OCR A2 F215 VARIATION (PART 2)
Specification
a) Explain, with examples, how environmental factors can act as stabilising or
evolutionary forces of natural selection
b) Explain how genetic drift can cause large changes in small populations
c) Explain the role of isolating mechanisms in the evolution of new species, with
reference to ecological (geographic), seasonal (temporal) and reproductive
mechanisms
d) Explain the significance of the various concepts of the species, with reference
to the biological species concept and the phylogenetic (cladistic/evolutionary)
species concept
e) Compare and contrast natural selection and artificial selection
f) Describe how artificial selection has been used to produce the modern dairy
cow and to produce bread wheat
Stabilising and Evolutionary (Directional) Selection

Genetic variation is the basis of natural selection

Within a population, there will be a range of different alleles known as the
gene pool

Each individual in the population can have any combination of these alleles
producing variation

The alleles that confer a survival advantage will become more common in
the population. Disadvantageous alleles will become less common or even
disappear

If the environment remains fairly stable, the same advantageous alleles will
be selected for in successive generations. This is called a stabilising
selection

If the environment changes, there may be a change in the selection
pressures on the population. A selection pressure is an environmental factor
that confers greater chance of survival of some members of the population.
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
A variation that was not advantageous may begin to confer better survival
value than another, resulting in evolutionary/directional selection

Another example is the fur colour of wild rabbits. In UK habitats, the majority
of rabbits have agouti coloured fur, because this coat colour camouflages
them better in their environment. Agouti colouration is a stabilising selection
and the alleles responsible are maintained from generation to generation

If the climate changes and the ground is covered in snow for most of the year,
white coated rabbits would have a selective advantage. The frequency of the
alleles resulting in white fur would increase resulting in an
evolutionary/directional selection
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Genetic Drift

Genetic drift refers to changes in the frequency of alleles in a population that
have occurred by chance and not as a result of natural selection

Genetic drift is more likely to occur in small populations where selection
pressures are not strong

When two heterozygotes produce two offspring, the frequency of alleles is
likely to be very different from the parental frequency. In a small population,
these changes in allele frequency are more significant.

Genetic drift is thought to occur frequently on islands with small populations.
There is geographical separation of the individuals from other members of
their species on other islands

Genetic drift explains why rabbits on offshore islands around Britain are more
likely to have white or black coats

In 1775, in the Pingelap atoll in the western Pacific ocean, a storm and famine
reduced the population to 30 people. Today, there are 2000 inhabitants, all
descended from the 30 survivors. About 5% of them have an eye defect
caused by a recessive allele. This condition is extremely rare in other
populations
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Speciation and Isolating Mechanisms

The formation of a new species from a pre-existing one is called speciation

Members of the new species cannot interbreed with members of the preexisting species to produce fertile offspring

Speciation increases biodiversity
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Time Scale for Speciation

This often takes a long time to occur, over many generations, and is usually
studied by looking at populations existing today compared with those in the
past

However, bacteria reproduce at a very fast rate, producing several
generations in a few hours. Speciation can be identified much more easily in
these species
Speciation Occurs because of Reproductive Isolation

A sub-group of a population are separated from the rest of the population

Changes may occur in the two separate groups, because of different
mutations

In time, if the members of the original population are re-united, they can no
longer interbreed. Speciation has occurred with a new species originating
from the original species
Reproductive Barriers leading to Reproductive Isolation
1. Geographical Isolation leading to Allopatric Speciation

A population of a species may be subdivided into two population groups by a
geographical barrier such as a mountain or river

Another example of geographical isolation would be a wild species separated
from a domesticated species in the same genus. Grey wolves (Canis lupus)
avoid human settlements and domesticated dogs (Canis familiaris) are
confined by humans. This geographical isolation has resulted in speciation

The two groups will have different selection pressures acting on them and so
different adaptations will be selected for

Over time, the inherited features of both groups or only one group will change
(by mutations) and perhaps this will lead to members of the two populations
being unable to interbreed

This type of speciation is called Allopatric Speciation
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Allopatric speciation allowed the evolution of new species that Darwin studied in the
Galapagos Islands
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2. Sympatric Speciation

Sympatric speciation is the development of a new species from an existing
one, that does not involve geographical isolation of members of the
original species

Members of the original population remain in the same area

The isolating mechanisms involved include ecological, temporal and
reproductive
Ecological Barriers

Members of two sub-groups of a population may live in the same area but
rarely meet

An example is a fly in Canada and North America whose maggots feed on
hawthorn berries and apples. Adult flies that fed on apples tend to mate with
adults that also fed on apples. Those that fed on hawthorn berries tend to
mate with other feeding on hawthorn berries. Perhaps over time, these two
population sub-groups will not be able to interbreed and will form two distinct
species
Seasonal/Temporal Barriers

Two subgroups of a species may share the same habitat but fail to interbreed
because they are not reproductively active at the same time of day or do not
reproduce at the same time of year

With the grey wolf/domesticated dog example. Wolves breed once a year
whereas dogs breed all year round
Reproductive Barriers

Members of the same population may share the same habitat and reproduce
at the same time but they cannot breed successfully because of the following
reasons

Behavioural differences - members of the two subgroups have different
courtship behaviour such that they are not stimulated to mate with each other

Mechanical differences – individuals of a subgroup may be much smaller
than another or they have different shapes or sizes of genitalia

Gamete incompatibility – perhaps the gametes of the two subgroups have
different chromosome numbers
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What is a Species?
Definition of the Biological Species Concept
A biological species is a group of similar organisms that can interbreed and
produce fertile offspring
Problems with the biological species concept:

Difficult to classify organisms that do not reproduce sexually

Some members of the same species look very different from each other eg
males look very different from females of the same species – often seen in
some bird species

Cannot be used to classify extinct organisms that are only known as fossils,
old bones or skins
Definition of the Phylogenetic Species Concept
A phylogenetic species is a group of organisms that have similar morphology
(shape), physiology, biochemistry, embryology (stages of development) and
behaviour and occupy the same ecological niche.
A monophyletic group is one that includes an ancestral organism and all of its
descendent species. Grey wolves and domesticated dogs share a recent common
ancestor and form a monophyletic group
Closely related organisms have similar molecular structures for DNA, RNA and
proteins. Grey wolves and domesticated dogs have very similar DNA.
With improvements in the techniques of DNA sequencing, biologists are able to
compare the base pair sequencies of chromosomes. A particular base sequence is
called a haplotype. Differences between species, caused by base substitutions are
expressed as % divergence.
% divergence = (number of substitutions /number of base pairs analysed) x 100
Any group of organisms with haplotypes that are more similar to each other than to
those in any other group is called a clade. Hence, using molecular analysis is a
cladistic approach to classification.
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Cladistics
Cladistics is the hierarchial classification of species based on their evolutionary
ancestry
Cladistics is different from taxonomic classification systems since:

It focuses on evolutionary/phylogenetic relationships rather than on similarities
between species

Molecular analysis is very important, including DNA and RNA sequencing

It uses computer programmes and the nucleic sequencing data to generate
cladograms to represent evolutionary trees

It includes extinct and extant (existing) species

The taxons, kingdom, phylum and class (from Linnaean classification) are not
used. Since the evolutionary tree is very complex, a fixed number of levels for
classification is too simplistic

The cladistic approach has often confirmed the Linnaean approach but
organisms have been reclassified. It has helped biologists to understand the
evolutionary relationships between species
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Comparison of Natural and Artificial Selection
Definition or Natural Selection

Natural selection is the process by which particular members of a population
are selected for because they have favourable adaptations to their
environment.

The environmental factors that cause the selection of individuals are called
selection pressures.

The individuals that survive the selection pressures will reproduce to pass
on their favourable alleles that were selected for, to their offspring

Natural selection is the basis of evolution
Definition of Artificial Selection/Selective Breeding

Humans select the animals or plants that they want to breed from

The individual organisms selected have desirable characteristics that are
beneficial to humans
Historical Background of Selective Breeding

Humans have been selecting animals and plants for breeding, for thousands
of years, since humans started agriculture (around 10,000 BC)

Darwin recognised selective breeding as artificial selection, in contrast to
the natural selection processes that occur in wild type communities (without
man’s interference)
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
In selective breeding, man applies the selection pressure
The procedure of selective breeding involves careful selection and controlled
reproduction:

Select animals and plants with the desired characteristics

Allow the parental animals or plants to reproduce

Select the offspring with the best combination of characteristics and allow
these to reproduce

Continue this selection and reproduction regime for many generations to
exaggerate the desired characteristics

Detailed records are kept to prove the ancestry of individuals
before the chosen characteristic is expressed (seen)
Table of Comparisons of Artificial and Natural Selection
Natural selection
Artificial selection
Agent of selection
Environment
Humans
Effect on allele
frequencies
Alters it
Alters it
Effect on evolution
Contributes to evolution of
the species
Contributes to evolution of
the species
Speed
Slow
Fast
Genetic diversity
High
Low
Inbreeding or
outbreeding
Outbreeding is common
leading to hybrid vigour
Inbreeding is common
leading to loss of vigour in
offspring
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Use of Artificial Selection to produce Bread Wheat (Triticum aestivum)

Wheat is grown to harvest the grains of wheat (seeds) so that flour can be
produced

Modern bread wheat is Triticum aestivum

The genus Triticum includes wild and domestic species of wheat

The genus Aegilops (wild goat grass) has contributed its genome to Triticum
aestivum

Most wild species of wheat are diploid with 14 chromosomes (n=7)

Grasses, like wheat, are able to undergo polyploidy – their nuclei can contain
more than one set of diploid chromosomes
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
Modern bread wheat is hexaploid (6n), having (6x7) 42 chromosomes in the
nucleus of each cell

Hexaploid cells are larger than diploid cells and the seeds therefore,
contain more nutrients for making flour. Growing hexaploid wheat varieties
produces a higher flour yield
T.aestivum is a hybrid containing three distinct genomes. The letters used below
represent different sets of chromosomes (not alleles):

The genome AuAu from a wild wheat species, referred to as einkorn

The genome BB has come from wild emmer wheat

The sterile hybrid offspring is made fertile by a mutation technique that
doubles the chromosome number, producing a tetraploid species

The genome DD has come from a wild goat grass
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Current Improvements of Triticum aestivum
Breeders continue to carry out selection programmes to improve bread wheat such
as:

Resistance to fungal infections

Increased protein content of wheat seeds (required for bread flour)

Increased flour yield

Resistance to logging (stems bending over in the wind and rain)
Artificial Selection to produce the modern Dairy Cow

Cattle have been domesticated for several thousand years

The main dairy cow bred for milk production Holstein-Friesian and Ayrshire

The original wild cattle that were first domesticated are thought to have looked
like modern Chillingham White cattle

High milk yielding cows were selected and bred over many generations
Breeding Programme

Cows with high milk yields were chosen to breed. The bulls chosen to breed
with them, were known to produce female calves that grew up to have high
milk yields

The offspring calves (F1 generation) were selected for further breeding on the
basis of their high milk yields. Again the cows were bred with the prize bulls

Offspring calves in the F2 generation were selected on the basis of high milk
yields and breeding continued for further generations

Note that it is not necessary to allow the cow and bull to mate. Semen can be
collected from prize bulls and frozen for long periods of time. Cows are
artificially inseminated with the chosen semen

When breeding animals, the health and welfare of the animals is important.
Cows that produce more milk have larger udders that are more prone to
mastitis (udder infection). The cows are also more likely to suffer lameness.
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Phenotypic traits selected for in Dairy Cows

Production of high volumes of milk

Long lactation periods (time that milk is produced for)

High milk quality

Large udders/correct udder shape for the milking machine

Resistance to disease such as mastitis

Calm temperament

Converts food to milk efficiently
Current Practices in Artificial Selection

Each cow’s milk yield is measured and recorded

The progeny of bulls is monitored – those that have fathered high milk yielding
daughters are selected. Their semen is collected and used for artificial
insemination
Current Practices involving Reproductive Technology

Some elite cows are hormone treated so that they produce many eggs

The eggs are fertilized in vitro (IVF)

The embryos are implanted into surrogate mothers

The embryos could also be cloned and divided into several embryos, to
increase the number of elite cows

Genetic screening using gene probes to identify desired genes. Once the
offspring are produced from the chosen parents, the offspring DNA is checked
for the marker. This allows selection at an early age

Sex selection techniques – screening for X and Y sperm to prevent production
of male offspring
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