17 Selection and evolution

17.1 Variation Genetic variation

Mutations that occur in body cells often have no effects
In Chapter 16 you saw how sexual reproduction
at all on the organism. If only one cell is affected, then
produces genetic variation among the individuals in
the overall function of the tissue of which the cell is part
a population. Genetic variation is caused by:
continues as normal. An exception is if the mutation
• independent assortment of chromosomes, and affects the control of the mitotic cell cycle, allowing
therefore alleles, during meiosis the cell to divide uncontrollably, which can lead to the
development of a tumour.
• crossing over between chromatids of homologous
chromosomes during meiosis However, mutations in cells in the ovaries or testes of an
animal, or in the ovaries or anthers of a plant, may be
• random fusion of gametes, and random mating
inherited by offspring. If a cell containing a mutation
between organisms within a species
divides to form gametes, then the gametes may also
• mutation. contain the mutated allele. If such a gamete is one of the
The first four of these processes reshuffle existing alleles two which fuse to form a zygote, then the mutated allele
in the population. Offspring have combinations of will also be in the zygote. This single cell then divides
alleles which differ from those of their parents and repeatedly to form a new organism, in which all the cells
from each other. This genetic variation produces will contain the mutated allele.
phenotypic variation. So genetic variation, whether caused by the reshuffling
Mutation, however, does not reshuffle alleles that of alleles during meiosis and sexual reproduction or
are already present. Mutation, which is described in by the introduction of new alleles by mutation, can
Chapter 6 (Section 6.6, Gene mutations), can produce be passed on by parents to their offspring, producing
completely new alleles. This may happen, for example, differences in phenotype. But variation caused by
if a mistake occurs in DNA replication, so that a new the environment is not passed on by parents to their
base sequence occurs in a gene. This is probably how offspring.
the sickle cell allele of the gene for the production of
the β-globin polypeptide first arose. Such a change in Continuous and discontinuous variation
a gene, which is quite unpredictable, is called a gene
Phenotypic differences between you and your friends
mutation. The new allele is very often recessive, so it
include qualitative differences such as blood groups and
frequently does not show up in the population until
quantitative differences such as height and mass.
some generations after the mutation actually occurred,
when by chance two descendants of organisms in which Qualitative differences fall into clearly distinguishable
the mutation happened mate and produce offspring. categories, with no intermediates – for example, you
have one of four possible ABO blood groups: A, B, AB
Genes are not the only cause of differences between
or O (Figure 17.2). This is an example of discontinuous
the phenotypes of organisms of the same species. The
variation.
environment also affects phenotype. For example, a
plant that has a genotype that allows it to grow tall
will only be able to grow tall if it has plenty of sunlight
and a good supply of minerals and water in the soil. If KEY WORDS
these are lacking, then it will not grow to the potential genetic variation: differences between the DNA
size determined by its genotype. The phenotype of an base sequences of individuals within a species
organism is a result of interaction between genetic and
environmental factors. phenotypic variation: differences between the
observable characteristics of individuals within
a species
Question discontinuous variation: differences between
1 Explain why variation caused by the environment individuals of a species in which each one
cannot be passed from an organism to its offspring. belongs to one of a small number of distinct
categories, with no intermediates

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35
KEY WORD
30 continuous variation: differences between
individuals of a species in which each one can lie
Percentage in population

25 at any point in the range between the highest

and lowest values
20

15 The genetic basis of continuous

10 and discontinuous variation
Discontinuous variation is caused entirely by genes,
5 with the environment having no effect. Your ABO blood
group depends on your genotype – the combination of
0 the IA, IB or Io alleles that you have in your cells.
O A B AB
Blood group
Continuous variation is also affected by genes, but
environment can also have an effect. For example, your
Figure 17.2: A bar chart showing variation in blood groups
height is a result of the genes affecting growth that you
in a population.
inherited from your parents, interacting with lifestyle
factors as you grew up – for example, the type of diet
In contrast, the quantitative differences between your that you had.
individual heights or masses do not fall into distinct
categories. When the heights of a large number of So both discontinuous variation and continuous
people are measured, there are no distinguishable height variation are affected by genes. Both may involve several
classes. Instead, there is a range of heights between two different genes. However, there are important differences
extremes (Figure 17.3). This is continuous variation. between them.

a b
Number of individuals
Number of individuals

Measurement Measurement

Figure 17.3: a A distribution curve and b a frequency diagram (histogram) showing continuous variation.

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 17 Selection and evolution

In discontinuous variation: Suppose that the height of an organism is controlled by

two unlinked (that is, on different chromosomes) genes:
• different alleles at a single gene locus have large
A/a and B/b. The recessive alleles of both genes (a and b)
effects on the phenotype.
each contribute x cm to the height of the organism. The
In continuous variation: dominant alleles (A and B) each add 2x cm.
• different alleles at a single gene locus have small Since the effect of such genes is additive, the
effects on the phenotype homozygous recessive (aabb) is therefore potentially
• different genes have the same, often additive, effect 4x cm tall and the homozygous dominant (AABB) is
on the phenotype potentially 8x cm tall. The other genotypes will fall
between these extremes.
• a large number of genes may have a combined
effect on a particular phenotypic trait; these genes Consider, for example, what might happen if these
are known as polygenes. two homozygous individuals interbreed, and if their
offspring (the F2 generation) also interbreed.

KEY WORD The number of offspring and their potential heights

according to their genotypes are summarised in
polygenes: a number of different genes at the histogram in Figure 17.4. These results fall
different loci that all contribute to a particular approximately on a normal distribution curve.
aspect of phenotype
6

In Chapter 16 you met a number of examples of

discontinuous variation where different alleles of a 5
single gene had a large effect on phenotype (Section
Number of offspring

16.2, The production of genetic variation). You also saw 4

how two different genes, at different loci, can interact
to produce phenotypic differences, a situation known 3
as epistasis. The inheritance of sickle cell anaemia and
haemophilia are examples of discontinuous variation
2
in humans, caused by variants of the alleles HBB and
F8 respectively. Flower colour in Salvia, stem colour
of tomato plants and feather colour of chickens are 1
examples of discontinuous variation in other species.
It is actually very difficult to find good examples of 0
4x 5x 6x 7x 8x
large effects of a single gene in an organism, such as Potential height from genotype / cm
those that you looked at in Chapter 16. It is much
more common to see a large number of different Figure 17.4: The added effects of alleles of two different
genes affecting a particular characteristic. Moreover, genes affecting height.
most genes have many more than the two alleles
that you considered in almost all of the examples in
Now imagine what the variation might be if the effects
Chapter 16. Multiple alleles, such as those involved in
of the dominant alleles of the two genes were not
the determination of blood group, are the norm – and
the same – for example, if allele A added 3x cm to
usually there are many more than three possible alleles
the potential height, rather than 2. (You might like
for any one gene.
to work that out, using the genotypes in the Punnett
Two of the typical effects of the inheritance of square above.) In fact, there are many more than
continuous variation – the small effects of the different two genes that affect height in humans, and in most
alleles of one gene on the phenotype and the additive plants. It is common for a large number of genes to be
effect of different genes on the same phenotypic involved, each of them having a small effect and all of
character – can be seen in a hypothetical example of the these effects adding together. The control of height is
inheritance of an organism’s height. therefore a polygenetic effect.

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parental phenotypes 8x cm tall × 4x cm tall

parental genotypes AABB aabb
parental gametes AB ab

F1 offspring genotypes AaBb

and phenotypes 6x cm tall

parental phenotypes 6x cm tall × 6x cm tall

parental genotypes AaBb AaBb
parental gametes AB Ab aB ab AB Ab aB ab
offspring genotypes and phenotypes

gametes of one parent

AB Ab aB ab
gametes of AABB AABb AaBB AaBb
the other AB
parent 8x cm 7x cm 7x cm 6x cm
AABb AAbb AaBb Aabb
Ab
7x cm 6x cm 6x cm 5x cm
AaBB AaBb aaBB aaBb
aB
7x cm 6x cm 6x cm 5x cm
AaBb Aabb aaBb aabb
ab
6x cm 6x cm 5x cm 4x cm

Moreover, each of these many genes may have generations of offspring resulting from the cross were
many more than two alleles, adding to the potential measured to the nearest centimetre. The number of cobs
for different heights. Now add in the effects of the in each length category was counted. The results are
environment, and it is easy to see why height shows shown in Table 17.1.
continuous variation. Any value of height can be seen,
lying between the two possible extremes.
In a classic experiment, the American geneticists Ralph Question
Emerson and Edward East crossed two varieties of maize 2 a In the classic experiment by Emerson and East,
which had distinctively different cob lengths. Both of the the Black Mexican parents were homozygous at
parental varieties (Black Mexican and Tom Thumb) were all gene loci affecting cob length. What caused
homozygous for most of their genes. The cob lengths the variation in their cob length?
of the plants used as parents and the first and second

Cob length / cm 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Number of Black Mexican parent cobs 3 11 12 14 26 15 10 7 2
Number of Tom Thumb parent cobs 4 21 24 8
Number of F1 cobs 1 12 12 14 17 9 4
Number of F2 cobs 1 10 19 26 47 73 68 68 39 25 15 9 1

Table 17.1: Variation in cob length of two varieties of maize and of the F1 and F2 generations.

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 17 Selection and evolution

b Was the variation in cob length of the F1 so great that they seriously affected the availability of
generation caused by genes, environment or grazing for sheep (Figure 17.6).
both? Explain your answer.
Such population explosions are rare in normal
c Was the variation in cob length of the F2 circumstances. Although rabbit populations have the
generation caused by genes, environment or potential to increase at such a tremendous rate, they do
both? Explain your answer. not usually do so.
Where you see variation in the same feature in different
As a population of rabbits increases, various
populations, you can use the t-test to compare the means
environmental factors come into play to keep down the
of the two populations – for example, the mean length of
rabbits’ numbers. These factors may be biotic factors
the cobs of the Tom Thumb and Black Mexican parents.
– caused by other living organisms such as through
The results of the test tell you whether the difference
predation, competition for food, or infection by
between the means is significant or could just be due to
pathogens – or they may be abiotic factors – caused
chance. The t-test is explained in Chapter P2 (Section
by non-living components of the environment such as
P2.8, Analysis, conclusions and evaluation).
water supply or nutrient levels in the soil.
For example, an increasing number of rabbits will eat an
increasing amount of vegetation, until food is in short
17.2 Natural selection supply. The larger population of rabbits may allow the
populations of predators such as foxes, stoats and weasels
All organisms have the reproductive potential to
to increase. Overcrowding may occur, increasing the ease
increase their populations. Rabbits, for example,
with which diseases such as myxomatosis may spread.
produce several young at a time, and each female
This disease is caused by a virus that is transmitted by
may reproduce several times each year. If all the
fleas, and it is fatal. The closer together the rabbits live, the
young rabbits survived to adulthood and reproduced,
more easily fleas, and therefore viruses, will pass from one
then the rabbit population would increase rapidly.
rabbit to another.
Figure 17.5 shows what might happen.
These environmental factors act to reduce the rate of
growth of the rabbit population. Of all the rabbits
born, many will die from lack of food, or be killed
by predators, or die from myxomatosis. Only a small
Numbers in population

proportion of young will grow to adulthood and

reproduce, so population growth slows.
This is true for most populations of organisms in the
wild. The number of young produced is far greater than
the number which will survive to adulthood. Many
young die before reaching reproductive age.

KEY WORDS
environmental factor: a feature of the
Time environment of an organism that affects its survival
Figure 17.5: The numbers in a population may increase biotic factor: an environmental factor that is
exponentially, if they are not checked by environmental caused by living organisms (e.g. predation,
factors. competition)
competition: the need for a resource by two
This sort of population growth actually did happen in organisms, when that resource is in short supply
Australia in the 19th century. In 1859, 12 pairs of rabbits
from Britain were released on a ranch in Victoria, as a abiotic factor: an environmental factor that is
source of food. The rabbits bred rapidly, as there was an caused by non-living components (e.g. soil pH,
abundance of food and there were very few predators. light intensity)
The number of rabbits soared. Their numbers became

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Figure 17.6: Attempts to control the rabbit population explosion in Australia in the mid- to late-19th century included ‘rabbit
drives’, in which huge numbers were rounded up and killed. Eventually, myxomatosis brought numbers down.

Selection pressures and very rare. As this selection continues to act over many
generations, the frequency of agouti alleles is likely to
survival increase, while the frequency of the alleles for white coat
will decrease and they may even disappear completely.
What determines which will be the few rabbits to survive
and which will die? It may be just luck. However, some The effect of such selection pressures on the frequency
rabbits will be born with a better chance of survival of alleles in a population is called natural selection.
than others. Variation within a population of rabbits Natural selection increases the frequency of alleles
means that some will have features which give them an conferring an advantage, and reduces the frequency of
advantage in the ‘struggle for existence’. alleles conferring a disadvantage.

One feature that can vary is coat colour. Most rabbits KEY WORDS
have alleles which give the normal agouti (brown)
colour. A few, however, may be homozygous for the fitness: the ability of an organism to survive and
recessive allele which gives white coat. Such white reproduce
rabbits will stand out distinctly from the others, and are selection pressure: an environmental factor that
more likely to be picked out by a predator such as a fox. affects the chance of survival of an organism;
They are less likely to survive than agouti rabbits. organisms with one phenotype are more likely to -
The chances of a white rabbit reproducing and passing survive and reproduce than those with a different
on its alleles for white coat to its offspring are therefore phenotype
very small. The term fitness is often used to refer to natural selection: the process by which
the extent to which organisms are adapted to their individuals with a particular set of alleles are
environment. Fitness is the capacity of an organism to more likely to survive and reproduce than
survive and transmit its alleles to its offspring. those with other alleles; over time and many
In this example, predation by foxes is an example of generations, the advantageous alleles become
a selection pressure. Selection pressures increase the more frequent in the population
chances of some alleles being passed on to the next
generation and decrease the chances of others. In this
case, rabbits with at least one allele for agouti coat have a Question
selective advantage over rabbits with the alleles for white.
3 Skomer is a small island off the coast of Wales.
The agouti rabbits have a better chance of reproducing
Rabbits have been living on the island for many
and passing on their alleles to their offspring. The
years. There are no predators on the island.
alleles for agouti will remain the commoner alleles in
the population, while the alleles for white will remain

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 17 Selection and evolution

a Rabbits on Skomer are not all agouti. There natural selection will ensure that this continues to be
are quite large numbers of rabbits of different the case.
colours, such as black and white. Suggest why
However, if a new environmental factor or a new
this is so.
allele appears, then natural selection may cause allele
b Suggest what might be important selection frequencies to change over successive generations. This
pressures acting on rabbits on Skomer. is called directional selection (Figure 17.7c).

Stabilising, disruptive and KEY WORDS

stabilising selection: natural selection that tends
directional selection to keep allele frequencies relatively constant over
Usually, natural selection keeps things the way they are. many generations
This is stabilising selection (Figure 17.7a and b). Agouti
rabbits are the best-adapted rabbits to survive predation, directional selection: natural selection that
so the agouti allele remains the most common coat colour causes a gradual change in allele frequency over
allele in rabbit populations. Unless something changes, many generations

a b
selection against selection against
Numbers in population

Numbers in population
very small animals very large animals

Body mass Body mass

c d
selection against
Numbers in population
Numbers in population

animals of middle size

selection against
very small animals

Body mass Body mass

Figure 17.7: If a characteristic in a population, such as body mass, shows wide variation, selection pressures often act against
the two extremes (graph a). Very small or very large individuals are less likely to survive and reproduce than those whose
size lies nearer the centre of the range. This results in a population with a narrower range of body size (graph b). This type
of selection, which tends to keep the variation in a characteristic centred around the same mean value, is called stabilising
selection. Graph c shows what would happen if selection acted against smaller individuals but not larger ones. In this case, the
range of variation shifts towards larger size. This type of selection, which results in a change in a characteristic in a particular
direction, is called directional selection. Graph d shows the result of selection that favours both large and small individuals but
acts against those whose size is in the middle of the range. This is disruptive selection.

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A third type of selection, called disruptive selection, The frequency of the allele for white coat increases at the
can occur when conditions favour both extremes expense of the allele for agouti. Over many generations,
of a population. This type of selection maintains almost all rabbits will come to have white coats rather
different phenotypes (polymorphism) in a population than agouti. There is a directional change in the
(Figure 17.7d). population, with a decrease in frequency of the agouti
allele, and an increase in frequency of the white allele, as
KEY WORDS a result of a change in selection pressure.

disruptive selection: natural selection that Mutation may also happen in an individual in the rabbit
maintains relatively high frequencies of two population. Mutations are random events, and the type
different sets of alleles; individuals with of mutation that occurs is not affected in any way by
intermediate features and allele sets are not the environment. As most organisms are already well
selected for adapted to their environment, most mutations are likely to
produce features that are harmful. That is, they produce
polymorphism: the continued existence of two organisms that are less well adapted to their environment
or more different phenotypes in a species than ‘normal’ organisms. Other mutations may be neutral,
conferring neither an advantage nor a disadvantage on
the organisms within which they occur. Very occasionally,
Directional selection leading to change mutations may produce useful features.
in allele frequency Imagine that a mutation occurs in the coat colour gene
Directional selection can happen when there is a change of a rabbit, producing a new allele which gives a better-
in selection pressures, or when a new allele arises by camouflaged coat colour than agouti. Rabbits possessing
mutation. this new allele will have a selective advantage. They will
be more likely to survive and reproduce than agouti
Imagine that the climate where the hypothetical rabbits, so the new allele will become more common
population of rabbits lives becomes much colder, so in the population. Over many generations, almost all
that snow covers the ground for almost all of the year. rabbits will come to have the new allele.
Assuming that rabbits can survive in these conditions,
white rabbits now have a selective advantage during Antibiotic resistance in bacteria
seasons when snow lies on the ground, as they are better The development of antibiotic resistance in a population
camouflaged (like the hare in Figure 17.8). Rabbits of pathogenic bacteria is an example of directional
with white fur are more likely to survive and reproduce, selection. As you saw in Chapter 10 (Section 10.2,
passing on their alleles for white fur to their offspring. Antibiotics), antibiotics are chemicals produced by
living organisms that inhibit or kill bacteria but do not
normally harm human tissue.
When someone takes the antibiotic penicillin to treat
a bacterial infection, bacteria that are sensitive to
penicillin die. In most cases, this is the entire population
of the disease-causing bacteria. However, by chance,
there may be among them one or more individual
bacteria with an allele giving resistance to penicillin.
This allele arises by random mutation. One example
of such an allele occurs in some populations of the
bacterium Staphylococcus, where some individual
bacteria have an allele that codes for the production of
an enzyme, penicillinase, which inactivates penicillin.
As bacteria have only a single loop of DNA, they have
only one copy of each gene, so the mutant allele will have
Figure 17.8: The white winter coat of a mountain hare an immediate effect on the phenotype of any bacterium
provides excellent camouflage from predators when possessing it. These individuals have a tremendous
viewed against snow. selective advantage in an environment where penicillin

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 17 Selection and evolution

is present. The bacteria without this allele will be killed, Alleles for antibiotic resistance often occur on plasmids
while those bacteria with resistance to penicillin can (Chapter 1, Section 1.7, Bacteria). Plasmids are quite
survive and reproduce. Bacteria reproduce very rapidly frequently transferred from one bacterium to another,
in ideal conditions and, even if there was initially only even between different species. So it is possible for
one resistant bacterium, it might produce 10 000 million resistance to a particular antibiotic to arise in one
descendants within 24 hours. A large population of a species of bacterium and be passed on to another. The
penicillin-resistant strain of Staphylococcus would result. more humans use antibiotics, the greater the selection
pressure we exert on bacteria to evolve resistance to
Such antibiotic-resistant strains of bacteria are
them.
continually appearing (Figure 17. 9). By using
antibiotics, humans change the selection pressures on
bacteria. A constant race is on to find new antibiotics Industrial melanism
against new resistant strains of bacteria. Another well-documented example of directional
selection producing changes in allele frequencies is the
peppered moth, Biston betularia (Figure 17.10), in the
Question UK and Ireland. This is a night-flying moth which
spends the day resting underneath the branches of trees.
4 These questions are about the bar chart in
It relies on camouflage to protect it from insect-eating
Figure 17.9.
birds that hunt by sight.
a Describe the trends in deaths from all types of
S. aureus between 1993 and 2012. Until 1849, all specimens of this moth in collections
b Describe the differences between the trends had pale wings with dark markings, giving a speckled
in deaths from meticillin-resistant S. aureus appearance. In 1849, however, a black (melanic)
(MRSA) and non-resistant S. aureus. individual was caught near Manchester (Figure 17.11).
c Many cases of MRSA develop in hospitals. During the rest of the 19th century, the numbers of
Suggest why this is so. black Biston betularia increased dramatically in some
d In the mid-2000s, healthcare professionals were areas, whereas in other parts of the country the speckled
asked not to prescribe antibiotics unless strictly form remained the more common.
necessary. Suggest how this could explain the
pattern shown by the graph between 2007 The difference between the black and speckled forms
and 2012. of the moth is caused by a single gene. The normal
speckled colouring is produced by a recessive allele
2500 Key of this gene, c, while the black colour is produced
resistant (MRSA) by a dominant allele, C. Up until the late 1960s, the
2000 not specified as resistant frequency of the allele C increased in areas near
Number of deaths

industrial cities. In non-industrial areas, the allele c

1500 remained the more common allele.
The selection pressure causing the change of allele
1000
frequency in industrial areas was predation by birds.
In areas with unpolluted air, tree branches are often
500
covered with grey, brown and green lichen. On such tree
branches, speckled moths are superbly camouflaged.
0
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012

However, lichens are very sensitive to pollutants such

as sulfur dioxide, and do not grow on trees close to or
Year
downwind of industries releasing pollutants into the air.
Trees in these areas therefore have much darker bark,
Figure 17.9: Meticillin is an antibiotic that is used to cure
against which the dark moths are better camouflaged.
infections caused by Staphylococcus aureus. However, many
Experiments have shown that pale moths have a much
populations of this bacterium have become resistant to this
higher chance of survival in unpolluted areas than dark
antibiotic and are known as meticillin-resistant Staphyloccus
moths, while in polluted areas the dark moths have
aureus, MRSA. The graph shows changes in cases and deaths
the selective advantage. As air pollution from industry
from MRSA in a European country between 1993 and 2012.

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is reduced, the selective advantage swings back in

favour of the speckled variety. So you would expect the
proportion of speckled moths to increase if there was a
reduction in the output of certain pollutants. This is, in
fact, what has happened since the 1970s.
It is important to realise that mutations to the C
allele have probably always been happening in B.
betularia populations. The mutation was not caused
by pollution. Until the 19th century there was such
a strong selection pressure against the C allele that it
remained exceedingly rare. Mutations of the c allele
to the C allele may have occurred quite frequently, but
moths with this allele would almost certainly have been
noticed and eaten by birds before they could reproduce.
Changes in environmental factors only affect the
likelihood of an allele surviving in a population; they
Figure 17.10: The dark and light forms of the peppered do not affect the likelihood of such an allele arising
moth, resting on dark and pale tree bark. by mutation.

17.3 Genetic drift and

the founder effect
Selection pressures are not the only factor that
determines which individuals survive and reproduce.
Chance can come into the equation, too. Chance is
most likely to have a significant effect if the population
is small. Imagine a very small population of plants in
Edinburgh
which two plants have white flowers and three plants
Newcastle
have yellow flowers. It could happen that, just by chance,
the seeds from the white flowers fall on unsuitable
ground, while a few seeds from the yellow flowers fall in
Dublin a better area where they can germinate and grow into
Manchester adult plants. Over several generations, the alleles for
white flowers could be completely lost, just by chance.
Birmingham This is called genetic drift. Genetic drift is a change in
allele frequency that occurs by chance, not as a result
Cardiff of natural selection. It is most likely to happen when a
prevailing London small number of individuals are separated from the rest
winds
of a large population.

KEY WORD

Figure 17.11: The distribution of the pale and dark forms genetic drift: the gradual change in allele
of the peppered moth, Biston betularia, in the UK and frequencies in a small population, where some
Ireland during the early 1960s. The ratio of dark to pale alleles are lost or favoured just by chance and
areas in each circle shows the ratio of dark to pale moths in not by natural selection
that part of the country.

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 17 Selection and evolution

Indeed, in the small population, some alleles may not The scientists decided that this was a good opportunity
be present at all. The gene pool – the complete set of to study the founder effect. They collected lizards
genetic information in a population – may be smaller belonging to the species Anolis sagrei from a larger
than that of the original population. Consequently, island nearby, where the lizards had survived. This
changes in the allele frequencies of this population may island had quite large shrubs and trees. These
take a different direction from that of the larger parent individuals showed variation in their leg length, but
population, just because of this chance effect, and not overall had quite long legs – it is known that lizards
because of the selection pressures acting on them. living on large shrubs and trees have an advantage if
they have long legs, because it is easier to grip the large
This process, occurring in a recently isolated small
branches. In 2005 the researchers selected one male and
population and resulting from only part of the gene
one female lizard from this island randomly, measured
pool being present in this small population, is called
their leg lengths, and placed them on one of the seven
the founder effect.
previously flooded islands. This was repeated for the
other six islands.
KEY WORDS
gene pool: the complete range of DNA base
sequences in all the organisms in a species or
population
founder effect: the reduction in a gene
pool compared with the main populations
of a species, resulting from only two or three
individuals (with only a selection of the alleles in
the gene pool) starting off a new population

For example, there is a very large number of small

islands in the Caribbean, and anole lizards live on
most of them. Lizards can be carried from one island
to another on floating vegetation. It is just chance
Figure 17.12: Anolis sagrei lives in bushes and trees.
which individual lizards land on an island and which
Lizards with longer legs have a selective advantage when
combination of alleles they will carry. There will be
the trees have large branches because they are better able
many other alleles in the gene pool of the population
to grip. In lower vegetation, lizards with shorter legs are
on the original island but only a small selection of them
more likely to survive and reproduce.
in the small number of lizards that arrive on the new
island. If the original population of lizards had a range
of colouration from green to brown, but the only two The two lizards placed on each island reproduced,
that floated to the new island were both green, then the so that the population on each island grew. The
population that develops on the new island might all researchers collected lizards from each of the islands
be green, with no brown individuals. This difference over the next four years. They measured their leg
between the two populations is not the result of natural lengths, and they also measured the sizes of plants on
selection but of a chance event that meant the alleles for the islands. They found that the mean leg lengths of
brown colour never arrived in the population. the lizards on each of the small islands gradually got
less. This is what you would expect. The original lizards
It is difficult to study the founder effect, because people came from an island with large vegetation, and were
are not often in the right place at the right time to see now living on an island where the vegetation was still
the individuals arriving in their new habitat. But in 2004 small, as it had not had time to grow back after the
a hurricane resulted in several islands off the Bahamas hurricane had damaged it.
becoming completely submerged in water for a time.
When the water subsided, scientists found that all of the However, the researchers also found that the mean leg
lizards on seven small islands had been killed. Not one length of the lizards was correlated with the leg lengths
living lizard was found. of the two randomly selected lizards that had been

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 CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK

placed on each island. For example, if the pair of

lizards placed on island A had relatively long legs, and
those placed on island B had relatively short legs, then
after four years the mean leg length on island A was
more than on island B. The leg length of the lizards had
been affected not only by the selection pressure exerted
by the small size of the vegetation on the islands, but
also by the set of alleles brought to the island by the
original parents – the founder effect.

The bottleneck effect

Something similar to the founder effect can happen if a
population that was originally large suffers a significant
fall in numbers.
The cheetah, Acinonyx jubatus, is a species that is in
danger of extinction. Today, all cheetahs are genetically
very similar to each other. They show little phenotypic
or genotypic variation. Many cheetahs are homozygous
at a very high proportion of their gene loci – in other
words, there are not many different alleles present in
a population. They have less than 5% of the genetic
variation that is normal within animal species. This lack
of genetic variation is of concern to conservationists,
as it puts cheetahs at a relatively high risk of extinction.
Without genetic variation, the species is unlikely to be
able to adapt to changes in its environment, such as
climate change. Figure 17.13: The cheetah is supremely adapted as a
fast-running predator, but its gene pool is so small that it
What could be the reason for this lack of genetic is at high risk of extinction.
variation in cheetahs? Scientists believe it results from
a period about 10 000 years ago, when all but a few
cheetahs were killed as a result of climate changes at the
end of the last Ice Age. Cheetahs found themselves living
17.4 The Hardy–Weinberg
in small populations, where the only mates available were
close relatives. Over time, much of the genetic variation principle
that was probably present in the earlier populations So far, you have considered allele frequencies
was lost. An event like this is called an evolutionary qualitatively. The Hardy−Weinberg equations allow you
bottleneck. This is a situation where the population falls to calculate allele frequencies, and to predict how these
so low that the gene pool is greatly reduced. might change in future generations.
When a particular phenotypic trait is controlled by two
KEY WORD alleles of a single gene, A/a, the population will be made
evolutionary bottleneck: a period when the up of three genotypes: AA, Aa and aa. Calculations
numbers of a species fall to a very low level, based on the Hardy–Weinberg principle allow the
resulting in the loss of a large number of alleles proportions of each of these genotypes in a large,
and therefore a reduction in the gene pool of randomly mating population to be calculated.
the species

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 17 Selection and evolution

The frequency of a genotype is its proportion of the

total population. The total is the whole population (that WORKED EXAMPLE
is 1) and the frequencies are given as decimals (e.g. 0.25)
of the total. 1 Imagine a population in which 1% of the
individuals show the recessive phenotype.
Use the letter p to represent the frequency of the Another way of stating this is to say that the
dominant allele, A, in the population and the letter q to frequency of these individuals is 0.01.
represent the frequency of the recessive allele, a. Then, These individuals must have the genotype aa.
since there are only two alleles of this gene: You therefore know that the frequency of aa
p+q=1 Equation 1 individuals, which is represented by q2, is 0.01.
Therefore, q = √ 0.01 = 0.1
You can also say that: Using Equation 1:
• the chance of an offspring inheriting a dominant p+q=1
allele from both parents = p × p = p2 so, p = 1 – q
• the chance of an offspring inheriting a recessive = 1 – 0.1
allele from both parents = q × q = q2 = 0.9
• the chance of an offspring inheriting a dominant In other words, you know that the frequency of
allele from the father and a recessive allele from the the dominant allele is 0.9 and the frequency of
mother = p × q = pq the recessive allele is 0.1.
• the chance of an offspring inheriting a dominant You can calculate the frequency of the
allele from the mother and a recessive allele from homozygous dominant and heterozygous
the father = p × q = pq genotypes:
frequency of homozygous dominant genotype
As these are the only possible combinations of alleles
= p2 = 0.92 = 0.81
that can be inherited, you can say that:
frequency of heterozygous genotype = 2pq =
p2 + 2pq + q2 = 1 Equation 2 2 × 0.9 × 0.1 = 0.18
You can check your calculations by adding up
Note that you do not need to remember these two the frequencies of the three genotypes, which
equations. They are given to you if you are asked a should come to 1.
question on this topic in your examination.
Worked example 1 shows you how to use these two
equations to calculate the frequency of the dominant
What is the use of these calculations? When the ratios
and recessive allele, and of the different genotypes, in a
of the different genotypes in a population have been
population.
determined, their predicted ratios in the next generation
These Hardy–Weinberg calculations do not apply when can be compared with the observed values. Any
the population is small or when there is: differences can be tested for significance using the χ2 test
(Chapter 16, Section 16.6, The chi-squared (χ2) test). If
• significant selective pressure against one of the
the differences are significant and migration and non-
genotypes
random mating can be discounted, this suggests that
• migration of individuals carrying one of the two directional selection is occurring in the population.
alleles into, or out of, the population
• non-random mating.

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Question 4 The two chosen parents are then bred together.

5 The resulting offspring are grown to adulthood
5 a A phenotypic trait is controlled by two alleles and tested. For example, if the breeder is trying to
of a single gene D/d. Explain why only the produce a variety of wheat with good resistance to
homozygous recessives, dd, can be recognised. a disease, she will ensure that all of the offspring are
b Calculate the proportions of homozygous exposed to the disease. She will then select the ones
dominant and of heterozygous individuals that show the greatest resistance. Alternatively, if
in a population in which the proportion of she is looking for both resistance and high yield, she
homozygous recessives is 16%. will select the ones that show the best combination
of these two characteristics.
6 This process continues for many generations, each
17.5 Artificial selection time selecting the ‘best’ individuals for breeding,
until all individuals show the desired characteristic or
Artificial selection is a process in which humans characteristics.
determine which individuals survive and reproduce.
Generally, at the end of this process, the breeder will
Artificial selection has been taking place for thousands
have produced a population of individuals that all show
of years and, for most of that time, people knew
the desired characteristics and that can be interbred
nothing at all about genetics. Today, scientists have a
among themselves to produce offspring that also
better understanding of the mechanisms that determine
show these characteristics. This means that they are
inheritance of desirable phenotypes.
all homozygous at all the gene loci that control these
features. The breeder does not have to know anything
KEY WORD about the genetics of this – she simply keeps selecting
individuals according to their phenotype, without
artificial selection: the selection by humans
knowing their genotype. This makes it possible to
of organisms with desired traits to survive and
work with characteristics that are controlled by a large
reproduce; also known as selective breeding
number of genes, where no one fully understands exactly
which genes are affecting the phenotype.
Artificial selection is done for many reasons. For
The breeding process outlined in steps 1 to 6 above
example, horse breeders may want to breed horses that
can be varied according to the breeder’s requirements.
can run fast or have the strength to pull heavy loads.
For example, she could breed two individuals from one
Cattle may be bred to produce more milk or more meat.
generation with each other, or bring in an individual
Maize may be bred to produce a variety with heavier
from a different variety – perhaps from the original
yields, or to survive in conditions where water is scarce,
variety with high yield or from a third variety that has
or to be resistant to attack from parasitic fungi. In every
another desirable feature.
case, the principles are the same.
1 The population that is to be used for artificial
selection must show some variation. For example, Introduction of disease
some individuals in a variety of wheat may have
resistance to a disease, while others are killed by it. resistance to varieties of
2 The breeder selects an individual that has the
feature that she wants future generations to have. wheat and rice
In this case, she would select a plant that has better Most modern varieties of wheat belong to the species
resistance to the disease than other individuals. Triticum aestivum. Selective breeding has produced
3 She then selects another parent plant. This could be many different varieties of wheat. Much of it is grown to
from the same variety and also have good resistance produce grains rich in gluten, which makes them good
to disease. However, in some cases, the breeder might for making bread flour. For making other food products
select a parent from a different variety that shows such as pastry, varieties that contain less gluten are best.
another beneficial characteristic, such as high yield, Breeding for resistance to various fungal diseases, such as
if she wants to produce a new variety with a good head blight, caused by Fusarium, is important, because
combination of disease resistance and high yield. of the loss of yield resulting from such infections.

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 17 Selection and evolution

Successful introduction of an allele giving resistance

takes many generations, especially when it comes from
a wheat grown in a different part of the world. To
help with such selective breeding, the Wheat Genetic
Improvement Network was set up in the UK in 2003 to
bring together research workers and commercial plant
breeders. Its aim is to support the development of new
varieties by screening seed collections for plants with
traits such as disease resistance, or climate resilience
(Figure 17.14). Any plant with a suitable trait is grown in
large numbers and passed to the commercial breeders.

Figure 17.15: Sheath rot, caused by Sarocladium oryzae,

can greatly reduce the yields of rice plants. Breeders are
attempting to produce rice varieties that have natural
resistance to this disease.

Inbreeding and hybridisation

in maize
Maize, Zea mays, is also known as corn in some parts
of the world. It is a tall grass with broad, strap-shaped
leaves (Figure 17.16). Maize grows best in climates with
long, hot summers, which provide plenty of time for its
cobs (seed heads) to ripen. It was originally grown in
Central and South America, but now it forms the staple
Figure 17.14: Selective breeding of wheat and rice takes crop in some regions of Africa and is grown as food for
place in closely controlled conditions, to prevent accidental people or animals in Europe, America, Australia, New
pollination between individuals that have not been selected Zealand, China and Indonesia.
to breed together. If maize plants are inbred (crossed with other plants
with genotypes like their own), the plants in each
Rice, Oryza sativa, is also the subject of much selective generation become progressively smaller and weaker
breeding. The International Rice Research Institute, based (Figure 17.17). This inbreeding depression occurs
in the Philippines, holds the rice gene bank and together because repeated inbreeding produces homozygosity.
with the Global Rice Science Partnership coordinates
research aimed at improving the ability of rice farmers to
feed growing populations. (You can read about seed banks KEY WORDS
in Chapter 18, Section 18.4, Protecting endangered species.) inbreeding depression: a loss of the ability to
The yield of rice can be reduced by bacterial diseases survive and grow well, due to breeding between
such as bacterial blight, and by a range of fungal diseases close relatives; this increases the chance of
including various ‘spots’ and ‘smuts’. The most significant harmful recessive alleles coming together in an
fungal disease is rice blast, caused by the fungus individual and being expressed
Magnaporthe. Researchers are using selective breeding to
inbreeding: breeding between organisms with
try to produce varieties of rice that show some resistance
similar genotypes, or that are closely related
to all these diseases.

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In maize, homozygous plants are less vigorous than pests and diseases, and good growth in nutrient-poor
heterozygous ones. Outbreeding – crossing with other, soils or where water is in short supply.
less closely related plants – produces heterozygous
plants that are healthier, grow taller and produce higher
yields. They show hybrid vigour.

KEY WORDS
outbreeding: breeding between individuals that
are not closely related
hybrid vigour: an increased ability to survive
and grow well, as a result of outbreeding and
therefore increased heterozygosity

However, if outbreeding is done at random, the farmer

would end up with a field full of maize in which there
was a lot of variation between the individual plants.
This would make things very difficult. To be able to Figure 17.17: The effects of inbreeding depression in
harvest and sell the crop easily, a farmer needs the plants maize over eight generations.
to be uniform. They should all be about the same height
and all ripen at about the same time.
So the challenge when growing maize is to achieve both
heterozygosity and uniformity. Farmers buy maize seed
Question
from companies that specialise in using inbreeding to 6 Explain why farmers need to buy maize seed from
produce homozygous maize plants and then crossing commercial seed producers each year, rather than
them. This produces F1 plants that all have the same saving their own seed to plant.
genotype. There are many different homozygous maize
varieties, and different crosses between them can
produce a large number of different hybrids, suited Improving milk yield of
for different purposes. Every year, thousands of new
maize hybrids are trialled, searching for varieties with
dairy cattle
characteristics such as high yields, resistance to more In some countries, particularly in Europe, America and
some parts of Africa, cattle are kept to produce milk.
For people who can digest lactose, milk is a valuable
food, rich in protein and calcium.
Milk production, like most characteristics of the crop
plants that humans grow and the farmed animals
that we keep, is influenced by both environment and
genotype. We still do not know which genes, let alone
which alleles of genes, contribute to the ability of cows
to produce large quantities of high-quality milk. But,
for selective breeding to be successful, we do not need to
know this. We simply choose the cows with the highest
milk yield – and the bulls with female relatives with high
milk yield – and breed them together, continuing to do
this over many generations.
Unlike natural selection, artificial selection (selective
breeding) often concentrates on just one or two
Figure 17.16: Maize plants in flower. characteristics. So, whereas natural selection tends to

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 17 Selection and evolution

result in a species that is well-adapted to its environment The graph in Figure 17.19 shows the large increase in
in many different ways, artificial selection risks milk yield that was produced in the selection line. The
producing varieties that show one characteristic to an results for the control line show that this increase must
extreme, while other characteristics are retained (or be due to genetic differences between the two groups,
even accidentally enhanced) that would be positively because environmental conditions were the same for
disadvantageous in a natural situation. both. It is interesting to see that the milk yield in the
control line actually went down. Why could this be?
An example of this is shown by a breeding experiment
Perhaps there is a selective disadvantage to having high
that was carried out with Holstein cattle in the USA
milk yields, so that the cows with lower milk yields were
(Figure 17.18). A large number of cows were used, and
more likely to have more offspring, all other factors
they were divided into two groups. In the first group,
being equal. Or perhaps this is just the result of random
only the cows that produced the highest milk yields
variation or genetic drift.
were allowed to breed, and they were fertilised with
sperm from bulls whose female relatives also produced 12 Key
high milk yields. This was called the ‘selection line’. population of cows selected for milk yield
The second group was a control, in which all the cows control population
were allowed to breed, and they were fertilised by bulls 10

Milk yield / 1000 kg

chosen more randomly. The selection was carried out in
each generation for 25 years. All the cattle were kept in
identical conditions and fed identical food. The results 8
are shown in Table 17.2 and Figure 17.19.

4
65 70 75 80 85 90
Year of birth

Figure 17.19: The results of selection for milk yield in

Holstein cattle, over a 25-year period.

The data in Table 17.2 support the hypothesis that very

high milk yields would be disadvantageous in a natural
situation. Health costs for every kind of ailment were
greater in the selection line than in the control line.
Figure 17.18: A Holstein cow. Her large udder shows that
Again, we do not know exactly why this is, but we can
she can produce large quantities of milk.
make informed suggestions. Mastitis is inflammation
of the udder, the organ in which milk is produced and

Health costs per year from

Selection line / USD Control line / USD
treatment for:
mastitis 43 16
ketosis and milk fever 22 12
reproductive issues 18 13
lameness 10 6
respiratory problems 4 1

Table 17.2: Health costs in the selection line and control line in Holstein cattle.

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stored. Very large quantities of milk in the udder could belong to the same species. Donkeys can interbreed with
make it more likely to become inflamed. Heavier udders organisms of another similar species, horses, to produce
could also put more strain on legs, so increasing the offspring called mules. However, mules are infertile; they
incidence of lameness. Perhaps, too, there are alleles that cannot breed and are effectively a ‘dead-end’. So, using
confer a greater likelihood of suffering these conditions, the definition above, donkeys and horses do not belong
and these were accidentally selected for along with the to the same species.
selection for high milk yields.
When a decision needs to be made as to whether two
organisms belong to the same species or to two different
species, the organisms should ideally be tested to find
17.6 Evolution out if they can interbreed successfully, producing fertile
offspring. However, as you can imagine, this is not
Evolution is the change of characteristics of species over always possible. Perhaps the organisms are dead; they
time, due to changes in allele frequency. It can lead to may even be museum specimens or fossils. Perhaps they
the formation of new species from pre-existing ones, as a are both of the same sex. Perhaps the biologist making
result of changes to the gene pools of populations over the decision does not have the time or the facilities to
many generations. attempt to interbreed them. Perhaps the organisms will
You have seen how natural selection, the founder not breed in captivity. Perhaps they are not organisms
effect and genetic drift can bring about changes in which reproduce sexually, but only asexually. Perhaps
allele frequencies in a population within a species. they are immature and not yet able to breed.
This section takes these arguments further, to see As a result of all of these problems, it is quite rare
how these changes could become so great that a new to test the ability of two organisms to interbreed.
species is produced. Biologists frequently rely only on morphological,
biochemical, physiological and behavioural differences
to decide whether two organisms belong to the same
Species and speciation species or to two different species. In practice, it may
be only morphological features which are considered,
It is not easy to define the term species. One definition
because physiological and biochemical ones, and to
of a species that is quite widely accepted by biologists
some extent behavioural ones, are more time-consuming
is a group of organisms, with similar morphological,
to investigate. Sometimes, however, detailed studies of
physiological, biochemical and behavioural features,
DNA sequences may be used to assess how similar two
which can interbreed to produce fertile offspring, and
organisms are to each other.
are reproductively isolated from other species.
It can be extremely difficult to decide when these
Morphological features are structural features, while
features are sufficiently similar or different to define two
physiological features are the way that the body works.
organisms as belonging to the same or different species.
Biochemical features include the sequences of bases in
DNA molecules and the sequences of amino acids in This leads to great uncertainty and disagreement about
proteins. whether to classify many slightly different varieties of
organisms together into one species, or whether to split
KEY WORDS them up into many different species. You can find more
discussion about the various ways of defining the term
evolution: a process leading to the formation of species in Chapter 18 (Section 18.1, Classification).
new species from pre-existing species over time
morphological: relating to structural features Genetic isolation
physiological: relating to metabolic and other Despite the problems described above, many biologists
processes in a living organism would agree that the feature which really decides
whether two organisms belong to different species is
their inability to interbreed successfully. In explaining
Thus, all donkeys look and behave like donkeys, and how natural selection can produce new species,
they can breed with other donkeys to produce more therefore, it is necessary to consider how a group of
donkeys, which themselves can interbreed. All donkeys

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 17 Selection and evolution

interbreeding organisms (that is, all of the same species)

can produce another group of organisms which cannot KEY WORDS
interbreed successfully with the first group. The two reproductive isolation: the inability of two
groups must undergo reproductive isolation from one groups of organisms to breed with one another;
another. This also means that they are genetically two populations of the same species may be
isolated – there is no interchange of genes between the geographically separated, or two different species
two groups because they can no longer interbreed. are unable to breed to produce fertile offspring
Reproductive and genetic isolation can take very
genetically isolated: no longer able to breed
different forms. Reasons for an inability to interbreed
together; there is no exchange of genes
successfully include:
• individuals not recognising one another as potential speciation: the production of new species
mates or not responding to mating behaviour geographical isolation: separation by a
• animals being physically unable to mate geographical barrier, such as a stretch of water or
a mountain range
• incompatibility of pollen and stigma in plants
• inability of a male gamete to fuse with a female
gamete They could not, of course, breed with the mainland
population, so all gene flow between the island and
• failure of cell division in the zygote mainland populations stopped. The island population
• non-viable offspring (offspring that soon die) was genetically isolated from its parent population.

• viable but sterile offspring. The selection pressures on the island were very different
from those on the mainland, resulting in different
So how can this happen? How can a species somehow alleles being selected for. Over time, the morphological,
become split into two groups that are no longer able to physiological and behavioural features of the island
interbreed with one another? In other words, how does population became so different from the mainland
speciation happen? The next sections describe how this population that the two populations could no longer
might occur in two types of situation: interbreed even if they were brought together. A new
• where a geographical barrier separates the species species had evolved.
into two groups
• where the reproductive isolation happens while the
species is still living in the same place.

Allopatric speciation
Geographical isolation has played a major role in the
evolution of many species. Many islands have their own
unique groups of species. The Hawaiian and Galapagos
islands, for example, are famous for their spectacular
arrays of species of all kinds of animals and plants
found nowhere else in the world (Figure 17.20).
Geographical isolation requires a barrier of some kind
to arise between two populations of the same species,
preventing them from mixing. This barrier might be
a stretch of water. You can imagine that a group of
organisms, perhaps a population of a species of bird,
somehow arrived on one of the Hawaiian islands from
mainland America. The birds might have been blown
off course by a storm. Here, separated by hundreds
of miles of ocean from the rest of their species on
mainland America, the group interbred with each other. Figure 17.20: Hibiscus clayi is found only on Hawaii.

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If the number of individuals that arrived on the island one or both of them, as they are both widely grown as
was small, then genetic drift and the founder effect, house plants (Figure 17.21).
as well as natural selection, could also contribute
to differences between the gene pools of this new The two species of palm look different from one another –
population and their parent population. that is, there are morphological differences between them.
H. forsteriana has many flower spikes and straight leaves,
This method of speciation, as a result of geographical
whereas H. belmoreana has only one flower spike and
separation, is called allopatric speciation.
curved leaves. The palms also grow on different soils on
the island. H. forsteriana tends to grow on calcareous
Sympatric speciation (alkaline) soil, while H. belmoreana grows on volcanic
Although allopatric speciation is probably the most soils, which are more acidic.
common way in which new species arise, there is The flowering times of the two species are also different
increasing evidence that it is also possible for speciation (Figure 17.22.) There is so little overlap between them
to happen without geographical isolation. This is called that it is almost impossible for one to be pollinated by
sympatric speciation. the other. The two species are reproductively isolated
from one another.
KEY WORDS Scientists have studied how this situation might have
allopatric speciation: the development of new arisen. There is evidence that the island was first
species following geographical isolation colonised by an ancestor of these two species of
palm about 5 million years ago, from Australia. This
sympatric speciation: the development of new species grew on neutral and acidic soils on the island.
species without any geographical separation However, at some point in time, some seeds germinated
on the more calcareous soils. The high pH of these
soils affects flowering time, making it occur earlier. So
For sympatric speciation to happen, something must these trees were unable to pollinate, or be pollinated
take place that splits a population into two groups, with by, the trees growing on volcanic soils. They became
no gene flow between them, while they are living in the genetically isolated. Over time, the different selection
same place. How might this happen? pressures imposed on them in their slightly new
Two species of palm trees, Howea forsteriana and H. environment resulted in differences in their morphology
belmoreana, provide an example. These palms are and physiology, so that they became better adapted to
endemic to Lord Howe Island. (‘Endemic’ means that growing in the calcareous soil.
they are found in no other place.) You may have seen

a b

Figure 17.21: a Howea forsteriana, the kentia or thatch palm; b H. belmoreana, the Belmore sentry palm.

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 17 Selection and evolution

0.30

0.25 Key
Howea forsteriana
Frequency of flowering

0.20 ovaries mature

anthers mature
Howea belmoreana
0.15 ovaries mature
anthers mature

0.10

0.05

0.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Weeks

Figure 17.22: Flowering times of Howea forsteriana and H. belmoreana.

This is an example of ecological separation. The two shorter jaws). Over time, the two groups of fish became
types of soil provide different ecological conditions, so different that today they no longer interbreed. They
which results in a difference in flowering time. As a have different courtship behaviours and will not mate
result, the plants growing on the two types of soil with each other.
became genetically isolated and developed into two
different species. This example shows a combination of behavioural
separation and ecological separation. The original
Sympatric speciation can also happen in animals. One
example is two species of cichlid fish, Amphilophus
citrinellus and A. zaliosus that live in Lake Apoyo KEY WORDS
in Nicaragua (Figure 17.23). It is thought that A.
ecological separation: the separation of two
citrinellus colonised this lake at least 10 000 years ago.
populations because they live in different
Within the lake, some individuals tended to feed on the
environments in the same area and so cannot
bottom, while others spent most of their time in the
breed together
open water. Scientists think that disruptive selection
may have occurred, with selection pressures resulting behavioural separation: the separation of
in advantages both for fish with features adapting them two populations because they have different
for bottom feeding (such as long jaws) and fish with behaviours which prevent them breeding together
features adapting them for feeding in mid-water (such as

a b

Amphilophus citrinellus Amphilophus zaliosus

Figure 17.23: a Amphilophus citrinellus and b A. zaliosus. A. zaliosus is thought to have split off from A. citrinellus as a result
of behavioural and ecological separation.

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species split apart because some fish had different are more similar than this, and they suggest that the
behaviour, tending to feed on the bottom of the lake Neanderthal and the modern human evolutionary lines
rather than in the open water. This also meant that split apart about 500 000 years ago.
they were living in different ecological conditions. This
Human mitochondrial DNA (mtDNA) is inherited
prevented gene flow between the two groups, resulting in
through the female line. A zygote contains the
the evolution of the two species that are present in the
mitochondria of the egg but not of the sperm. Since
lake today.
the mtDNA is circular and therefore cannot undergo
When thinking about how one species might split into any form of crossing over, changes in the nucleotide
two, it is important not to confuse the original factors sequence can arise only by mutation. Mitochondrial
that caused the separation with the factors that prevent DNA mutates faster than nuclear DNA, acquiring one
the two species from breeding after they have become mutation every 25 000 years.
genetically separated. These fish, for example, are now
Different human populations show differences in
prevented from interbreeding because they have different
mitochondrial DNA sequences. These provide evidence
courtship displays, so will not mate with one another.
for the origin of H. sapiens in Africa and for the
But this is not what caused the new species to form in
subsequent migrations of the species around the world.
the first place. This difference in mating behaviour has
These studies have led to the suggestion that all modern
arisen after the two groups became separated, as a result
humans, of whatever race, are descendants from one
of the genetic isolation brought about by the differences
woman, called Mitochondrial Eve, who lived in Africa
in feeding behaviour of two groups of fish in the original
between 150 000 and 200 000 years ago. This date is
A. citrinellus population that colonised the lake.
derived from the ‘molecular clock’ hypothesis, which
assumes a constant rate of mutation over time and that
the greater the number of differences in the sequence
17.7 Identifying of nucleotides, the longer ago those individuals shared
a common ancestor. The ‘clock’ can be calibrated by

evolutionary relationships comparing nucleotide sequences of species whose date

of speciation can be estimated from fossil evidence.
Molecular evidence from comparisons of the nucleotide Fossils can be dated by methods such as carbon dating,
sequences of DNA can be used to reveal similarities which can provide a good idea of how long ago the
between related species. organism lived.
DNA is mostly found in the nucleus, but there are also In Section 17.3, Genetic drift and the founder effect,
small circles of DNA in mitochondria and chloroplasts. you looked at how the founder effect and genetic drift
All of these sources of DNA can give information about could affect the phenotypes of lizards isolated on
the relationships between species. different islands on the Caribbean. Analysis of mtDNA
of the different species of anole lizards that are found
The more similar DNA nucleotide sequences of two throughout the Caribbean and the adjacent mainland
species are, the more closely related the species. In other provides evidence of their relationships (Figure 17.24).
words, two species with very similar DNA split apart Each island species of lizard is found only on one
more recently than two species with less similar DNA. island or a small group of islands. Table 17.3 shows the
Differences in the nucleotide sequences of DNA can be results of comparing part of the mtDNA of four of the
used to study the origin and spread of our own species, species. These results show that the three species Anolis
Homo sapiens. For example, humans share about 98% brunneus, A. smaragdinus and A. carolinensis are more
of our DNA with chimpanzees, suggesting that we are closely related to A. porcatus than they are to each other.
quite closely related to them in evolutionary terms. This suggests that these species have each originated
It is not possible to determine exactly how long ago from separate events in which a few individuals of A.
our common ancestor lived, but these DNA sequence porcatus spread from Cuba to three different places. The
differences suggest that it was a few million years ago. mtDNA analysis shows that allopatric speciation has
The DNA sequences of modern humans and DNA occurred.
extracted from the remains of Neanderthal people

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 17 Selection and evolution

A. brunneus
A. brunneus A. smaragdinus
A. smaragdinus 12.1 A. carolinensis
A. carolinensis 16.7 15.0 A. porcatus
A. porcatus 11.3 8.9 13.2

Table 17.3: The results of comparing part of the mitochondrial DNA of four of the species of anole lizards. The smaller the
number, the smaller the differences between the base sequences of the two species.

some very ‘primitive’ features, called the tuatara, which

now lives only in New Zealand (see Figure 17.26).
The other branch splits again, giving rise to the genus
U.S.A. 0 50 100 km
Florida
Heloderma, which contains species such as the Gila
monster (see Figure 17.27).

Anolis The remaining branches split again, producing the

carolinensis genera Agama (agama lizards), Chamaleo (chameleons),
Oplurus (which all live in Madagascar) and, of course,
Anolis.
Andros
Sphenodon
Anolis
brunneus

Acklins
Cuba Anolis
Anolis porcatus smaragdinus
Agama

Chamaleo

Figure 17.24: Distribution of different Anole lizard species Oplurus

in the Caribbean.
Anolis
We can use the results of DNA analysis, and a
comparison of base sequences in different species, to
draw a ‘family tree’ showing the relationships between
different species, genera or even larger classification
groups. The family tree in Figure 17.25 shows how Heloderma
Anolis lizards are thought to be related to five other
genera of lizards. Figure 17.25: Family tree showing the evolutionary
relationships of six general of lizards.
Work from left to right to think about the tree. Starting
at the left, the diagram indicates that all these six genera
had a common ancestor (whose name we do not know),
whose population then split to form two species. One
species gave rise to the genus Sphenodon – a lizard with

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 CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK

Figure 17.26: A tuatara lizard, one of two living species Figure 17.27: A Gila monster, Heloderma suspectum
belonging to the genus Sphenodon

REFLECTION
When describing natural selection, the term struggle for existence is often used.
• Using what you have learnt in this chapter, do you think this is a useful term or is it misleading?
• What did you learn about yourself as you worked on this question?

Final reflection
Discuss with a friend which, if any, parts of Chapter 17 you need to:
• read through again to make sure you really understand
• seek more guidance on, even after going over it again.

SUMMARY

Phenotypic variation may be continuous (as in the height or mass of an organism) or discontinuous (as in the
human ABO blood groups). The genotype of an organism gives it the potential to show a particular characteristic.
In many cases, the degree to which this characteristic is shown is also influenced by the organism’s environment.
Genetic variation within a population is the raw material on which natural selection can act.
Meiosis, random mating and the random fusion of gametes produce genetic variation within populations of
sexually reproducing organisms. Variation is also caused by the interaction of the environment with genetic
factors, but such environmentally induced variation is not passed on to an organism’s offspring. The only
source of new alleles is mutation.
All species of organisms have the reproductive potential to increase the sizes of their populations but, in the
long term, this rarely happens. This is because environmental factors come into play to limit population growth.
Such factors decrease the rate of reproduction or increase the rate of mortality so that many individuals die
before reaching reproductive age.

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