CORE P
SELECTION AND EVOLUTION
In organism, there is overproduction of offspring and yet…

Size of population remains relatively constant. Therefore there is…

A struggle for existence with
individuals competing with each other
to breed
+
There is individual variation between
members of the population (genetic and
phenotypic variations)


Some individuals will be better suited to survive the prevailing conditions than others =
survival of the fittest

Those that survive and breed are likely
to produce offspring similar to
themselves (like produces like)
+
A variety of forms of isolation
mechanisms leads to groups of
individuals being incapable of
interbreeding


Individuals with favoured characteristics survive and breed in preference to others.
Reproductive isolation leads to evolution of new species.
(a) Explain how natural selection may bring about evolution.
Natural selection: Choosing done by nature with its abiotic and biotic components
- abiotic = non-living components – pH, temperature, nutrient
availability in soil, water availability
- biotic = living components – predation by other organisms, infection
by pathogens
Evolution:
- Change in allele frequencies
- in a gene pool of a population
- from generation to generation over time.
Gene pool: Sum of…
- all the various alleles of all the genes
- in all the individuals
- that make up the population
Population: Group of organisms of the same species living in the same place at the
same time.
Allele frequency: Relative occurrence of an allele in a gene pool.
1
CORE P
SELECTION AND EVOLUTION
Explain how natural selection may bring about evolution.
Overproduction/high reproductive rate/potential
In organisms, there is overproduction of offspring and yet…
Constancy of numbers
… the size of populations remains constant therefore there is…
Struggle for existence
… a struggle for existence with individuals competing with each other to breed
Variations among offspring
There is individual variety between (competing) members of population
Ref. to sources of variation caused by mutation/pre-existing/enhanced by
meiosis/crossing over/independent assortment/random mating
Survival of the fittest
Some individuals will be better suited/adapted/to survive the prevailing conditions
than others
i.e. individuals/phenotypes adapted to the environment are selected/have selective
advantage
Those less suited die/less reproductive potential and subsequently their allele/allele
frequency decrease
Those that survive and breed are likely to produce offspring similar to
them/reproduce/pass on gene to next generation
Like produces like
Therefore, their alleles/allele frequency/genotype frequency/phenotype frequency
increases. This means there is a change in gene pool.
Change in allele frequency takes place due to selection/pressure/factor
Ref. to selection pressure/abiotic and biotic factors
Ref. to direction/disruptive selection
Natural selection/microevolution/brings about a change in the allele frequency due to
selection pressure
A variety of forms of isolation mechanisms lead to a group of individuals being
incapable of interbreeding
Ref. to allopatric/geographical isolation
Ref. to sympatric isolation/polypoidy
Reproductive isolation leads to the evolution of a new species from an old species
Formation of new species
Ref. to flow chart of evolution (ref. to page 1)
2
CORE P
SELECTION AND EVOLUTION
(b) Explain why variation is important in selection.
ESSAY QUESTION: Why is variation important in selection
NOT asking: What is variation? Genetic (heritable) and environmental variation leads
to phenotypic variation in a population.
NOT asking: How does variation occur? Independent assortment, random fertilization.
1. Different individuals respond differently to, environment/suitable factor/biotic and
abiotic factors/with adaptive features.
2. Those best suited survive/gives selective advantage in struggle for
existence/breed.
3. Pass alleles conferring favourable characteristics to successive generation.
4. Those less suited do not survive/breed.
5. Variation is the raw material on which natural selection can act/to change allele
frequency of a gene pool.
6. No variation, no selection.
7. It is undesirable for population to be genetically identical (without variation).
8. They could be wiped out if condition unfavourable/all respond in the same way.
9. Ref. to genetic engineering.
(c) Explain how all organisms can potentially overproduce.
All organisms have the reproductive potential to increase their population.
Example: RABBITS.
1. Several young produced in a litter and each female may produce several litters
each year.
2. If all survived to adulthood and reproduced, then the rabbit population would
increase exponentially.
3. But such an exponential increase in population rarely happens in a normal
circumstance. Why?
4. ENVIRONMENTAL FACTORS: biotic (disease by viral pathogen –
myxomatosis, competition from other species, predation) and abiotic (water, light,
temperature, space) factors.
5. If the environmental factor is sufficiently great, then te population size will
decrease.
6. Over a period of time, the population will oscillate about a mean value (for most
populations. E.g. lemming)
7. The number of young produced greater than the number that will survive to
adulthood.
8. Many young die before reaching reproductive age.
9. Some survive.
3
CORE P
SELECTION AND EVOLUTION
What determines which will be the rabbits to survive and which will die? Determined
by favourable features (phenotype) due to variation, will confer an advantage in
“struggle for existence: and will survive and reproduce.
10. Example: rabbit coat colour. Agouti vs white. White, conspicuous  predation by
fox.
11. Therefore the chances of a white rabbit surviving, reproducing and passing of
white allele to the offspring are very small, so that the white allele will remain
very rare in the population.
12. Predation (by fox) is an example of selective pressure.
13. Agouti alleles are said to have selective advantage over white alleles.
14. The effect of selection pressure on the frequency of alleles in a population is
called natural selection.
15. NS raises the frequency of alleles conferring an advantage, and reduced the
frequency of alleles conferring a disadvantage.
(d) Explain, with examples, how environmental factors can act as stabilizing or
evolutionary forces of natural selection.
Natural selection
1. NS is one of the agents that changes the allele frequency.
2. NS is responsible for most evolutionary change by reducing and changing genetic
variation.
3. Some combinations of alleles are more likely to help survival and reproduction
than others, therefore the frequency of these alleles in the population will
steadily increase generation after generation.
4. Individuals with these alleles are said to have greater ‘fitness’.
Types of natural selection
1. Every population has a large range of phenotypes and these usually fall into a
normal bell-shaped pattern of distribution (for continuous variation, e.g. fur length
of rabbits, height).
2. The selective forces (selective pressures) such as predation, competition,
disease, lack of food, water, climate conditions etc act on the phenotypes in the
following way:
a. STABILISING SELECTION
b. DIRECTIONAL SELECTION
c. DISRUPTIVE SELECTION
4
CORE P
SELECTION AND EVOLUTION
Stabilising Selection
1. When environmental condition remain same/unchanged
2. Average/mean/intermediate phenotype selected/favoured at the expense of the extremes,
i.e. the lower and the upper extreme phenotypes
3. Therefore, the average phenotype (fittest) survive and reproduce successfully compared
to both extremes.
4. Frequency of alleles of average phenotype/selective advantage INCREASES
5. Frequency of alleles of both extremes/selective disadvantage DECREASES
6. Class mode remain SAME
7. Class mean remain SAME
8. Variation/standard deviation around mean REDUCED
9. Effect of stabilizing selection is to:
a. Reduce variation around mode/mean of the distribution range
b. Maintain population in stable form
c. Prevent evolutionary change
Examples
of
stabilising
selection
Mass of new
born babies
vs mortality
rate
Selective
pressure /
environment
factors
Size of babies
head relative
to the cervix
size
What is
selected / of
selective
advantage
Alleles for
conferring
babies head
size smaller
that cervix
size
What is not
selected / of
selective
disadvantage
Alleles for
conferring
babies head size
larger than
cervix size
How is it advantageous?
Intermediate phenotype selected: average
weight / average size head compatble to
cerix size which the baby has o pass
through during birth. No perinatal problems
/ baby survives / reproduces successfully /
low mortality
Extreme phenotypes: Underweight. Easily
delivered BUT organs underdeveloped,
perinatal complications, low survival rate,
high mortality
Sickle cell
trait &
anaemia
Malarial
parasire,
Plasmodium
Heterozygot
e HbAHbS /
sickle cell
trait
Homozygotes
HbAHbA and
HbSHbS
Overweight. Body/head size too bid/unable
to pass through cervix, may need caesarian
section.
Intermediate phenotype selected:
Heterozygote. Mild sickle cell trait,
protected against malaria. Although RBC
not efficient oxygen carrier.
Extreme phenotypes:
Homozygotes HbAHbA. Possible death due
to malaria, but RBC excellent oxygen
carrier.
Flowering
time
Pollination
period
Alleles
conferring
flowering at
same/similar
time
Alleles
conferring too
early or too late
flowering
Homozygotes HbSHbS. Possible death due
to complete sickle cell trait and RBC poor
oxygen carriers.
Intermediate phenotype selected:
synchronized flowering ensures successful
pollination and fertilization / greater
survival.
Extreme phenotypes: Out of time / too
early and too late / low success of
pollination and low fertilization, low
survival.
5
CORE P
SELECTION AND EVOLUTION
Directional Selection
1. Due to environmental condition(s)/factor(s).
2. ONE EXTREME (not average/other extreme) phenotype selected/favoured/of
selective advantage.
3. Selected/favoured phenotype fittest, survive and
4. breed/reproduce more successfully
5. compared with unfavoured phenotypes.
6. Therefore, frequency of allele with selective advantage/phenotype INCREASES
and
7. frequency of allele with selective disadvantage/phenotype DECREASES.
8. Hence, class mode shifts towards selectively advantageous phenotype.
9. Class mean also shifts towards selectively advantageous phenotype.
10. Eventually the population will have a new mean and mode.
11. The effect of directional selection is
a. To move the distribution of the phenotype,
b. So that the mode/mean coincides with a new environmental optimum
c. This MAY lead to speciation.
12. Ref. to graph showing the effects of directional selection.
13. Ref. to named specific examples.
Examples of
directional
selection
DDT / Pesticide
/ Warfarin
resistance
e.g. rats /
mosquitoes
Antibiotic
resistance
e.g. bacteria
Industrial
melanism /
peppered moth
Selective
pressure /
environmental
factors
DDT / Pesticide
What is
selected / of
selective
advantage
Pest with
resistant allele
What is not
selected / of
selective
disadvantage
Pest without
resistant allele
How is it advantageous?
Antibiotic /
Penicillin
Bacteria with
resistant allele
Normal bacteria
without resistant
allele
Predatory birds
Alleles
conferring
black colour
against dark
background
Alleles
conferring white
colour against
dark
background
One extreme phenotype selected:
resistant allele enables the coding
for penicilinase / confers
protection against antibiotics
One extreme phenotype selected:
Black moths well camouflaged
against black background hence
protected against predation,
survives and reproduces
successfully.
And
And
Alleles
conferring
white colour
against clear /
white /
unpolluted
background
Grasses with
copper toxicity
tolerance
Alleles
conferring black
colour against
white
background
N.B. Industrial
melanism
started as a
directional
selection
mechanism and
now exist as
disruptive
selection
mechanism
Heavy metal /
copper / zinc
tolerance
e.g. grass
High copper
concentration
Grasses without
copper toxicity
tolerance
One extreme phenotype selected:
resistant allele enables the
breakdown of pesticide / confers
protection against pesticide
One extreme phenotype selected:
White moths well camouflaged
against white background hence
protected against predation,
survives and reproduces
successfully.
One extreme phenotype selected:
Resistant allele allows the grass to
survive / confers protection
against copper toxicity
6
CORE P
SELECTION AND EVOLUTION
Disruptive selection
1.
2.
3.
4.
5.
6.
7.
8.
9.
Reverse of stabilizing selection
Happens when a population lives in two or more contrasting habitats
Both extreme phenotypes are favoured or selected
Therefore both extreme phenotypes (fittest) survive and reproduce successfully
compared to intermediate phenotype.
Frequency of alleles of extreme phenotypes/selective advantage INCREASES
Frequency of alleles for intermediate phenotype/selective disadvantage
DECREASES.
This leads to two class modes
And two means.
The effects of disruptive selection:
a. Produce a bimodal population
b. Each mode has a different appearance and is distinct morph (form) of the
species
c. This is the beginning of a balanced polymorphism (same population,
members of same species, but have many different shapes/size/colours)
d. May lead to speciation
Examples
of
stabilising
selection
Banded
snails
Cepaea
nemoralis
Male
Pacific
Salmon
Selective
pressure /
environmental
factors
Predatory birds
Compatible size
in the
competition of
spawning a
female salmon
What is
selected / of
selective
advantage
Alleles
conferring
colour band
matching
against
background /
vegetation /
sandy beach /
its
environment
What is not
selected / of
selective
disadvantage
Alleles conferring
colour band nonmatching
conspicuous against
background /
vegetation / sandy
beach / its
environment
Alleles
conferring
large size and
alleles
conferring
small size
Alleles conferring
intermediate size
How is it advantageous?
Extreme phenotypes selected:
individuals with colour band
matching the two different
environment, i.e. vegetations and
sandy beach, protected from
predation, survive and reproduces
successfully.
Intermediate phenotypes:
conspicuous, dies of predation, less
survive to reproduce, hence less or no
intermediate phenotype.
Extreme phenotypes selected: large
size salmon, aggressive, can fight
vigorously among themselves for
position closer to a female for
successful fertilization, passing on
alleles for big size.
Small-sized males sneaks between
rocks of river bed closer to spawning
female and release sperm to
successfully fertilize eggs released by
female, passing on alleles for small
size.
Intermediate phenotypes: cannot win
competition with big males and not
small enough to sneak between rocks.
Industrial melanism/peppered moth is an example under disruptive selection only AFTER directional selection
has occurred, and now, two areas are established as industrial and non-industrial for disruption selection to
occur. (see DIRECTIONAL STABILISATION table above)
7
CORE P
SELECTION AND EVOLUTION
(e) Describe the processes that affect allele frequencies in populations with respect to
the global distribution of malaria and sickle cell anaemia. (See Q8, Core P worksheet)
Factors that cause gene frequency to remain the same over time.
Five “ideal conditions” must exist if a phenotype is to remain at genetic equilibrium.
1. There must be an absence of mutation so that no new alleles appear in the
population.
2. Individual cannot migrate into or out of population so that no new alleles enter or
existing alleles leave the population.
3. The population must be very large so that it is not affected by random changes in
allele frequency. (e.g. large natural disasters will not cause allele frequency to
change since population is so large! – Brunei vs Bangladesh)
4. All individuals in the population must have an equal chance of survival that is,
there are no genetic traits that give individuals a survival advantage.
5. Mating must combine genotype at random, that is, no preference is shown in the
selection of a mate.
ESSAY POINTS…
1. SIX processes that affect allele frequency in a population are:
a. Natural selection: selection by environmental factors/biotic and abiotic
factors. Increased reproduction of individuals that have phenotypes that
makes them better to survive and reproduce in a particular environment.
NS is the driving force behind adaptation. It changes allele frequency.
b. Gene flow: emigration and immigration. Population can gain alleles when
they are introduced from other gene pools. This is immigration. Population
can also lose alleles when there is emigration.
c. Genetic drift: bottle neck effect – where population size is dramatically
reduced by a catastrophic event - and founder’s effect – where a small
number of individuals colonise new area and descendants will inherit their
alleles.
d. Mutation: randomly produced inheritable changes in DNA that introduce
new alleles (or new genes, as the result of chromosome rearrangements)
into a gene pool. Mutation is the source of new alleles. Mutation is
important in evolution, because it is the original source of genetic variation
that provides new material for natural selection.
e. Mate selection/non-random mating: individuals may not select their
mate randomly, and may seek out particular phenotypes, increasing allele
frequency of these favoured alleles in the population.
f. Geographical barriers: isolate gene pool and prevent regular gene flow
between population.
2. Ref. to gene pool:
Sum of…
- all the various alleles of all the genes
- in all the individuals
- that make up the population
3. Ref. to allele frequency:
Relative occurrence of an allele in a gene pool.
8
CORE P
SELECTION AND EVOLUTION
4. Sickle cell anaemia can be used to illustrate how natural selection/environmental
factor (oxygen carrying capacity of Hb and parasitic Plasmodium) can cause a
change in the allele frequency.
5. Ref. to sickle cell anaemia
6. Sickle cell anaemia is due to mutated gene for B-polypeptide (HbS)
7. Caused by a gene/point mutation by substitution
8. Ref. to table below:
Level
At DNA
At mRNA
At B-polypeptide
Phenotype/shape of RBC
HbA
CTT or CTC
GAA or GAG
Glutamic acid
Normal, biconcave
Genotype
Phenotype
Type of Hb
Type of RBC
HbAHbA
Normal
Normal
Normal
Oxygen-carrying
capacity
Resistance to
malaria
HbS
CAT or CAC
GUA or GUG
Valine
Sickle
HbSHbS
Sickle cell anaemia
Mutant
Sickle-shaped
Normal
HbAHbS
Sickle cell trait
50% normal, 50% mutant
Usually normal, but
sickle-shaped at low [O2]
Reduced (mild anaemia)
None
Moderate
High
Poor (severe anaemia)
9. In normal circumstance/non-malarial region,
a. Frequency of HbAHbA >> frequency of HbAHbS / HbSHbS
b. i.e. HbAHbA is a selectively advantageous allele.
c. HbAHbS and HbSHbS are selectively disadvantageous alleles.
d. Ref. to directional selection
e. Ref. to bar chart for non-malarial region.
10. Yet, in malarial-prone areas, e.g. Africa,
a. Frequency of HbAHbS >> frequency of HbAHbA / HbSHbS
b. WHY? HbAHbA individuals (homozygotes) are of selective disadvantage!
c. Because Plasmodium parasite can enter RBCs and cause malaria.
d. Therefore, often death due to malaria not due to anaemia.
e. HbSHbS individuals (homozygotes) are of selective disdvantage always in
all regions.
f. Because of sickled RBC/poor oxygen transport
g. Therefore, often death due to severe anaemia, not due to malaria.
h. HbAHbS individuals (heterozygotes) are of selective
advantage/heterozygote advantage
i. Mild sickling of RBC/sickle cell trait
j. Plasmodium unable to cause malaria
k. Only mild anaemia
l. Oxygen can still be transported though not as efficient as in HbAHbA
individuals
m. Therefore, protected against severe anaemia and malaria/heterozygote
superiority!
11. Hence, frequency of HbAHbS >> frequency of HbAHbA / HbSHbS
12. Ref. to stabilizing selection
9
CORE P
SELECTION AND EVOLUTION
13. Ref. to bar chart for malarial region
14. TWO selective pressures: sickle cell anemia and malaria!
15. Homozygous recessive individuals still remain in the population
16. Because a recessive allele is always present in each heterozygote of the population
17. When heterozygous couple produces offspring, there is a 1 in 4 chance that one
will be homozygous recessive (HbSHbS).
18. Because greater proportion of the population in malarial regions are heterozygotes
than non-malarial regions, it follows that frequency of the sickle cell allele (HbS)
is greater in areas where malarial is present than in areas where it is absent.
(f) Explain the role of isolating mechanisms in the evolution of new species.
(Refer back to original flow chart… natural selection responsible for allele frequency
or evolution. When added with isolating mechanisms, then, speciation may occur!)
What is an isolating mechanism?
It is a means of which causes genetic isolation and prevents interbreeding.
There are two types of isolating mechanisms:
1. Prezygotic (before fertilization) isolating mechanisms, barriers to the formation
of hybrids.
a. Temporal/seasonal isolation
b. Ecological islation
c. Mechanical isolation
d. Behavioural isolation
e. Geographical isolation
2. Post-zygotic (after fertilization) isolating mechanism, barriers affecting hybrids.
a. Hybrid inviability
b. Hybrid infertility/sterility
c. Hybrid breakdown
Role of isolating mechanisms:
I. Isolating mechanism leads to
 genetic isolation and
 reproductive isolation.
II. No exchange of genes/allele/gene flow
III. Gene pool isolated
IV. Over time, speciation occurs.
Isolation allows genetic drift in a small population and selection of different
adaptation from those of the main population.
When enough difference have built up, the population may be reproductively
isolated, i.e. they don’t interbreed.
10
CORE P
SELECTION AND EVOLUTION
Speciation is a process of formation of 2 or more species from an existing one.
1. Sympatric speciation: same land
a. Behavioural isolation (diurnal/nocturnal)
b. Ecological isolation (different habitat)
c. Temporal/seasonal isolation
d. Mechanical isolation (genital size/position not compatible)
2. Allopatric speciation: Different land
a. Geographical isolation (river/mountain)
SPECIATION ESSAY POINTS:
1. Ref. to intraspecific speciation: variation within a single species and reproductive
isolation leads to formation of several new species.
2. Originally there is ONE population of a particular spcies.
3. Part of the population becomes reproductively isolated from the rest due to
isolating mechanisms.
4. Different mutations occur in the two isolated groups.
5. Different features are selected for each population, in other words, they become
adapted to the local conditions.
6. The two groups become increasingly genetically different.
7. Eventually, successful reproduction is no longer possible between individuals
of the two groups.
8. A new species has evolved/formed. This is speciation.
9. Ref. to interspecific speciation. Species A + Species B -----hybridization
Species C.
(g) Describe one example of artificial selection.
Definition of artificial selection:
 selection of organism with desired trait
 deliberately by human
 human/breeder is the selective pressure/selective agent
 not nature/environmental factors as in nature selection.
“Artificial selection” is also described as evolution at the will of man!
A person with a desired product in mind, makes the decision about which organisms
will produce offspring with desired traits.
Artificial selection occurs when a breeder chooses individuals with desirable
phenotype for breeding and/or prevent those with less desirable phenotypes from
breeding, thus changing allele frequencies in the population.
Artificial selection by man is another agent that changes the allele frequency in a
population done by selective breeding. HOW?
11
CORE P








SELECTION AND EVOLUTION
Individual showing one or more desired feature(s) to a larger degree than other
individuals are chosen for breeding
i.e. organisms to be bred are chosen by man, NOT BY NATURE
They are mated/crossed.
Offspring showing best desirable traits selected
They are mated/crossed again
selection and breeding/mating repeated over a number of generation until
preferable traits attained/expressed
Over many generations, alleles conferring desired characteristics increase in
frequency.
while alleles conferring undesirable characteristics decrease in frequency and may
be lost entirely.
Artificial selection
Man is selective agent
Few/single traits selected
Only done a few species (e.g.
domesticated organisms with commercial
value)
Traits advantageous to man but not
necessarily to organism
Speed of selection is fast
Reduction is variations as gene pool
decreases
Product of selection known and fitness
may decrease if inbreeding depression
occurs
Natural selection
Total environment is selective agent
Total fitness selected
All species evolve
Traits advantageous to organism is
selected
Speed of selection is slow and may take a
few years
Variations still exist (mutations are
important)
Product of selection is unknown and
fitness of selected individuals increases if
environment remains constant
Write about an example of artificial selection in the space below…
12