates
based on the size of their beaks so these two factors - ability to find food
and ability to attract a mate - seem to be key influences in the eventual
development of a new species. The Grants have continued to study the
finches of Daphne Major and gathered more data on other factors co
provide further evidence of natural selection.
Figure 5.7 Photographs of som
an s: t e tree finch , ground finch and cactus finch.
Industrial melanism
The peppered moth (Bisro11 bet11/aria) .
.
.
during the day on th b k f
is a mght-flymg moth that restS
e ar o trees p · 1 1
covered with grey-g
. h
'. articu ar Y on branches that are
reen 1IC en It is a li 11
kl
camouflage against the t
b ·
g t spec ed grey, and relies on
ree ranches to
. fr
. els
In Britain in the mid-l 9 h
protect It om predatory bir .
.
t century a bl k fi
noticed (Figure 5.8) . Th
'
ac orm of the moth was
e appearance of th ·
•h
th e period of the Ind
.al
is new colour coincided wJt
.
US tn
Revolutio
h
and comnbuted to g
.
n w en many factories were but1t
.
row1ng pollution i th
.
.
n e atmosphere. Tl11S polluoon
kill e d the lichens that
·h
.
grow on the bark 0 f
d
wit parncles of soot.
tre es, w hich becam e blackene
166
Figure 5.8 Light and melanic forms of peppered moths on light and dark tree bark.
The colour of the moth is due to a single gene, which can be present
in two forms. The common recessive form gives rise to a light speckled
colour. The much less common dominant form gives rise to the black,
melanic moth.
In the polluted areas, the speckled form was no longer camouflaged
on the blackened tree bark, and was easily seen by birds that ate speckled
moths. The black moths were better suited to the changed environment as
they were camouflaged. Black moths survived and bred and the proportion
of black moths with the dominant allele grew in the population.
In 1956, the Clean Air Act became law in Britain and restricted air
pollution . Lichen grew back on trees and their bark became lighter.As a
consequence, the speckled form of the peppered moth has increased in
numbers again in many areas , and the black form has become less frequent.
Antibiotic resistance
Antibiotics are drugs that kill or inhibit bacterial growth. Usually, treating
a bacterial infection with an antibiotic kills every invading cell. But,
because of variation within the population, there may be a few bacterial
cells that can resist the antibiotic. These individuals will survive and
reproduce. Because they reproduce asexually, all offspring of a resistant
bacterium are also resistant, and will survive in the presence of the
antibiotic. In these conditions, the resistant bacteria have enormous
selective advantage over the normal susceptible strain, and quickly outcompete them.
Treating a disease caused by resistant strains of bacteria becomes very
difficult. Doctors may have to prescribe stronger doses of antibiotic or try
different antibiotics to kill the resistant bacteria.
The problem of antibiotic resistance is made more complex because
bacteria frequently contain additional genetic information in the form
of plasmids, which they can transfer or exchange with other bacteria,
even those from different species. Genes for enzymes that can mact1vate
antibiotics are often found on plasmids, so potentially dangerous bacteria
can become resistant to antibiotics by receiving a plasmid from a relatively
harmless species . Many bacter ia are now resistant to several antibiotics
Figure 5.9 The grey-green areas on the
agar jelly in this Petri dish are colonies of
the bacterium Escherichia coli. The white
card discs are impregnated with different
antibiotics. Th is strain of E.coli is resistant
to the antibiotics at the bottom left and has
been able to grow right up to the discs, while
the other discs have a 'zone of inhibition'
around them.
0(ffl]•unein+#i•R:HMM•M
167
(Figure 5 _9 ), so pharmaceutical comp~nies are constantly trying_ to
develop new antibiotics to treat infect1ons caused by these mult1ple
resistance forms of bacteria.
So-called 'superbugs', such as MRSA (methicillin-resistant Staphylococa 15
aureus) and Clostridium difficile, are bacteria that are resistant to many
antibiotics. They have arisen partly as a result of overuse of antibiotics.
Antibiotics used incorrectly, or too frequently, help to 'select' the resistant
individuals, which then increase in numbers. Patients failing to take a
complete course of medication can also encourage the survival of slightly
resistant bacteria that might have been killed if the antibiotic had been
taken properly.
Scientific theories and evidence
Natural selection is a theory. In science, the term 'theory' has
a very specific meaning. Scientific theories require a hypothesis that
can be tested by gathering evidence. If any piece of evidence does not
fit in with the theory, a new hypothesis must be put forward and more
scientific evidence gathered. It is important to recognise the differen ce
between a theory and a dogma, which is a statement of beliefs chat are
not subject to scientific tests. Since Danvin 's time evidence has been
collected to support his theory. With new techn ologies such as DNA
profiling and carbon dating, further evidence continues co accumulate.
Questions to consider
• How much evidence is needed to
support a theory?
• What kind of evidence is needed
to refute a theory?
• Will it ever be possible to prove
ch at evolution has taken place?
Nat ure of science
Using theories to explain natural phenomena_ the evolution of
antibiotic resistance in bacteria
Read the information in the preceding paragraphs and use it to formulate
an answer to this question:
• How important is it for s · c·
d
·
· I ·
.
cien 1sts to evelop theories to exp lam c1eir
proposals to a Wider audience?
1J Test yourself
4 Explain why sexual rep d
• . .
. .
ro uct1on 1s important for evolution.
5 lnd1v1duals
in a pop 1 ·
.
. ,
u atwn are often said to be 'struggling for
survival . State the key fact that
h.
.
causes t 1s struggle.
6 If an environment
changes 111
· d. .d al
. .
•
I V ! u s with particular
comb111at1ons of genes are
lik
.
.
more
ely to survive. State the na111e
given to this phenomenon.
7 Describe two examples of e
. .
.
vo 1ut1on 111 response to
environmental change.
S,3 Classification of biodiversity
Learning objectives
You should understand that:
• The binom.ial system for naming
• .
.
species 1s used unive all b
biologists and has been agreed at 1
. .
rs Y Y
.
.
nany scientific meetin
• Each new species that 1s discover d · •
gs.
.
.
e 1s given a twousing the bmo1111al system.
part name
• Taxonom.ists classify species using h.
h
.
.
•
a ierarc Y of eight taxa·
domam, kingdom, phylum, class, order f: .
· .
·
l .
' ami1Y, genus and species
All orgamsms are c ass1fied into one 0 f h
.
·
t ree domams· A h
Eubacteria and Eukarya.
· re aea,
In a natural classification, each genu
d h' h
.
.
s an ig er taxon contams
all the species evolved from one common ancestor
Taxonom.ists reclassify groups of species h
.d·
.
wm=m~~M
that species have evolved from different ancestors.
• Natural classification helps to identify
·
.
. .
.
new species and predict
charactenstICs shared by species with.in a group.
Natural (biological) classification attempts to arrange Iivmg
· orgarusms
· mto
·
groups that enable them to be identified easily and that show evolutionary
Jinks between them. The system of classification we use today has its origins
in a method devised by the Swed.ish scientist Carolus Linnaeus (1707-1778).
The binomial system of classification
The classification of living organisms is simply a method of organising
them into groups to show similarities and differences between them.
More than 2000 years ago, the Greek philosopher Aristotle (384-322
BCE) classified organisms into two groups - plants and animals. This was
useful as a starting point, but as the two main groups were sub-divided,
problems started to appear. At that time, organisms were seen to be
unchanging, so there· was no understanding of evolutionary relationships.
Many organisms discovered later did not fit into the scheme very well.
Birds were separated into a group defined as 'Feathered animals that
can fly' so no place could be found for the flightless cormorant, a bird
that does not fly. Bacteria, w hich were unknown at the time, were not
included at all.
In 1735, Carolus Linnaeus (Figure 5.10) adapted Aristotle's work, and
his system forms the foundation of modern taxonomy. Taxonomy is the
science of identifying, nam.ing and grouping organisms.
Linnaeus gave each organism two Latin names - the first part of the
name is a genus name and the second part a species name. Thus the
binomial, or two-part, name for the American grizzly bear is Ursus
americanus whereas a polar bear is Ursus maritimus. Linnaeus used Latin for
J
Figure 5.1O Carolus Linnaeus, also known
as Carl Linnaeus, was a Swedish botanist,
physician and zoologist, who laid the
foundations for the modern scheme of
binomial nomenclature.
By convention, the genus name
starts w.ith a capital, while the
species does not. Both are written
in italic or underlined. Once an
organism has been referred to by its
full Latin name in a piece of text,
further references abbreviate the
genus to the first letter only - for
example, U. maritimus (Table 5.1).
169
-
,;( f
,., •-~
.
Polar bear Lemon tree
Domain
Eukarya
Eukarya
Kingdom Animalia
Plantae
Phylum
Angiospermata
Chordata
Class
Mammalia
Dicoyledoneae
Order
Carnivora
Geraniales
Family
Ursidae
Rutaceae
Genus
Ursus
Citrus
Species
maritimus
limonia
Table 5.1 The taxonomic hierarchy for a
plant species and for an animal species.
Thermophilic bacteria
inhabit hot sulfur springs and
hydrothermal vents and survive at
temperatures in excess of 70 °C and
up to 100 °C in some cases.
Halophilic bacteria live in
very salry environments such as
the Dead Sea where the Sun has
evaporated much of the water.
They are also found in salt mines.
Methanogenic bacteria are
anaerobes found in the gut of
ruminants, as well as in waste
landfills, paddy fields and
marshland. They produce methane
as a waste product of respiration.
nguage of medicine and science
I Ia
.
Ion been ne
.
,and
his names. Lat111 has
g
. . is mentioned anywhere m the world
. If Urs11s 111arit111111s
•
d
,
it is unchangmg.
I b rs are being d1scusse •
·""
kn
chat
po
ar
ea
'
f
.
h
scientists ww ow
. dicates a group o species t at are Ve
f the name 111
ry
• The genus part O
mmon ancestor.
I
d
and
share
a
co
.
di
"d
a1
h
I
close y re ate ,
fi d as a group of 111 v1 u s t at are capab!
• 1s· ust1ally de me . ffi ·
e
The species
•
.
. to reduce ferule o sprmg.
of 111terbreeding P
. h" classification system - for example
I d structure m is
'
Linnaeus deve ope
.
f
wading birds and perching birds.
O prey,
b.
ds
into
b1rds
·fli
he groupe d tr .
.. ng things in many d1 erent ways, ov
h· ·
sible co group 1!VI
er
Althoug it is pos .
hi a] classification system has emerged, through
the last 200 years a hi~rarcfi c
tings which is now used by biologists
any snenu ic mee
'
agreement at m
. ts all over the world classify species using
eve vhere Modern taxonom1s
.
f}'\
·
all d
(singular: tax on). There are eight levels to
a hierarchy of groups c e taxa
the hierarchy:
• domain
• kingdom
• phylum (plural: phyla)
• class
• order
• family
• genus (plural: genera)
• species.
_
Two examples of how species are classified are shown m Table 5.1 .
Aristotle's original grouping of organisms into just two kingdoms has
also been refined. Today the most widely accepted method of classification
uses three domains - the Archaea, Eu bacteria and Eukarya - divided into
six kingdoms.
• Archaea:
Kingdom Archaebacteria (ancient bacteria) - methanogens, halophiles
and thermoacidophiles
• Eubacteria:
Kingdom Eubacteria (true bacteria) - bacteria and cyanobacteria
• Eukarya:
Kingdom Plantae - plants
Kingdom An.imalia - animals
Kingdom Fungi - fungi
Kingdom Protista - red algae and d.inoflagellaces
You will notice that viruses are not included in the classification of
living things. They are not considered to be living because they cannot
reproduce independently and need to take over a host cell to do so.
Viruses differ ,videly in shape and appearance and are usually divided
into groups and described by the shape of their protein coat.
? ..
WhY is classification important?
- ~- ----'-L/7
.....
I
I
N.1niral cla,sifirations group together organisms with mauy of the same
and arc. pred1ct1ve, so that by studying th c characcenstlcs
- - of
cI1.\r:icrerisncs
·
. .
org•
\l\iSill
1t
1s
possible
to
predict
the
natural
aroup
·t
b
I
.111
•
_
_
_
t.
1 e ongs to.
Ph)•logenenc class1ficat10us
_
_ are natural
' classificat"
, ions tI1at attempt to
·dencify the. evolutionary h1storv· of species · The rol,t' o f taxonomy 1.s to
1
\
1
I
croup speoes that are related by common ancestry.You can find out more
;i,out this in Subtopic 5.4 .
A natural classification such as the one devised bv Linnaeus is based
on identification of homologous structures that ind.icate a conunon
evolutionary history. If these characteristics are shared by organisms then
it is likely that those orgamsms are related. So the binomial system can be
both .1 natural and a phylogenetic classification.
In summary, then , there are four main reasons why organisms need to
be classified:
to impose order and organisation on our knowledge
to give each species a unique and universal name, because common
names vary from place to place around the world
to identify evolutionary relationships - if two organisms share particular
characteristics then it is likely that they are related to each other, and
rhe more characteristics they share then the closer the relationship
4 to predict characteristics - if members of a particular group share
characteristics then it is likely that other newly discovered members of
that group will have at least some of those same characteristics.
In 2013 , previously unknown
bacteria were found living in a
deep lake nearly 500 111 below the
ice in Antarctica.These organisms
have new charJcteristics unlike
archaeans and other bacteria.
Discoveries like this may mean
chat further changes will have to
be made to our current system of
classification in the future .
Classifications change
New species are being found and new discoveries are being made about
existing species all the time. Such discoveries may force us to rethi11k
the way we classify living things. For example, i11 the past, the name
'bacteria' was given to all microscopic single-celled probryotes. Bur
recent molecular studies have shown that probryotes can be divided into
two separate domains, the Eu bacteria and the Archaea, which evolved
independently from a common ancestor. Molecular biology, genetics
and studies of cell ultrastructure have shown that the Archaea and
Eukarya (eukaryotes) are in fact more closely related to one another than
either is to the Eubacteria. Similar principles are applied to all levels of
dassificatio11 .Taxonomists reclassify organisms when new evidence shows
that they have evolved from different ancestral species.
The Ki11gdom Protista, which is not accepted as a true taxon by all
taxonomists, includes eukaryotes that are either unicellular or, where they
do form colonies, are not differentiated into tissues. The taxonomy of this
group is likely to change because the group is paraphyletic, which means
that aU of its members may not share a common ancestor. A paraphyletic
group 1s defined by what it does not have and may consist of an ancestor
but nor aU of its descendants.
Car!Woese
Carl Woese introduced the three
domains oflife - the Archaea,
Eubacteria and Eukarya - in
I 977. In proposing these new
classifications, he sep.iratcd the
prokaryotes into t\\"o groups - the
Eubactcria and the Archaea - based
011
di"covcrtl's about the structure
of their ribosomal ltNA. His
cbssific:.1tion reflects thl! current
vie\\" of the ancestrv of the three
m.,jor groups of organisms, but it
w,1s 11 ot accepted :md t1'ied widt!y
until the 1990s, " ·hen scientists'
knowledge about RNA structure
increased. (Note that the name
I
Bacteria is now often LE,ed for
the domain comprisiuu the true
bacteria, r.lther tlun W~ese's
original name, Eubacteria.)
, 11 II
,i,11 $1@ ,' [111)
(
II
r-
9'W
ev
Acceptance of a new paradigm
. . Ob'~cctcd to Wocscs
SC\-'Cr.lI l'l'Spt•ctcd SCIClltlSts
,
.
.
.
f
I
k
.
.. .1,·ns· , including
d1V1s1on o t 1c pro ·aryotes mto two d0 111,
microbiologist Salvador Luria (I912-1 99 1) and
evolutionary biologist Ernst Mayr (1904-2005).
. to consider
. .
Questions
.
servacism m science desirabl ,
•nt 1s con
e.
, To what exte .
must be presented before a
h evidence
• HoW n1uc
be accepted'.
heory
can
new t
f the plant kingdom
The main phyla O
ukaryotic, have cellulose cell Walls
ki dom are e
. .
.
b rs of the plant ng
ki dam is divided mto several
Mem e
nthesis.The ng
.
and carry out photos)'
·miJaricies (Figure 5.11) .
d other s1
different phyla base on
flowering Plant
conifer
moss
fern
5mm
Figure 5.12
Amoss, Grimmia pulvinata.
Figure S. l l Acladogram showing the relationship_ between the main plant phyla.
You can find out more about cladograms in Subtopic 5.4.
Bryophyta
Plants in this phylum include the mosses (Figure 5.12) and liverworts.
These are the simplest land plants and are probably similar to the first
plants to colonise the land some 400 million years ago.
• Bryophyta are usually small and grow in damp places because they have
no vascular system to carry water.
• They reproduce by way of spores and these are contained in capsules
on small stalks held above the plants.
• They have no roots.j ust chin fJamentous outgrowths called rhizoids.
They have no cuticle and absorb water across their whole surface.
• Liverworts have a flattened structure called a thallus but mosses have
small simple leaves.
Filicinophyta
Figure 5.13
Cl ub moss hangi·ng from a
This group includes the club mosses, horsetails and ferns (Figure 5.13).
• Filicinophyta have roots, stems and leaves and possess internal structures.
• Because
.
,
. of the stip port firom woody tissue,
some tree ferns grow to over
..U-SA_._ _ _ _____S
_m
. _1_n_h_ei_g_ht_.._._,__~--
J
S.001 ,. have: fibrous roots, whik others prodLicc
an UIILIergrounu stl'111
calkd a rhizome.
·k clic brvophvta, they also 1\·nrodL1c, \)
d ·
·
:
,
L Y pro ucmg spores. In the krns,
, L1 e
rhcse ;m• found m clusters called sori on thL' undcrsirb of the leaves.
coniferophyta
Coniferophyta include shrubs and trees, such as pine trees, fir and cedar.
which are often large and evergreen (Figure 5.14) . Some of the world's
h~est forests are comprised of conifers .
Coniferophyta produce pollen rather than spores, often in huge
amounts, as conifers are wind- pollinated plants.
, They produce seeds, which are found in cones on the branches.
, Most have needle-like leaves to reduce water loss.
Angiospermophyta
This group includes all the flowering plants, which are pollinated by wind
or animals (Figure 5.15). They range from small low-lying plants to large
trees. Many of them are important crop plants.
, Angiospermophyta all have flowers, which produce pollen.
, They all produce seeds, which are associated with a fruit or nut.
Figure 5 _14 A western white pine tree (Pinus
monticola) in the Sierra Nevada, USA.
some phyla of the animal kingdom
Organisms in the animal kingdom are characterised by being able to move
and getting their nutrition by eating plants, other animals or both. Animals
flower
are divided into two groups - those that have a backbone (vertebrates)
and those that do not (invertebrates).
Porifera
'--=--
-
fruit
This group contains the sponges (Figure 5.16) .They have different types
of cell, but no real organisation into tissues and no clear symmetry. All
leaf
t-<:...---- root
Figure 5.15 Shepherd's purse, Capsefla
bursa-pastoris - an example of a flowering
plant.
· tes tud'naria)
in the Sulawesi Sea of
1
Figure 5.16 A giant barrel sponge (Xestospong,a
Indonesia is around 10-20cm in diameter and 10-20cm tall.
.'
iii 17
a skeleton of calcium carbonate
pro d uce,
.
or
,,.., aquatic and lllany
h numerous pores m the body Wal}
sponges " ,
through t e
.
l
They pump water
rves or muscular tissue.
s1ICon.
-,es have no ne
and filter out food. Sp 011 g
Figure 5, 17 Dendrophyllia coral polyps
in the Red Sea. These polyps are 2--4cm in
diameter.
Cnidaria
dJ·ellyfish (Figure 5.17) .Alrnost
,ones coraIs an
.
b d 1
These are the sea anen
,
. d into tissues 111 two o y ayers.
.
cells orgamse
.
all are marine and have
. . g them with special cells called
,
I
nimals by sungm
h
They feed on oner a
. h ir tentacles. They ave a mouth
ing them mt e
nematocysts and trapp
·ng to get rid of waste. The box
d
the same opem
to take in food an use
f war are two of the most venomous
e man-a jellyfish and the Portugues
animals on Earth.
Platyhelminthes
.
.
h ve a body cavity with a mouth and an
of ce Us an d a
.
. . .
Th ese I1ave t h re e layers
. . .
hile others are parasites, livmg mside
fr
livmg
m
water
w
S
anus. ome are ee
fl
d appearance hence their common
other or nisms. They have a attene
,
ga
, .
l8) Most flatworms are small, but the tapeworm
5
name 'flatworms (Figure · ·
eraJ metres long
found in the intestines of animals may grow to sev
.
Annelida
Figure 5. 18 The Hawaiian spotted flatworm
(Pseudobiceros sp,) is about 5 cm long.
This. group, k nown as the 'segn1ented worms' , contains
. . lugworms,.
earthworms and leeches. Some are aquatic, living m nvers, estuanes and
mud and others inhabit soil. All annelids have bodies that are d1v1ded
into ~ections called segments. All of them have a simple gut with a
mouth at one end and an anus at the other. Earthworms are rmportant
in agriculture because their burrowing aerates the soa and brings down
organic matter from the surface, which helps to fertilise 1t.
Mollusca
This is the second largest animal phylum, containing over 80000 species.
The group includes small organisms like slugs and snails, as well as large
marine creatures like the giant squid and octopuses (Figure 5.19) . Many
produce an outer shell of calcium carbonate for protection.
Arthropoda
Figure S. 19 The giant octopus (Enteroctopus
sp.) is one of the largest invertebrates and
can be up to 5 m long.
The arthropods comprise the largest animal phylum, and include aquatic
animals such as the crustaceans (the crabs and lobsters) , and terrestrial
animals such as insects, spiders and scorpions. All have an exoskeleton
made of chitin. They have segmented bodies and jointed limbs for
walking, swimming, feeding or sensing. An exoskeleton places a restriction
on their size: arthropods are never very big because they must shed their
exoskeleton and produce a new, larger one in order to grow. The largest
arthropod is the Japanese spider crab, which can be 4 m long. These crabs
are marine so water gives their bodies buoyancy, enabling them to move.
Well over 1 million arthropods are known and it is estimated that there
may be at least as many more that have not yet been identified.
ct,ordata
hylum Chordata includes humans and othe
b
b
fhe P
r verte rates, ut not a11
. tes are vertebrates. The features shared by all h d
hO rd •1
•
c or ates are:
c .1 dorsaJ nerve cord ' which is a bundl e O f nerve fib
•
I res connectmg the
' '
-
br:H 0
to the organs and muscles
.
.
notochord, which 1s a rod of cartilage supporting the nerve cord
il
ost-anal ta
' ap
1·
h' h
·
]laryngeal s its, w IC are openmgs connecting the inside of the
' :]Jaryn,"< (throat) to the outside. These may be used as gills.
f\]l chordates have these four features at some stage in their lives but, in the
be seen in the embryo •The phylum
case Of 111 ost vertebrates, they may only
.
. eludes the vertebrates (mammals, birds, amphibians, reptiles and fish) as
'.~,ells the tunicates (sea s_quirts and salps) and the cephalochordates Oancelets).
About half the phylum is made up of the bony fish (Class Osteichthyes).
, a
vertebrates
.
.
Vertebrate groups can be quickly identified from their external features
'{their skin and method of breathing are examined (Figure 5.20).
~other important feature is their method of reproduction. The young of
mammals develop inside the body of the mother and are fed with milk
from mammary glands after they are born. Birds lay hard-shelled eggs,
while the eggs of reptiles are covered by a leathery shell. Bird and reptile
gs contain nutrients for the development of their embryos. Both fish
:d amphibians must reproduce in water - their eggs are released into the
water and fertilised outside the female's body.
Annelida,
Tlie. Platyhelrn.inthes,
' Arthropoda an d Cliordata
Mollusca,
• ,
are animals that are 'bilaterally .
. I' . Th ey have
symmetnca
• a defirute
al
front and back end, and a dors
(back) and ventral (belly) side.
All vertebrates have cartilage in
addition to, or instead of, bone. .
Cartilage can either be flexible, hke
the cartilage in the human nose
and ears, or hard and firm, like the
cartilage in the larynx (voicebox).
Cartilage also covers surfaces of
bones at joints to ease movement.
Calcified cartilage makes up the
vertebrae and teeth of sharks.
This is not true bone as it is dead
material, whereas bone is a living
tissue.
Designing a dichotomous key
A dichotomous key is a series of steps, each involving a decision, which
can be used to identify unknown organisms. The key prompts us to ~ec1de,
through careful observation, whether or not a specimen displays parti~ular_
visible features, and allows us to distinguish between specimens on this basis.
vertebrates
no hair or
feathers
hair or
feathers
smooth skin
scales
hair
feathers
mammals
birds
nostrils
gills
reptiles
fish
.' 11
amphibians
d identify the vertebrate groups.
Figure 5.20 A simple key like this can be use to
5 EVOLUTION
AND BIODIVERSITY
-'""'-"•'I·-·
. a dichotomous key
(onstruct1n9
. ·denrify organisms
such as those sh
. ' ,, key ro I
.
I
own .
When consrruct1ng · , , ·h specinien 111 t 1e set carefully, and 01n
. •x•11111J1C cac
f h . d' 'd
ch
fi,•un· 5.21. f,r,;t <, •
. bo ut balf o t em 1v1 uals and b ose
"
. resent in a
a
a
1
5
ch:1r.icrcrisric rh:it P ·
sent in
rh,• otlwrs.
Start
wings absent
wings present
-:"
,~
more than 4 pairs of legs
1 pairofwings
4 pairs of legs
~lu ,
fly
r
claws absent
claws present
f M "'~
legs all about same length
legs not all same length
locust
crab
wings transparent
f
dragonfly
wings with scales
r
equal-sized body segmentation
t
'
'" '~I
segm~ntat·,o . h' d
n,n
/ /.
butterfly
Figure S.21 A dichotomous tree diagram distinguishing ei'ght organisms.
.
'
centipede
prawn
. e:--arnpk, the presence of wings could lw 1 . .
. . .
fol
.
I . I ff- . I . .
t lls first d1st111gu1shi11 '
erisnc, w uc l e ect1ve y d1v1dcs the s .
.
g
char.1ct
. pec1111cns mto two s111allcr
roups~, w for each group, another diagnostic fcatu , .
b
1,0
d. ·d
.
rl must e chosen whose
e or absence 1v1 es the specnnens int 0
~senc
.
two ,urther groups A
I'
•ng cree diagram can be constructed as I
. .
··
[lr.u1c 111
• . •
.
•
.
'
s iown 111 Figure 5.21,
....
ssively
dividing
the
specimens
mto
smaller
d
.
ll
,rog••
.
.
an sma er groups until.
I h end of each branc11 a smgle mdividual 1
·
s
·d
.6 d .
'
.
. ,
• 1 ent11e
11 1 e
. f1·oally,
the
tree
diagram
is
translated'
into
a
writte11
k
.
hi
h
I
,
ey,m w c tie
points
are
expressed
as
alternative
statem
E
h
.
br.lnc 11
.
.
.
'
ents. ac a1ternative
or leads th e user to a subsequent
eit. 11er names the 1dent1fied
.
.specimen
.
is reached •A we11-written
.
key 1s
.
pair. of statements, .until an 1dent1fication
.
osed
of
a
sen
es
of
questions
or
steps
such
ti
t
.
.
1
coOlP
'
a an orgamsm that 1s
being stu~ed can only be placed in one of two groups. The style of the
queSn·ons 1s therefore very
. important m the design of a goo d key.
So, for example, the dichotomous key arising from the tree diagram in
figure 5.21 would be as follows.
Wings present
go to 2
1
No wings
go to 5
Two pairs of wings
go to 3
2
One pair of wings
fly
Legs all approximately the same length
go to 4
3
Hind pair oflegs much longer than front two pairs
locust
butterfly
4 Wings covered in scales
Wings transparent, not covered in scales
dragonfly
spider
5 Four pairs of legs
More than four pairs oflegs
go to 6
crab
6 Pair of claws present
go to 7
No claws
centipede
7 Body clearly divided into equal-sized segments
prawn
Body in two regions, segments only clear on hind region
Nature of science
Cooperation and collaboration - an international naming
system
Local names for different species can cause confusion. What do you think
is being described here: armadillo bug, cafuer, wood bug, butchy boy,
gamersow, chiggley pig, sow bug, chuggypig and pill bug?
All these terms are local names for the woodlouse Porcellio scaber or its
relative Armadillidium vulgare and are used in different parts of Europe and
North America.
Cooperation and collaboration between international scientists
provided an agreed binomial name for the woodlouse so that wherever
these organisms are studied, information about them can be attributed to
the correct species.
'U'liest yourself
0
8
9
10
11
12
13
. h hierarchy of taxa.
els 111 t e
order the 1ev
he hierarchy of taxa that are used.
List 111
mes from t
1n
cwo na
h
State t e
binomial system- I
that is characterised by producin
ne
1
ofp ants
g
Identify the group ds in cones.
ollen and having see I t is characterised by producing
p
.
up of p an s
.
State which gro
stem and no cuticle.
· no root sy
· db h · ·
spores, having
·mals is characterise y avmgJointed
hi h group of aru
State w c
k I ton.
limbs and an exos e e . als is characterised by having
. h oup of amm
State wh1c gr
.h
outh at one end and an anus at the
segmented bodies wit a m
.
.
other.
a dichotomous key to identify leaves,
. 'ls the leaflarge?' would not be useful
. why the quesnon
·
exp Iam
ki
14 If you were ma ng
Learning objectives
You should understand that:
• A group of organisms that have
evolved from a common ancestor
is called a clade.
• Evidence used to place a
species in a clade can come
from base sequences of genes
or corresponding amino acid
sequences in proteins.
• Differences in base sequences
accumulate gradually so there is
a positive correlation between
the number of differences
between the sequence of a gene
in two species and the time since
they diverged from a common
ancestor.
• Characteristics (traits) can be
analogous or homologous.
• A cladogram is a tree diagram
that shows the most likely
sequence of divergence of clades.
• Evidence from cladistics has
shown that the classification
of some groups based on their
structure does not correspond
with their evolutionary origins.
s.4
Cladistics
The universality of ONA and prot~in structures
.
.
dibl
mplexity oflife, the buildmg components of
Despite the mere
e co
.
.
.
t only simple m structure but are also umversal.
Jivina orgarusms are no
Jiving organisms use DNA built from the same four bases to store
0
·
t. •nfiormation and most use the same tnplet code dunng
the1r gene 1c 1
.
.
· The cew exceptions include rrutochondna, chloroplasts and
trans1anon.
"
a group of bacteria.
Proteins are built up from amino acids and living organisms make use
0
All
of the same 20. In most cases, if a gene from one organism is transferred
into another, it will produce the same polypeptide (if the intrans have
been removed from it - Subtopic 7.1).
These facts indicate a conunon origin of life and provide evidence
to support the view that all organisms have evolved from a common
ancestor. Study of th e genetic code and amino acids of an organism can
provide evidence that links it to its close relatives and enables us to build
up diagrams called cladograms, which show how species are related to
one another in clades.
Ci.atl~s ;md da.r.frSi~ics
Cladistics is a method of classification that groups organisms together
according to the characteristics that have evolved most recently. Diagrams
called cladograms divide groups into separate branches known as clades
(Figure 5.22 and 5.28).
One branch ends in a group that has characteristics the other group
does not share. A clade contains the most recent common ancestor of th e
group and its descend an t s, so a c1ad e contams
· all the organisms that l13ve
evolved from a conm1on ancestor.
~ -------::=::::;;;~~~~
~-~
~
·---· - - - - - - .
3
7
1j
figure 5.22 A cladogram with four clades.
Cladc ., gro up of orga11i1111 <, horh
livi11g and extinct, rhar 1ncludc1 a ll
:rnccsror a11d all the ckscrndants of
that ancestor
Cladistics a method of classifyin g
or!(anisms usi ng cladograms
to analyse :i range of their
characteristics
Cladogram a diagram that shows
species' evolutionary relationships
to one another
,:-_
Figure .5.22 shows five organi
c · part of an evolutiomry tree.
· sms 10rrrung
• Orgamsms 1, 2, 3, 4 and 5 belong to the yellow clade.
Organisms 1 and 2 belong to the blue clade.
• Organisms 3, 4 and 5 belong to the green clade.
• Organisms 4 and 5 belong to the red clade.
• The common ancestor for each clade is shown by the coloured spot at
the branch point, or node.
Why do biologists need cladistics?
There are three important reasons for using cladistics to organise and
discuss organisms.
• It is useful for creating systems of classification so that biologists can
communicate th eir ideas about species and the history of life.
• Cladograms are used to predict the properties of organisms. A
cladogram is a model that not only describes what has been observed
but also predicts what might not yet have been observed.
• Cladistics can help to explain and clarify mechanisms of evolution
by looking at similarities between the DNA and proteins of different
species.
Finding evidence for clades and constructing
cladograms
Phylogenetics is the study of how closely related organisms are and it is
used to establish clades and construct cladograms. The modern approach
is to use molecular phylogenetics, which examines the sequences of DNA
bases or of amino acids in the polypeptides of different species to establish
the evolutionary history of a group of organisms. Species that are the most
genetically similar are likely to be more closely related. Genetic changes
are brought about by mutation and, provided a mutation is not harmful ,
it v.~ll be retained within the genome. Differences in DNA accumulate
over time at an approximately even rate so that the number of differences
be tween genomes (or the polypeptides that they specify) can be used as
an approximate evolu tionary clock. This information can tell us how far
back in time species split from their common ancestor. A greater number
t
I
=-~---·-J... }
of differences in a polypeptide indicates that there has been mor_e_t-im_e _ _ _ _ _ _ __.._____
han if the number is smaller. Ther ,
.
·1cct11nulate t .
'fli
b
c
for DNA nmtatwns to '
I wmber of dt erenccs etween "··
. between t ie I
""O
is a positive correlauon
d from a common ancestor.
. ti ey evolve
species and the ume 1
Ev1'd ence fro mamino acids . will have the same molecules
h
I ted organisms
We can expect t at re a
.
d that these molecules will have
.
. ular funcuons an
.
carrymg out paruc ·
.
roteins in different groups of
. .
S0 by comparing p
.
.
sumlar structures.
fi •nu'larities in anuno acid sequences ·
.
h k.i them or s1
. .
.it
orgamsms and c ec ng
Chlorophyll, hemoglobm, msulin and
.1s poss1'bl e to trac eilirn~m~
.
d . nany different species, have all been
h'ch are foun 111 1
cytoc hrome c, w 1
. found in the electron transport ch •
. d. h
Cytochrome c 1s
a1n
studte m t is way.
.
. ole in cell respiration. Its prim
. . h dria where it plays a vita1r
.
ary
m nutoc on . •
and 111 amino ands and the sequence has
structure conta111s between 100
d . al
lants an arum s.
been determined for a great many P
•
.
'd sequences of correspondmg parts of the
Below are the anuno act
.
6 five animal species. Each letter represents
cytochrome c mo Iecu Ie rom
.
.
.
. H
d chimpanzees have 1denucal molecules,
one anuno acid. umans an
.
. di . h h tw species are closely related. There ts only one
111 cat111g t at t e o
•
(h
· d) between the human cytochrome c and that
difference s own 111 re
.
.
bits and mice have rune differences when
of a rhesus monkey but rab
'th h mans which indicates they are less closely related. This
compare d w1
u
,
.
biochemical evidence supports the classification of the arumals that has
been made from morphological observations.
Human:
mgdvekgkki fimkcsqcht vekggkhktg pnlhglfgrk tgqapgysyt aanknkgiiw gedtlmeyle npkkyipgtk mifvgikkke eradliaylk katne
Chimpanzee:
mgdvekgkki fimkcsqcht vekggkhktg pnlhglfgrk tgqapgysyt aanknkgiiw gedtlmeyle npkkyipgtk mifvgikkke eradliaylk katne
Rhesus monkey:
mgdvekgkki fimkcsqcht vekggkhktg pnlhglfgrk tgqapgysyt aanknkgitw gedtlmeyle npkkyipgtk mifvgikkke eradliaylk katne
Rabbit:
mgdvekgkki fvqkcaqcht vekggkhktg pnlhglfgrk tgqavgfsyt danknkgitw gedtlmeyle npkkyipgtk mifagikkkd eradliaylk katne
Mouse:
mgdvekgkk.i fvqkcaqcht vekggkhktg pnlhglfgrk tgqaagfsyt danknkgitw gedtlmeyle npkkyipgtk mifagikkkg eradliaylk katne
rhesus chimpanzee,
monkey
human
Figure 5.23 Acladogram for five mammal
species.
The differences in the amino acid sequences in cytochrome c represented
by the letters shown above can be tabulated as shown in Table 5.2. From
these data, it is possible to construct a cladogram showing the relationships
between these five organisms, as shown in Figure 5.23.There are no
differences between rabbit and mouse so they have to be drawn together
at the end of a branch, and the same applies to the chimpanzee and
human. Rhesus monkey differs from chimpanzee and human by only
one amino acid and so the branch point must be one unit from the end.
Rabbit and mouse differ by nine amino acids and so the branch point
must be nine units furth er down. Biochemical analysis of other molecules
n1parison of DNA sequences would be needed to complete the
or co . n of ~bbit from mouse and human from ch'
nnpanzee.
;cpi rJOO
Number of amino acid differences in cytochrome c
compared with human
organism
hU111an ---i---------_______j
chir11Pa:'.,'.:nz::.::e=:e- r_ _ _ _ _ _ _ _o:__ _ _ _ _ _ __J
rhesus m~o:::nk::e~y+ _ _ _ _ _ _ __..:,_ _ _ _ _ _ _ _J
---t--------...:
9
rabbit
:__ _ _ _ _ __j
se_ __.J._ _ _ _ _ _ _ _. :,9_ _ _ _ _ _ _ _J
11100
S2 Table comparing cytochrome c in five species
fable ·
·
Evidence from DNA
oNA molecules can be compared in a technique known as hybridisation.
If a specific DNA sequence from an insect, a reptile and a mammal are
cornPared in this way, the number of differences between the mammal
od the reptile might be found to be 40, and between the insect and the
:aIJ1I11al 72. This provides evidence that the reptile is more closely related
to the mammal than the insect is and we can construct a cladogram that
shows the relationship between these three species (Figure 5.24).
The diagram in Figure 5.25 has been constructed in a similar way from
DNA analysis of the Canidae (dog and wolf family) . It shows that the
domestic dog and the grey wolf are very closely related, but the grey wolf
and Ethiopian wolf are more distantly related. The black-backed jackal and
golden jackal are also very distantly related. Complex diagrams like this
are usually constructed using specially designed computer software.
_ _ _ _ _ insect
reptile
mammal
Figure 5.24 Acladogram showing the
phylogenetic relationship between insects,
reptiles and mammals.
- - - - - - - - - - - - - - black-backedjackal
- - - - - - golden jackal
domesticated dog
grey wolf
' - - - - - coyote
' - - - - - - - - Ethiopianwolf
L - - - -- - -- - - - -
African wild dog
Key
6-7.S mya
3-4 mya
mya; million years ago
Figure 5.25 A cladogram showing the phylogenetic relationships in the Canidae family.
5 EVCiUTIONAND BIODIVERSITY
Worked example
- - - hu_mans, 90rillas,
chimpanzees
5.1 Whi ch :ipes :ire the closest living relatives of /-fo,110 sapicns?
Gibbons, orangutans, gorillas and chimpanzees have many .
physical similarities to human beings. For example human bemgs,
chimpanzees and gorillas all have a cavity in the skull just above
the eyes known as a frontal sinus. Gibbons, orangutans and 0th er
primates do not have this, so the physical evidence sugge st5 th at
chimpanzees and gorillas are more closely related to human beings
than gibbons and orangutans are. Evidence from the analysis of
blood proteins also suggests that orangutans are more closely related
to humans than gibbons are. This evidence can be shown as in
Figure 5.26.
.___ _ orangutans
L - - - - - - - gibbons
Figure S.26 A cladogram showing the
relationship between five apes.
Chimpanzees and gorillas are more closely related to humans than
other living animals are but which are our closest living relatives?
""'
h
1 ·
·
•
h'
d gorillas ' we must assess the evidence
10 sort out t e re at10nsh1ps between human bemgs, c 1mpanzees an
and check which features are shared. We can construct a table to summarise the evidence (Table 5.3).
Consider which of the three cladograms shown in Figure 5.27 is supported by moS t evidence.
Characteristic
Other primates
Gorillas
Human beings
Chimpanzees
Cladogram
supported by
such as baboons
evidence
DNA evidence:
number of
chromosomes
42 or more
48
46
48
C
structure of
chromosomes 5
and 12
different from
other primates
like other primates
different from
other primates
C
chromosome Y
and 13
same as human
beings
same as gorillas
like other primates
A
1.6%
-
1.2%
B
alpha hemoglobin several differences
compared with
human
one amino acid
difference
-
identical to
humans
B
protein factor in
blood
not variable
same variability as
chimpanzees
same variab ility as
humans
B
same variab il ity as
humans
C
% genetic
difference from
humans
orangutan 3.1%
rhesus monkey 7%
Molecular evidence:
amino acid
sequence in
myoglobin
not variable
like chimpanzees
Table 5.3 Summary of molecular and DNA evidence for the relate
like other primates
dness of primates,
baboon chimpanzee gorilla
baboon gorilla chimpanzee
human
baboon chimpanzee
gorilla
human
CladogramA
Cladogram B
Cladogram C
5 27
Figure · Three possible cladograms to show the relationship between human beings, chimpanzees and gorillas.
th
None of e cladograms can be proved to be correct from this evidence but cladogram B is the best supported
th
based on e data and is therefore hypothesised to best reflect our current understanding of the evolutionary
relationships of human beings. If further evidence is collected in future the hypothesis may be changed.
The shapes of cladograms
sharks amphibians
reptiles
birds
mammals
Cladograms can be drawn in one of two ways, as shown in
Figure 5.28, which shows two formats for a cladogram of
Jiving vertebrate animals. By looking at the upper diagram
we can see that the organism with the greatest number
of differences from mammals branches off first. These
organisms are the least closely related to mammals.
The relationship between reptiles and birds is the subject
of much debate amongst scientists. Some reptiles (e.g.
crocodiles and dinosaurs) are more closely related to birds
than other reptiles (e.g. lizards and turtles). Cladists have
suggested 'reptiles' is not a clade and a better grouping
would be Archosaurs (crocodiles, dinosaurs and birds)
Lepidosaurs (snakes and lizards) and Testudines (turtles and
tortoises).
sharks amphibians
I
I
i
reptiles
birds
mammals
J
Figure 5.28 A cladogram shown in two different formats.
Cladograms and the classification of living organisms
Cladograms have been produced using information from fossils,
morphology, physiology and molecular data. In most cases, they agree
with each other, or the differences are small. When there is uncertainty, a
second cladogram, built \JP using a different feature, can be constructed
c,or companson.
·
Eac h cladogram can be thought of as a hypothesis about
the relationships of the organisms it contains.
5 EVOLUTION AND BIODIVERSITY