Biology SME

Key unit competence
Explain variation and mutation as a source of biodiversity, the role of artificial
and natural selection in the production of varieties of animals and plants with
increased economic importance

Introductory activity 5

Human population is classified as a single species ”homo sapiens” .Small
group of individuals with different skin colour do not look the same ,here is
a representation of human population observe it carefully then answer to the
questions below..

a) Some individuals in the figure above look like they are an intermediate
of other skin colour, from your observation is there any cause of this?
b) All of us we are human being but we do not look the same, why?

5.1 Variation
All living organisms on the earth are unique, individuals of different species are
easy to differentiate and even those of the same species present differences
(morphological, physiological, cytological and behaviouristic). Such differences
among individuals of the same species are referred to as “Variation”. These
differences between cells, individual organisms, or groups of organisms of any
species are caused either by genetic differences (genotypic variation) or by
the effect of environmental factors on the expression of the genetic potentials
(phenotypic variation). So, organisms that have helpful variations tend to survive
better, and reproduce more. As they reproduce, their genes (including the
helpful genes) become more common in the gene pool, and these variations
spread out more and more.

5.1.1 Genetic variation

Activity 5.1.1

A mutation in one gene causes the shell of the Japanese land snail (Euhadra)

to spiral in the opposite direction from others. Snails with opposite spirals
cannot mate, resulting in reproductive isolation

Using the knowledge acquired in genetics, what types of variation is indicated
by the Japanese Land snail?

Genetic variation result from the differences in DNA sequences of individuals
(gene make up), those variations can be inherited by the transfer of genes.

There are three primary sources of genetic variation:
• Variation from mutations are changes in the DNA. A single mutation
can have a large effect, but in many cases, evolutionary change is based
on the accumulation of many mutations. Ex, base substitution (Glu →Val),
deletion and insertion
• Variation from gene flow/gene migration/allele flow is any
movement of genes from one population to another and is an important
source of genetic variation. Ex, a bee carrying pollen from one flower
population to another
• Variation from recombination/ reproduction can introduce new
gene combinations into a population. This genetic shuffling/ genetic
recombination (meiosis & crossing over) is another important source of
genetic variation. At meiosis, the process that generates a haploid product
of meiosis whose genotype is different from either of the two haploid
genotypes that constituted the meiotic diploid. The creation of genetic
variation by recombination can be a much faster process than its creation
by mutation. For example, when just two chromosomes with “normal”
survival, taken from a natural population of Drosophila, are allowed to
recombine for a single generation, they produce an array of chromosomes
with 25 to 75 percent as much genetic variation in survival as was present
in the entire natural population from which the parent chromosomes were
sampled. This outcome is simply a consequence of the very large number
of different recombinant chromosomes that can be produced even if we
take into account only single crossovers.

• Why is genetic variation important for evolution?
Variation is one of the main things that drive evolution. First, there are limited
resources available, and there is just not enough; food, water, shelter, etc.
available for all organisms. Second, to make matters worse, most species have
many offspring that can possibly survive. Just think of how many insect eggs are
laid compared to the number that make it to adulthood. This leads to competition
for the limited resources.

Not all individuals in a species are the same. There are variations in; size, speed,
coloration, etc. These small variations can help or hinder individuals in their
survival. These variations are caused by small differences in genes. Organisms
that have helpful variations are more likely to survive. On average, they
get more food, get better shelter, etc. Coloration can help a predator get closer
to prey and eat better. Or, for the prey species, coloration can make it harder for
predators to find and eat it. So, organisms that have helpful variations tend to
survive better, and reproduce more. As they reproduce, their genes (including
the helpful genes) become more common in the gene pool, and these variations
spread out more and more.

Variation can be influenced by numerous factors including:

i. Independent assortment of chromosomes.
Due to the law of independent assortment, traits are transmitted from parents to
offspring independently of one another.
This occurs at the time of gamete formation. At the time of gamete formation
during meiosis, the parental chromosomes separate randomly hence forming
different gametes with different chromosomes. This independent assortment
gives a wide variety of different gametes and hence individuals.

ii. Crossing over
Crossing over allows the alleles on DNA molecules to change positions from
one homologous chromosome segment to another in other word is the transfer
or exchanges of genetic material from one homologous chromosome to another
during gamete formation known as meiosis. The new formed chromosomes are
known as recombinant chromosomes; Genetic recombination is responsible for
genetic diversity in species or population.

iii. Random mating
Random mating involves individuals pairing by chance, not according to their
genotypes or phenotypes. Random mating is a source of variation in a population.
For example, a population in which mating only occur between organisms of
similar phenotypes as red beetles mating with red beetles and yellow beetles
mating with yellow beetles, will tend to show less variation than a population
where crosses are random.

iv. Random fertilization of gametes
Random fertilization means that the collection of genes within one gametes,
each gametes contain a unique set of gene combination, and the ova is fertilized
randomly by the male gamete as a result each zygote is unique hence the
variation among individuals.

v. Mutations
Mutation is a random change in the sequence of DNA, either due to errors
during DNA replication or by the influence of environmental factors. Mutations in
gametes cell can be inherited while somatic mutations are not transmitted from
generation to generation (not inherited).

vi. Environmental factors
These variations caused by environmental factors are not inherited, environmental
variation are not prominent in animals as in plants, and this is due to the
environmental effect on the meristems of various parts. Some environmental
factors that can induce variation include, availability of food, light intensity,
Temperature, water, minerals etc…

Application activity 5.1.1

1) Which of the following give rise to genetic variation in a population?
a) Crossing over and independent assortment in meiosis
b) Different environmental conditions
c) Random mating and fertilization.
d) Mutation.
i) a, b, c and d
ii) a, b and c only
iii) a, c and d only
iv) b, c and d only
2) Variation caused by environmental factors are not inherited. Why?
3) What is random mating?

5.1.2 Phenotypic variation

Activity 5.1.2

1) Observe the following figures of students and make analysis on
their size (weight and height). These students live together in the
same school which means that the type of food they consume is
the same.

Figure of students

a) Write down your observation
b) Try to form 3 groups according to their height
c) By looking on their size can you try to make 3 groups according to
their weight?
d) By considering weight and height, why are not the same to those
soldiers

Phenotypic variations can be brought about by genes or environmental
factors or a combination of both genes and environment they are not inherited.
So, there are characteristics that are not inherited but influenced by the
environmental factors, a child that get insufficient food will not grow to the size
expected, a cat with a skin disease may have bald patches in its coat. Those
conditions are not inherited. Such Phenotypic variations can be divided into two
types such as continuous variations or quantitative and Discontinuous variations
or Qualitative variations.

a) Continuous variation
Continuous variation is variation which does not show clear cut differences i.e. it
shows a gradual change from one extreme to another. Characteristics such as;
human height and weight show continuous variation, and are usually determined
by a large number of genes (i.e. polygenic) and/ or considerable environmental
influence. Some examples of continuous variation are: Height, weight, heart
rate, finger length, and leaf length. They are also called fluctuating variations

In continuous variations/quantitative:
• Different alleles at a single gene locus have a small effect on the phenotype
• Different genes have the same, often additive, effect on the phenotype
• A large number of genes may have a combined effect on a particular
phenotypic trait, these genes are known as polygenes

A typical example of continuous variation is height. There are no distinct
categories of height; people are not either tall or short. There are all possible
intermediates between very short and very tall (Figure 5.1).

Continuously variable characteristic is greatly influenced by environment. A
person may inherit tallness trait and yet not get enough food to grow tall. A plant
may have a gene for large fruits but not get enough water, minerals and sunlight
to grow large fruits.

b) Discontinuous variation.
Discontinuous variation is indicated as a variation where there is a clear difference
among individuals there is no intermediates, in human you are male or female
apart from abnormalities, Sex are inherited in a discontinuous way, some people
are able to roll they tongue in a tube other can’t do it.

There are many characteristics that are difficult to classify as continuous or
non-discontinuous such as human eye colour people can be classified roughly
as having blue eyes or brown eyes, but there are also categories described as
grey, Hazel or green.

A typical example of discontinuous variation is human blood group; discontinuous
variations is controlled by a single pair of alleles or small number of genes.

A person is one of four blood group: A, B, AB and O there is no blood group
between

The major distinctions between continuous and discontinuous variations in
inheritance are as follows:

Continuous variations have the following characteristics:
– The variations fluctuate around an average or mean of species.
– Direction of continuous variations is predictable.
– They are already present in the population.

– Continuous variations are formed due to chance segregation of
chromosomes during gamete formation, crossing over and chance
pairing during fertilization.
– They can increase adaptability of the race but cannot form new species.
– Continuous variations are connected with the mean or average of the
species by intermediate stages.
– The continuous variations are also called fluctuations.
– When represented graphically, continuous variations give a smooth bell
shaped curve - They are very common
– Continuous variations do not disturb the genetic system.

Discontinuous variations have the following characteristics:
– A mean or average is absent in discontinuous variations.
– The direction of discontinuous variations is unpredictable.
– Discontinuous variations are new variations though similar variations
might have occurred previously.
– Discontinuous variations are produced by changes in genome or genes.
– Discontinuous variations are the fountain head of continuous variations
as well as evolution
– These variations are not connected with the parental type by intermediate
stages.
– Discontinuous variations are also known as mutations or sports.
– A curve is not produced when discontinuous variations are represented
graphically.
– These variations appear occasionally.
– They disturb the genetic system of the organism
Table 5.1. Comparison between Discontinuous and continuous variation

Application activity 5.1.2

The histogram shows the height of wheat plants in an experiment plot.

Based on the figure
a) Which type of variation is shown by the height of each of the strains of
wheat plants
b)Give other example of discontinuous variation

5.2 Natural selection
Natural selection is a process that results in the adaptation of an organism to
its environments by means of selectively reproducing changes in its genotype
or genetic constitution. In 1858, Charles Darwin and Alfred Russel Wallace
published a theory of evolution by natural selection.

Individuals with certain variants of the trait may survive and are capable to
reproduce more than less successful individuals with unfavorable characters;
therefore, the population evolves. Over time, this process can result in
populations that specialize for particular ecological niches (microevolution) and
may eventually result in speciation (the emergence of new species also known
as macroevolution). In other words, natural selection is a key process to change
organisms and make them suitable to different environment.

The allele responsible for the variation that help individuals to survive better is
inherited by the offspring, they will survive and transmit the trait to its offspring,
in time this particular variety outnumber and finally replace the original variety.

This is known as “the survival of the fittest” but this doesn’t indicate good health
to an organism but the one which is well fitted to the conditions of environment.

Factors of natural selection

Activity.5.2.1

The puffball can produce billions of offspring, if all offspring produced survived
to maturity they would carpet the surrounding land surface.

For your observation, how can the factor of producing a high number of
offspring for an organism have an impact on surviving in the environment?

Natural selection which is one of the evolution means is due to several factors in
a population including over production and environmental factors.

Role of over production and variation in natural selection

Over production is the production of more offspring that can be supported
by the available resources (food, light, space…). Individuals possessing genes
that help them to survive in an environment and this trait is transmitted from
generation to generations.

Darwin appreciated that all species have the potential to increase their numbers
exponentially, he realized that, in nature, population rarely, if ever, increased
in size at such rate. The reason why reproductive rate is high is because an
individual cannot control the climate, availability of food, rate of predation etc.
Therefore, the production of sufficient offspring ensures a sufficient large
population surviving during hash conditions.

Application 5.2.1

1) In the natural selection species that is able to produce a high number
of offspring ensure the possibility of great number offspring to survive.
How do we call such process?
2) How does over production lead to competition?

Environmental factors

Activity 5.2.2

The figure above represents plants into different environments, observe them
carefully and
a) Write down your observations according their environments and their
structures.
b) Mention your differential observation of the plant (a), (b), (C) according
to their environment.

Environmental factors as forces of natural selection, Environment is a responsible
agent of natural selection. Thus, it selects and determines individuals in different
ways according to different types of natural selections.

Selection pressure
Selection pressure are environmental factors that limit the population of species
it is also known as environmental resistance.
It includes:

• Availability of resources:
– Competition for food
– Competition for a space in which to live, breed and rear young
– Competition for mating etc.

• Environmental conditions:
– Need for light, water, oxygen,
– Climate changes temperature, whether conditions or geographical
    access

• Biological factors: Predators and diseases(pathogens)
Selection pressure can be density dependent or independent density factor,
and it extent varies from time to time and place to place.
The selection is of three main types:
• Stabilizing selection
• Direction selection
• Disruptive selection

a) Stabilizing selection
Stabilizing selection is a type of natural selection in which a population mean
stabilizes on a particular non-extreme trait value as result of genetic diversity
decreases as illustrated in the figure below.

As illustrated in the above figure (Fig. 5.3), in stabilizing selection, natural selection
favors the individuals in the population with the intermediate phenotypes. These
individuals have greater survival and reproductive success. Individuals with
extreme phenotypes are less adaptive and are therefore eliminated.

b) Directional selection
Directional selection is a mode of natural selection in which a single or new
fit phenotype is favored when exposed to environmental changes, causing a
population genetic variance or allele frequency to continuously shift in one
direction or one end of the spectrum of existing variation.

c) Disruptive or diversifying selection
In disruptive selection, both the extreme phenotypes in the population are
selected and become more prevalent. The individuals with extreme phenotypes
or end- phenotypic spectrum have greater survival and reproductive success.
The disruptive selection pressure increases the chances of the advantageous
alleles to be passed on to the next generation. By disruptive selection, the
intermediate phenotype is selected against and gradually decreases in number
from generation to generation, and may become extinct.

From the above figure, disruptive selection many generations may cause the
formation of two separate gene pools and the formation of new species.
From the factors of natural selection some examples that indicate natural
selection today are known and include:
• Antibiotic resistance,
• industrial melanism
• pesticides resistance in insect and mammals

Application activity 5.2.2

1) Classify the following figures according to the types of natural selection

Key: Blue line indicates a given population after natural selection while red
line indicates a given population before natural selection.
2) Give 2 examples of natural selection due to environment?

A. Antibiotic resistance in bacteria

Activity 5.2.2.A

Those Petri dishes contain gel that are nutrient for the growth of bacteria, the
red gel in each of these petri dishes has been inoculated with bacteria. The
small blue circles are discs impregnated with antibiotics but some bacteria
can grow around those antibiotics that are supposed to kill them.

From the observation above what can you say on those bacteria that are able
to grow in the presence of Antibiotics?

Antibiotic resistance occurs when an antibiotic has lost its ability to effectively
control or kill bacterial growth; in other words, the bacteria are “resistant” and
continue to multiply in the presence of therapeutic levels of an antibiotic.

When the antibiotic is used the bacteria can develop the ability to defeat the
drugs designed to kill them. These bacteria survive to this antibiotic continue to
live and produce offspring that are resistant to this antibiotic.

Antibiotic resistance is a natural process even though a number of bacteria drug
resistance is attributed to human being by overuse and misuse of antibiotics. In
some countries and over the Internet, antibiotics can be purchased without a
doctor’s prescription. Patients sometimes take antibiotics unnecessarily, to treat
viral illnesses like the common cold.

How do bacteria become resistant?

Some bacteria are naturally resistant to certain types of antibiotics. However,
bacteria may also become resistant in two ways: by a genetic mutation or by
acquiring resistance from another bacterium.

How does antibiotic resistance spread?

Genetically, antibiotic resistance spreads through bacteria populations both
“vertically,” when new generations inherit antibiotic resistance genes, and
“horizontally,” when bacteria share or exchange sections of genetic material
with other bacteria. Horizontal gene transfer can even occur between different
bacterial species. Environmentally, antibiotic resistance spreads as bacteria
themselves move from place to place; bacteria can travel via air, water and wind.

People can pass the resistant bacteria to others; for example, by coughing
or contact with unwashed hands.

Can bacteria lose their antibiotic resistance?

Yes, antibiotic resistance traits can be lost, but this reverse process occurs more
slowly. If the selective pressure that is applied by the presence of an antibiotic
is removed, the bacterial population can potentially revert to a population of
bacteria that responds to antibiotics.

Application activity 5.2.2.A

1) Compare the two diagram below and differentiate them in the two
categories of resistance transmission in bacteria.

B. Pesticides resistance in insect and mammals

Activity 5.2.2.B

Discuss and explain the pesticides resistance in insect and mammals and
present your findings.

Pesticide resistance means a decreased ability of pesticides to kill pest, Pest
species evolve pesticide resistance via natural selection and it can be passed
from one generation to the other trough reproduction.

a) Pesticide resistance in insect
The intensive use of insecticide and genetics are the fundamental factors of
insecticide resistance. By natural selection insect with the genes that confer
the resistance to the insect survive and transmit the trait to the next generation,
most of pest species including insect produce large broods which increase the
probability of mutations and ensures large resistant populations.

Resistance can be for a single insecticide, but it is more common that insect

resistance can be developed to the pesticide with the same mode of action.

This is known as cross resistance while multiple resistance is when insects
resist to two or more pesticides. In addition to resistance there is Tolerance, it
is not a result of selection pressure but a natural tendency, for example mature
caterpillar are tolerant to many insecticides than their young one.

b) Pesticide resistance in mammals
Resistant weed species have now been reported and about 10 species of
small mammals and plants attacking nematodes are known to be resistant.
Resistance in mammals has affected the control of rat populations. A chemical
called warfarin has been used to kill rats since the 1950s.It is given to rats in the
form of food baited with the chemical. Once ingested, warfarin prevents blood
clotting, causing hemorrhaging and death of the rats. A warfarin –resistant
allele arose in the rat population that allow them to survive when warfarin was
ingested.

Application 5.2.2.B

Answer by true or false
1) Due to the development of exoskeleton in some insect like caterpillar
they become more susceptible to pesticides than their young ones?
2) In our house there is a lot of mice, we use to kill them using ‘sumu ya
Panya”but it can’t kill them nowadays, they are resistant to it.
3) To fight against malaria we can use a mosquito net which kills
Anopheles, Anopheles do not resist on it.

C. Industrial melanism

Activity 5.2.2.C

1) Observe the following diagram of moth and answer to the question
below

a) Write down your observation about the colour of moth and its back
      ground.
b) Why those moth do not have the same colour?

Industrial melanism is an effect of evolution prominent in several arthropods
where melanism has evolved in an environment where the air is greatly affected
by Sulphur dioxide and dark soot deposits. Darker pigmented individuals are
better fitted into those polluted environments which favors their camouflage.
Other explanation links this industrial melanism with immunization (strengthening
of immunity), absorption of heat at high rate in reduced sunlight and ability to
excrete trace element into melanic scales and feathers.

Industrial melanism can be observed in more than 70 species of Lepidoptera
(butterflies and moths).

Application 5.2.2.C

In the light forest, after the end of the simulation 76% light moths and 24%
dark moths were observed. Relate this to the industrial melanism and find an
explanation to those number.

5.3 Artificial selection

Activity 5.3

1. The original Rwandan cattle (a) from which individuals were first
domesticated use to have long horns, but now days many cattle (b) do
not look the same as the original one.

With the daily life experience and the observation on the figures
above, why does the number of traditional cows (a) are decreasing in
Rwandan societies.

Artificial selection or selective breeding involves the selection of trait of interest
done by human being not environment and use them as the parent of the
next generation Artificial selection has been practiced by humans for several
centuries. It has played an important role in the evolution of modern crop plants,
farm animals and domestic pets from the wild ancestors. This led to a population
with desired traits. Artificial selection is used by humans to produce varieties of

animals and plants that have an increased economic importance. It is considered
as a safe way of developing new strains of organisms, compared with genetic
engineering, and is a much faster process than natural selection. However,
artificial selection removes variation from a population, leaving it susceptible
to disease and unable to cope with changes in environmental conditions.
Potentially, therefore, artificial selection puts a species at risk of extinction.

Comparison between natural selection and artificial selection

Two methods of carrying out selective breeding are known as in breeding and
out breeding

Application 5.3

1) At break time one of the student who come from Mamba sector in
Gisagara district told us about the agriculture activity, she said that
before sowing ground nuts their parents select seeds that look good.
I was asking myself why they can’t saw any seed.
a) From your knowledge is this selection, artificial or natural selection
b) Give other examples of artificial selection that you know

Methods of Artificial selection

Activity 5.3.1

1. Carefully observe the following diagram and write down what are your
observation of the activity that take place.

a) Inbreeding
In breeding methods, is a method of artificial selection in which there is a
breeding of closely relative’s individuals with a desired trait, in this case the
chances to obtain offspring showing the desired characteristics are greater.
This characteristic of interest is retained as far as possible and the origin of the
desirable trait is spontaneous mutation. The inbreeding presents some negative
consequences like the loss of vigour, with the population being weakened by
lack of diversity, the increase of expression of recessive allele that is why there is
a need of introducing new genes from outside to make the population healthier
and stronger.

b) Outbreeding
This method of artificial selection involves crossing unrelated individuals showing
two different characteristics in order to obtain an individual which combine both
characteristics for example by crossing a crop plant that gives an excellent yield
with the one which resist to disease in the expectation of a plant with a high
yield and disease resistance. It frequently produces tougher individuals with a
better chance of survival. This is called hybrid vigour

Selective breeding in cattle: Nowadays milk dairy industry are interested in
modern-day cattle for milk production and farmers involved in it use selective
breeding by artificial insemination.

In the selection farmers follows some factors including:
• Volume of milk produced each day
• Length of milking(lactation) period
• Protein and fat content of milk

• Disease resistance,

The selective breeding process
• Selecting a suitable cow and bull by consulting the pedigree records of
each and trough progeny testing.
• Collection of sperm from the selected bull and storing them by freezing
• Detection of when the cow is in oestrus by observing changes in her
behavior, e.g. Increase restlessness, feeding less.
• Artificially inseminating the defrosted semen into the cow.
• Checking that fertilization has occurred and the calf in cow.
Both artificial insemination and embryo transplantation can be used.

Application 5.3.1

Before leading the following passage observe carefully the fig of animals
above.

The hybrid offspring of donkey (I) and horse (J) is a mule (k), which is robust
but sterile, animals can be crossed according to the characteristic needed in
artificial selection.
a) Which type of artificial selection indicated above?
b) What can you do, to prevent the sterility of your hybrid(mule)

Skills lab

This exercise illustrates the effect of natural selection on populations of
predators and prey. Students-teacher, in groups of four, will represent
predators, each with a different adaptation for capturing their prey. The prey
will consist of different species represented by different colored beans.

Procedure.
1) Each team of 4 students will count out exactly 100 dried beans of each
color.
2) Thoroughly mix the beans and spread them evenly over your’habitat.’your
habitat depends on the weather.
i) If the weather is poor, it is dark outside, or your instructor would
rather, your habitat will be a tray of sediment in the classroom.
ii) If the weather is lovely,or your instructor is adventurous,you will do
this about lab outside.Each team will mark off 1m*1m ‘’habitat’’ in
the grass using yarn, a meter stick,and wood stakes.
iii) All ‘prey’ are confined to the habitat,wherever it is!
3) Each student (predotor) will have a different feeding apparatus:A
fork,spoon,Knife or forceps.
4) When everyone is ready, predators will spend 60 seconds capturing
prey with their devices and depositing them into a cup while obeying
the following rules:
i) Predators must only use their capture device to capture prey
ii) Predators may not scoop prey up with their cup.
iii) If predators ‘eat’ too much of the environment, they will become
constipated and die.

5) Each predator determines the number of prey captured and records
results in data
Sheet: Generation1.

4) Calculate and fill the statistics on the data sheet (see example
below).
Data sheet: Generation1

End unit assessment 5

1) Suggest how each of the following might decrease the chances of
     an antibiotic resistant strain of bacteria developing
a) Limiting the use of antibiotics to cases where there is a real need
b) Regularly changing the type of antibiotic that is prescribed for a
     particular disease
c) Using two or more antibiotics together to treat a bacterial infection.
2) Differentiate between natural selection from artificial selection
3) Some individuals of the swallowtail butterfly scientifically known as
Papilio machaon of the family papilionidae pupate on brown stems
or leaves; others pupate on green stems or leaves. Two distinct
colour forms of the pupae are found, namely brown and green, with
very few intermediates.
a) What type of natural selection does this example show?
b) Explain why the intermediate colour formed would be at selective
disadvantage.
4) Copy and complete the table to compare artificial selection with
natural selection

Distinguish inbreeding and outbreeding
6) Explain why artificial breeding is beneficial to man?