UNIT 16 NATURAL AND ARTIFICIAL SELECTION
UNIT 16: NATURAL AND ARTIFICIAL SELECTION
Key Unit Competence
Explain the role of artificial and natural selection in the production of varieties of
animals and plants with increased economic importance
Learning objectives
By the end of this unit, I should be able to:
– Explain that natural selection occurs as populations have the capacity to
produce many offspring that compete for resources.
– Explain, with examples, how environmental factors can act as either stabilising,
disruptive and directional forces of natural selection.
– Explain how selection, the founder effect and genetic drift may affect allele
frequencies in populations.
– Explain how a change in allele frequency in a population can be used to
measure evolution.
– Describe how selective breeding (artificial selection) has been use to improve
the milk yield of dairy cattle.
– Outline the following examples of crop improvement by selective breeding:
– The introduction of disease resistant varieties of wheat, tomatoes, Irish
potatoes, and rice.
– Inbreeding and hybridization to produce vigorous, uniform varieties of maize
– Interpret graphs on how fur length affects the number of individuals at
different temperatures.
– Use the Hardy-Weinberg principle to calculate allele, genotype and phenotype
frequencies in populations.
– Differentiate between natural and artificial selection.
– Appreciate that the environment has considerable influence on the expression
of features that show continuous (or Quantitative) variation.
– Appreciate the importance of selective breeding (artificial selection) to
improve features in ornamental plants, crop plants, domesticated animals andlivestock.
Introductory activity
Some species of plants and animals such as Dinosaurs no longer exist today
but it is common to find new species such as lemon-orange. According to you,
what would be the cause of some species to disappear and some new speciesto appear?
16.1 Natural selection
Activity 16.1
1. Observe the graphs below, analyse and interpret them and then deducedifferent types of natural selection.
Key: Blue line indicates a given population after natural selection while Red
line indicates a given population before natural selection
16.1.1 Natural selection as a means of evolution as well as capacity
to survive and reproduce
Throughout the lives of the individuals, their genomes interact with their
environments to cause variations in traits from genotypic to phenotypic variations
among the individuals in a population because of differences in their genes.
Individuals with certain variants of the trait may survive and are capable to
reproduce more than less successful individuals with unfavourable characters;
therefore, the population evolves. Over time, this process can result in populations
that specialise 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 variants that are best adapted to their natural environment such as abiotic
conditions, predation, competition to food, space, light, water and resistance
against diseases will be selected for survival and can reproduce. By reproduction,
organisms transmit their physical traits contained within their genes or alleles to
their next generation. The individuals that best suited or fitted to the stated before
environmental conditions will have the best chance to survive and produce fertile
offspring due to characteristic features or favourable characteristics that give
them an advantage in the struggle for existence being intraspecific or interspecific
competition. However, those with unfavourable characteristics are more likely to
die due to lack of resources or not having access to resources. The high or birth
rate gives a selective advantage whereas high mortality or death rate gives them aselective disadvantage.
As environmental conditions gradually change, certain characteristics within a
population also gradually change; thus, randomly varying population are favoured,
and natural selection occurs. This is known as the survival of the fittest. The fittest in
evolution is defined as the ability of an organism to pass on its alleles to subsequentgenerations, compared with other individuals of the same species.
16.1.2 Types of natural selection
As it has been mentioned, environment is a responsible agent of natural selection.
Thus, it selects and determines individuals in different ways according to different
types of natural selections. Those natural selections are stabilizing selection,
directional selection, and disruptive selection among other.
a. Stabilising selection
Stabilising selection is a type of natural selection in which a population mean
stabilises on a particular non-extreme trait value as result of genetic diversitydecreases as illustrated in the figure below.
Figure 16.1.a: Illustration of stabilising selection
As illustrated in the above figure, in stabilizing selection, natural selection favours
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. An example is the newlyborn
human babies who are under 2.27 Kg or over 4.54 kg are less likely to survive
than babies weighing between 2.27 and 4.54 kg. Despite of this, with advances
in medical science, the survival chances of newly-born underweight or overweight
babies have now been improved.
These individuals with extreme phenotypes have greater survival and reproductive
success.
b. Directional selection
Directional selection is a mode of natural selection in which a single or new
fit phenotype is favoured when exposed to environmental changes, causing a
population genetic variance or allele frequency to continuously shift in one directionor one end of the spectrum of existing variation.
Figure 16.1.b: Illustration of directional selection
A classical description of directional selection has been identified in eighteenth and
nineteenth century in England as illustrated in the figure 16.1.b above. Prior to the
industrial revolution, the moths were predominately light in colour, which allowed
them to blend in with the light-coloured trees and lichens in their environment. As
soot/black powder began spewing from factories, the trees darkened and the lightcoloured
moths became easier for predatory birds to spot.
Over time, the frequency of the melanic form of the moth increased because their
darker coloration provided camouflage against the sooty tree; they had a higher
survival rate in habitats affected by air pollution. The result of this type of selection
is a shift in the population’s genetic variance towards the new and fit phenotype.
These individuals with extreme phenotypes have greater survival and reproductivesuccess.
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 endphenotypic
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.
Figure 16.1.c: Illustration of disruptive selection
From the above figure, disruptive selection many generations may cause the
formation of two separate gene pools and the formation of new species.
Disruptive selection is mostly seen in many populations of animals that have
multiple male mating strategies such as; rabbits, mice, and lobsters among others
and is often the source of speciation or drives to speciation.
In rabbits as illustrated in the figure 16.1.c, a hypothetical population in which
grey and Himalayan (grey and white) rabbits are better able to blend with a rocky
environment than white rabbits. Large dominant alpha lobster males obtain mates
by brute force, while small males can sneak in for furtive copulations with the females
in an alpha male’s territory. In this case, both the alpha males and the sneaking males
will be selected for, but medium-sized males, which cannot overtake the alpha malesand are too big to sneak copulations, are selected against.
In scenario case of mice, those living at the beach where there is light-coloured sand
interspersed with patches of tall grass. Light-coloured mice that blend in with the
sand would be favoured, as well as dark-coloured mice that can hide in the grass.
Medium-coloured mice, on the other hand, would not blend in with either the grass
or the sand, thus, would more probably be eaten by predators. The result of this typeof selection, is increased genetic variance as the population becomes more diverse.
Figure: 16. 1.d: Comparison of three types of natural selection
The three types of natural selection are summarized in the figure 16.1.d above. It
shows populations of species which are selected by the environment particularlythe temperature on fur colour and those which decreases to extinction.
Application 16.1
1. Distinguish among the different of natural selection.
2. Describe what is meant by industrial melanism and how is beneficial to
peppered moth
3. Discuss how natural selection is one way of evolution and allows individualcan survive and reproduce
16.2 Artificial selection
Activity 16.2
From your daily experience and or carry out project work on plants (cabbage,
banana, wheat, maize, tomatoes, irish potatoes, and rice) and animals (cattle
and chicken) at your school or home. Do also research through internet and
textbooks and then answer to the questions below:
1. Discuss what is meant by artificial selection
2. Distinguish between inbreeding and outbreeding selection
3. Discuss how selective breeding or artificial selection has been used to
improve the yield or production of plant crops such as maize, wheat,tomatoes, and rice as well as milk and meat
Artificial selection is selective breeding that occurs when humans instead of
environmental forces select and determine the desirable alleles of plants or animals
to be passed on to successive generations. 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. For
example, farming took place about 7000 years ago. The first crops humans selected
and domesticated include barley and wheat. By artificial selection, some scientists
argue that artificial selection and biotechnology can combine characteristics within
a short period of time that natural selection would require thousands or millions of
years to carry out.
It exerts/influences a directional selection pressure which leads to changes in the
frequencies of alleles and genotypes which have been selected by nature in thepopulation.
16.2.1 Advantages of artificial selection
Some of the advantages of artificial selection are:
– It is the quickest and more certain method of producing offspring for a
desirable characteristic.
– It selects and breeds animals and plants that can adapt and tolerate to live in
certain habitats or different environmental conditions such as heat, cold, day
length, and salinity or pH changes in the soil.
– It produces organisms that are resistant to pests, diseases or herbicides.
– It selects and breeds crop plant such as wheat, barley, rice, and maize plants
for high productivity or yield per unity area.
– It selects and breeds farm animals for better quality and quantity of milk, meat
production and wool quality.
– It leads to plants of fast germination seeds capacity, higher growth rate, early
maturation, better absorption of water, mineral salts or fertilizers. This allows
the planting of the same type of crop two or three times in one season and
therefore increases their production.
– Animals for sports or hobbies such as horses for racing and transport; pigeons
for flight capacity and plumage type; dogs as guardians or for hunting, racing
and appearance; orchids, roses and other flowers to produce more colourful
bloom; koi (a beautiful ornamental fish of striking colours-reds, golds, blues,
yellows, metallic silvers and even greens) fish for appearance from colouredmutants of common food carp are produced.
Figures 16.2 (a) Japanese Koi fish (b) Columbia livia of Europe of artificial breeding
16.2.2 Types of artificial selection
Inbreeding and outbreeding are the two distinguishable types of artificial selection.
a. Inbreeding
Inbreeding is the selective crossing between individuals that have a similar genotype
or are more closely related than if they had been chosen at random from the entire
population. Examples of inbreeding include; selfing in plants, mating between
offspring with one of the parents, among siblings or closely related individuals.
It has noticed that after several generations, the force of selection of inbreeding
increases the frequency of homozygous genotypes. Thus, the organism is probably
purebred, or homozygous for the selected characteristics. By inbreeding, organism
tends to maintain the desirable characteristics such as increase the quantity and
quality of milk by jersey cows (high cream content), produce maize plants and others
of uniform height to facilitate mechanical harvesting, increase oil content of linseed
oil to reduce cost of production and extraction, increase yields from plant crop and
livestock, use less land for farming or raising livestock but increase, breading of
horses for racing, and produce varieties of dogs for competition or as security guard
for example.
Even though, inbreeding is advantageous as described in above; it also presents
disadvantages that include:
– After several generations of excessive inbreeding, it results into inbreeding
depression. The inbreed progeny have decreased/loss vigor resulting from
excessive selective inbreeding between closely related organisms which
increases homozygosity (production of individuals with harmful or undesirable
phenotypic characteristics), poor growth and yield and decline in fertility than
non-inbred individuals.
– There is an increased risk of lowered diseases resistance as genetic variation is
reduced. Thus, inbreeding is not encouraged by animal breeders.
b. Outbreeding
Outbreeding is the controlled mating or crossing between distantly related
individuals (plants and animals) with desired characteristics e.g. the cross between
Elaeis guineensis (African oil palm or macaw-fat) variety dura with Elaeis guineensis
variety pisifera to produce the hybrid oil palm Elaeis guneesis variety tenera, with
fruits of high oil content and do not drop off easily. They may come from two breeds
of the same species or may come from different species. Outbreeding is more
advantageous than inbreeding because:
– The progeny also known as hybrids usually show more variation than
progeny produced by inbreeding. The hybrids usually have new and superior
phenotypes and have greater potential to adapt to environmental changes
for example wheat, tomatoes and rice produced by outbreeding are capable
to resist to diseases.
– Increases heterozygosity and new opportunities for gene interaction. Harmful
recessive alleles are masked by dominant alleles.
However, in some cases outbreeding results in hybrid vigour; healthier; or larger
offspring. And the hybrid produced between genetically different species are often
sterile. They do not have sets of homologous chromosomes and meiosis cannotproceed to produce fertile gametes.
Application 16.2
1. Explain how artificial selection is beneficial to man.2. Distinguish between inbreeding from outbreeding.
16.3 Allele frequency and its causes
Activity 16.3
Use available school resources such as internet, library, and teachers; search
information about allele frequency, selection, the founder effect and genetic
drift and or use pictures (a) and (b) given in question of this activity or use bean
seeds of different colour and play a game as instructed:
a. Take 15 bean seeds and then put all in one plastic bottle such as the one of
mineral water or power soap
b. Take other three empty bottles
c. Shake the bottle containing bean seeds and randomly distribute seeds into
the three bottles. Record and discuss the observations
d. Repeat events in step c) at least three times.
e. Draw the conclusion by linking the discussion to what they have read on
allele frequency, founder effect, and genetic drift
Then, do the following:
1. Discuss what is meant by allele frequency
2. Discuss how forces of mutation and natural selection affect the allelefrequencies
3. Analyse the figures below and then describe how the founder effectand genetic drift affect the allele frequencies in populations
16.3.1 Allele frequency in a population as determinant of
evolution
Genetic variation which confirms evolution is determined by; mutation, natural
selection, the founder effect, and genetic drift among others.
a. Mutation and natural selection
In a particular period, why do some organisms survive while others die? These
surviving organisms generally possess traits or characteristics that bestow / give
them traits or benefits of great value benefits that help them survive (e.g. better
camouflage, mating, faster swimming or running, or digesting food more efficiently)
as discussed before. Each of these characteristics is the result of a mutation or a
change in the genetic code.
Mutations occur spontaneously, but not all mutations are heritable; they are passed
down to offspring only if the mutations in the gametes. These heritable mutations
are responsible for the rise of new traits in a population. Populations or gene pools
evolve as gene frequencies change otherwise individual organisms cannot evolve.
Variation in populations is determined by the genes present in the population’s
gene pool as illustrated in figure below, which may be directly altered by mutation.
In natural selection, those individuals with superior traits will be able to compete
and get more resources as there are more organisms than resources and produce
more offspring. The more offspring an organism can produce, the higher its fitness.
As novel traits and behaviours arise from mutation, natural selection preserves thetraits that confer a benefit.
Figure 16.3a. Mutation and natural selection
As mutations create variation, natural selection gradually affects the frequency of
that advantageous trait in a population.
b. The founder effect
The founder effect occurs when part of a population becomes isolated and
establishes a separate gene pool with its own allele frequencies. When a small
number of individuals become the basis of a new population, this new population
can be very different genetically from the original population if the founders are
not representative of the original. Therefore, many different populations, with very
different and uniform gene pools, can all originate from the same, larger population.
Together, the forces of natural selection, genetic drift, and founder effect can lead tosignificant changes in the gene pool of a population.
Figure 16. 3b. three possible outcomes of the founder effect, each with gene pools separate from the
original populations
c. Genetic drift
Genetic drift is an overall shift of allele distribution in an isolated population,
due to random fluctuations in the frequencies of individual alleles of the genes.
When selective forces are absent or relatively weak, gene frequencies tend to drift
or change due to random events. This drift halts when the variation of the gene
becomes “fixed” by either disappearing from the population or replacing the other
variations completely. Even in the absence of selective forces, genetic drift can cause
two separate populations that began with the same genetic structure to drift apartinto the two divergent populations.
Figure 16.3c. Genetic drift and gene fixation in beetles
In the above simulation, there is fixation in the blue gene variation within five
generations. As the surviving population changes over time, some traits (red) may
be completely eliminated from the population, leaving only the beetles with othertraits (blue).
16.3.2 Allele frequency
Natural selection affects a gene pool by increasing the frequency of alleles that give
an advantage, and reducing the frequency of alleles that give a disadvantage. The
allele frequency (or gene frequency) is the rate at which a specific allele appears
within a population. In population genetics, the term evolution is defined as a
change in the frequency of an allele in a population. Frequencies range from 0,
present in no individuals, to 1, present in all individuals. The gene pool is the sum of
all the alleles at all genes in an interbreeding population.
A gene for a particular characteristic may have several variations called alleles. These
variations code for different traits associated with that characteristic. For example,
in the ABO blood type system in humans, three alleles (IA, IB, or i) determine the
particular blood-type protein on the surface of red blood cells. A human with a type
IA allele will display A-type proteins (antigens) on the surface of their red blood cells.
Individuals with the phenotype of type A blood have the genotype IAIA or IAi, type Bhave IBIB or IBi, type AB have IAIB, and type O have ii.
A diploid organism can only carry two alleles for a particular gene. In human blood
type, the combinations are composed of two alleles such as IAIA or IAIB. Although each
organism can only carry two alleles, more than those two alleles may be present
in the larger population. For example, in a population of fifty people where all the
blood types are represented, there may be IA alleles than i alleles. Population genetics
is the study of how selective forces change a population through changes in alleles
and genotypic frequencies.
Using the ABO blood type system as an example, the frequency of one of the alleles,
for example IA, is the number of copies of that allele divided by all the copies of the
ABO gene in the population, i.e. all the alleles. Allele frequencies can be expressed
as a decimal or as a percent and always add up to 1, or 100 percent, of the total
population. For example, in a sample population of humans, the frequency of the
IA allele might be 0.26, which would mean that 26% of the chromosomes in that
population carry the IA allele. If we also know that the frequency of the IB allele in
this population is 0.14, then the frequency of the i allele is 0.6, which we obtain
by subtracting all the known allele frequencies from 1(thus: 1-0.26-0.14=0.6). A
change in any of these allele frequencies over time would constitute evolution inthe population.
Application 16.3
1. What is allele frequency?
2. Explain how mutation and natural selection are important in gene
frequency?
3. In a situation where a trait is determined by two allele forms. What is
the frequency of each allele form?4. Using illustrations, explain genetic drift and founder effect.
16.4 Study of population genetic variation by Hardy-Weinbergprinciple
Activity 16.4
Use available school resources such as internet, library, search information
about Hardy-Weinberg principle, allele, genotype and phenotype as well as
allele frequency and then do the following:
1. What is Hardy-Weinberg principle
2. If the frequency of a recessive allele is 0.2. What is the frequency of a
dominant allele?
3. Cross one homozygous dominant individual of yellow colour with
one homozygous recessive pea plant of green colour. Calculate both
genotype, phenotype and allele frequencies by using Hardy-Weinbergprinciple if the recessive allele is equal to 0.4.
The Hardy-Weinberg principle is a mathematical baseline way used to estimate
the frequency of alleles, genotypes and phenotypes in a population. The principle
assumes that in a given population, the population is large and is not experiencing
mutation, migration, natural selection, or sexual selection.
The Hardy- Weinberg principle states that the frequency of alleles in a population
can be represented by P + Q = 1, with P equal to the frequency of the dominant
allele and Q equal to the frequency of the recessive allele.
The principle also states that the frequency of genotypes in a population is
represented by
p2 + 2pq + q2 = 1, with p2 equal to the frequency of homozygous dominant
genotype, pq equal to the frequency of the heterozygous genotype, and q2 equal
to the frequency of the Homozygous recessive genotype.
The frequency of alleles can be estimated by calculating the frequency of the
recessive genotype, then calculating the square root of that frequency in order todetermine the frequency of the recessive allele.
Figure 16.4a: Proportions of two alleles by Hardy-Weinberg principle
By referring to the above chart, by applying the expression of Hardy-Weinberg
principle, if the dominant allele is illustrated below as Y is equal to 0.7 while the
recessive allele noticed as y is equal to 0.3; then by using the Hardy-Weinberg
principle p2+2pq+q2=1, if the number of individuals is given as 500 and number
of alleles in a gene pool is 1000; genotypic and allelic frequencies are calculated asillustrated follow by Y2 + 2Yy + y2 = 1 and p + q = 1 respectively:
16.4.1 Hardy-Weinberg analysis
Figure 16.4b: Illustration showing analysis of Hardy-Weinberg principle and calculation of allele/
genotypes frequencies
The Hardy-Weinberg principle states that a population’s allele and genotype
frequencies will remain constant in the absence of evolutionary mechanisms.
Ultimately, the Hardy-Weinberg principle models a population without evolution
under the conditions such as; no mutations, no immigration/emigration, no natural
selection, no sexual selection and a large population. Although there is no realworld
population can satisfy all of these conditions, the principle stiff offers a useful
model for population analysis.
16.4.1 Hardy-Weinberg equations and analysis
According to the Hardy-Weinberg principle, the variable p often represents the
frequency of a particular allele, usually a dominant one. For example, assume that p
represents the frequency of the dominant allele, Y, for yellow pea pods. The variable
q represents the frequency of the recessive allele, y, for green pea pods. If p and q are
the only two possible alleles of this characteristic, then the sum of the frequencies
must add up to 1, or 100 percent. This can also be written as p+q=1, if the frequency
of the Y allele in the population is 0.6, then we know that the frequency of the y
allele is 0.4.
From the Hardy-Weinberg principle and the known allele frequencies, we can also
infer the frequencies of the genotypes. Since each individual carries two alleles per
gene (Y or y), we can predict the frequencies of these genotypes with chi square. If
two alleles are drawn at random from the gene pool, we can determine the possibilityof each genotype.
Application 16.4
1. Calculate the allelic, genotypic and phenotypic frequencies:
a. When a tall plant is crossed with a short one
b. When a heterozygous is crossed with another heterozygous
c. When heterozygous is crossed with a dominant homozygous. Note
that 0.2 is given as a value of recessive allele
2. Calculate the phenotype, genotype and allele frequencies of
populations/ hybrids obtained when the crossing is done between YYand Yy individuals. Note that the dominant allele is assumed to be 0.7.
End of unit assessment 16
1. Differentiate between natural selection from artificial selection
2. 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.
3. Why are heavy-metal tolerant plants rare in unpolluted regions?
4. What effect did the industrial pollution have on the frequency of the C
(melanic) allele within a population of peppered moths.
5. Explain what is meant by heterozygous advantage, using the sickle-cellallele as an example.