VARIATION AND SELECTION

Species
• A species is often defined as a group of similar
organisms that can reproduce and create fertile
offspring.
VARIATIONS WITHIN A SAME
SPECIES
•Variation is the differences between
individuals of the same species.
• Variation between individuals can be due to their genetics
or their environment.
CAUSES OF VARIATION
Variation in species can be caused by

 Genetic
 Environmental
 Or a combination of both.
 CAUSES OF VARIATION
Why do members of the same family look similar?

Mum Dad

son daughter

Humans, like all living things, inherit characteristics from their parents.
How are characteristics passed on?
 1. GENES
• Genes are the portions of a DNA that
codes for a specific protein
• The unique set of genes you have
inherited from your parents.
• For example: natural hair colour , eye
colour are an inherited features.
2. ENVIRONMENT
Examples for environmental variation

• Scars - caused by accidents personal to you.

• Muscle strength - dependent on how much exercise you do.
• Accent
• Tattoo
3. COMBINATION OF BOTH FACTORS

Differences in some characteristics are due to a

combination of
both inherited(Gene)
and environmental factors.

Weight is caused partly by its genes – inherited

- and partly by what it eats environmental.
Height
 TYPES OF VARIATION

Discontinuous variation Continuous variation

1. DISCONTINUOUS VARIATION
Discontinuous variation = genotype alone
• In discontinuous variation, the variations take
the form of distinct, alternative phenotypes with
no intermediates.
• Discontinuous variation is under the control of a
single pair of alleles or a small number of
genes.
 Gender - male or female
 Tongue rollers or non tongue rollers
 Blood group A, AB, B or O
 Free or attached ear lobes
• Discontinuous variation cannot usually be
altered by the environment.
• You cannot change your eye colour by altering
your diet. A genetic dwarf cannot grow taller by
eating more food. You cannot learn how to roll
your tongue.
2. CONTINUOUS VARIATION There might be five pairs of alleles
for height – (Hh), (Tt), (Ll), (Ee) and
(Gg) – each dominant allele
Continuous variation = genotype + envt. adding 4cm to your height. If you
• Continuous variation, results in a range of inherited all ten dominant genes
(HH, TT, etc.) you could be 40cm
phenotypes with a number of intermediates in taller than a person who inherited
between. all ten recessive genes (hh, tt, etc.).
The actual number of genes that
• Continuously variable characteristics are usually control height, intelligence, and
controlled by several pairs of alleles. even the colour of hair and skin, is
not known.
• EXAMLE:
 Height
 Weight
 Body mass….
• Continuously variable characteristics are greatly
influenced by the environment
• A person may inherit genes for tallness and yet
not get enough food to grow tall. A plant may
have the genes for large fruits but not get enough
water, minerals or sunlight to produce large fruits.
 Continuous variation Discontinuous variation
a range of phenotypes limited no. of phenotypes
intermediates in between no intermediates in
between
normally represented by normally represented as
histogram bar graph
Influenced by genes and Influenced only by genes
environment
Under the control of a
Controlled by several pairs single pair of alleles or a
of alleles; small number of genes
TOPIC: MUTATION
 CAUSES OF GENETIC VARIATION

 A mutation is a random, spontaneous genetic change

in the base sequence of DNA
 new alleles are formed
 MUTAGEN: an agent, such as ionizing radiation( X -
rays, gamma rays, etc) or a chemical substance,
which causes a genetic mutation.
 An abrupt change in a gene-defective enzyme disrupt
the complex reactions in the cells.
 Most mutations, therefore, are harmful to the
organism.
 T YPES OF MUTATION

 GENE MUTATION – a random change in the base

sequence of DNA [when genes are not copied
correctly]

 CHROMOSOME MUTATION – when a chromosome part

is lost, damaged or even gaining an extra
chromosome due to errors during mitosis or meiosis
A mutation that happens in a somatic cell [normal cell] or
a gamete [sperm or ovum], which one is more dangerous?

If mutation occurs in a gamete, it will affect all

the cells of the individual that develops from
the zygote. Thus the whole organism will be
affected. e.g. hemophilia, cystic fibrosis Cystic fibrosis is a
genetic condition. It's
caused by a faulty gene
that affects the
If the mutation occurs in a somatic cell (a movement of salt and
water in and out of
normal body cell), it will affect only those cells cells. This can result in
a build-up of thick,
produced, by mitosis, from the affected cell. sticky mucus in the
body's tubes and
e.g. Cancer [caused by uncontrolled cell passage ways –
particularly the lungs
division] and
digestive system
TOPIC: ADAPTATIONS
 ADAPTATION
ADAPTATION: The ability of an organism
to survive and reproduce in its
environment / habitat is called
adaptation

ADAPTIVE FEATURES: The inherited

features of organisms that helps them
to survive and reproduce in their
environment / habitat
 ADAPTATIONS TO ARID
CONDITIONS
In both hot and cold climates, plants may suffer
from water shortage.
high temperatures - rate of transpiration is
high
very low temperatures - the soil water
becomes frozen and therefore unavailable to
the roots of plants.
Plants modified to cope with lack of water are
called xerophytes.
 PINE TREE

The pine tree (Pinus) is an evergreen

tree that survives in cold climates.
It has small, compact, needle-like
leaves. The small surface area of
such leaves offers little resistance to
high winds. This helps to resist wind
damage and can reduce the amount
of water lost in transpiration.
However, photosynthesis can
continue whenever water is
available.
Sunken stomata create high
humidity and reduce transpiration.
A thick waxy cuticle is present on the
epidermis to prevent evaporation
from the surface of the leaf.
 CACTI
Cacti are adapted to hot, dr y conditions in
several ways.

 Often they have no leaves, or the leaves

are reduced to spines. This reduces the
sur face area for transpiration .
 Cacti are succulent, i.e. they store water
in their fleshy tissues and draw on this
store for photosynthesis.
 The stomata of many cacti are closed
during the day when temperatures are
high, and open at night when
evaporation is at a minimum. This
strategy requires a slightly different
form of photosynthesis. At night, carbon
dioxide diffuses in through the open
stomata and is ‘fixed’ (i.e. incorporated)
into an organic acid. breaks down to
yield carbon dioxide, which is then built
into sugars by photosynthesis. Closure
of the stomata in the daytime greatly
reduces water loss.
MARRAM GRASS
Marram grass Marram grass
(Ammophila) lives on sand dunes, where
water drains away very quickly.
 It has very long roots to search for
water deep down in the sand .
 Its leaves roll up into straw like tubes
in dry weather due to the presence of
hinge cells, which become flaccid as
they lose water.
 Leaf rolling, along with the fact that
the stomata are sunken, helps to
increase humidity around the
stomata, reducing transpiration.
 The presence of fine hairs around the
stomata reduces air movement so
humidity builds up and transpiration
is reduced.
 SUNKEN STOMATA

The stomata
of xerophytes are sunk into
pits in the epidermis. The
pits trap moist air. This
decreases the water
potential gradient between
the inside of the leaves and
the outside. This slows the
rate of diffusion of water out
of the stomata.
 HYDROPHYTES

Plants adapted to live in water are

called hydrophytes. An example is the
water lily (Nymphaea ) [picture on the
right side]

 The leaves contain large air spaces to

make them buoyant, so they float on
or near the surface. This enables them
to gain light for photosynthesis.
 The lower epidermis lacks stomata to
prevent water entering the air spaces,
while stomata are present on the
upper epidermis for gas exchange.
 The roots of hydrophytes, which can be
poorly developed, also contain air
spaces. This is because the mud they
grow in is poorly oxygenated and the
root cells need oxygen for respiration.
Pygmy seahorses have
slightly different
colouration – better
camouflaged than others

Some individuals have more

mitochondria to release
more energy-to swim faster
than other population

Adaptive features are caused by genes

How natural selection could affect a population of cacti?
 Variation-There is
variation within a
population of organisms
Overproduction- Many
more offspring are produced
than will survive to become adults and reproduce.
Best-adapted individuals more likely to survive and reproduce- Within
the population, it is the individuals that have the best adaptive features
that have the best chance of surviving and reproducing.

Alleles that confer useful adaptations more likely to be passed on- These
better-adapted individuals are therefore the ones that pass on their
alleles to the next generation
 Selection pressure

External agents which affect an organism’s

ability to survive in a given environment
 The better adapted
varieties are ‘selected’ by
the pressures of the
environment - selection
pressures
 Selection pressures
are external agents which
affect an organism's
ability to survive in a
given environment.
 Selection pressures can
be negative (decreases
the occurrence of a trait)
or positive (increases the
proportion of a trait)
A drug is a substance that affects metabolic
reaction in the body.

Antibiotics are medicinal drugs that kill bacteria

in the body, without harming our own cells
Many populations of pathogenic bacteria have
become resistant to antibiotics due to natural
selection

Antibiotic resistance occurs when

bacteria adapt and become resistant to
the effects of antibiotics, making
medications ineffective.
• Within that population, there are
likely to be at least one or two
individuals that have an allele
that makes them resistant to
penicillin.

• These individuals will have a

tremendous selective advantage.

• They will be able to go on

reproducing while all the others
cannot.

• Soon, their descendants - all

containing the allele that confers
resistance to penicillin - may form
a huge population of penicillin-
resistant bacteria
How the development of resistant bacteria
can be minimized?

Prescribe /use antibiotics less often

Do not use it for viral or fungal
infections
Make sure to complete the course of
antibiotics
Develop new antibiotics
Do not use new antibiotic for too long
Use combinations of antibiotics
Isolation of patients with antibiotic
resistant infections
Good hygiene to spread of disease
Describe natural selection with all key words

 Variation exists in all populations.

 Whenthere is a selection pressure, some organisms will have a selective
advantage over the other.
 Theadvantageous organisms are considered to be “fit” in that
environment.
 Theseorganisms with a selective advantage thus can grow, survive and
reproduce more in that environment.
 This increases the allele frequency of the advantageous group.
 This is called natural selection.
 Natural selection leads to evolution.
Variation. The garden tiger moths in
this picture are all from the same
family. There is a lot of variation in
the pattern on the wings.
  The process of selective breeding involves humans selecting individuals with
desirable features.
 These individuals are then cross-bred to produce the next generation.
 Offspring with the most desirable features are chosen to continue the breeding
programme and the process is repeated over a number of generations.

OUTLINE - ARTIFICIAL
SELECTION
STEP 1: Select parents with
desired characteristics
STEP 2: Breed them together
STEP 3: Select offspring with
the desired characteristic
STEP 4: Repeat STEP 2 for many
generations
SELECTIVE BREEDING EXAMPLES:
SELECTIVE BREEDING EXAMPLES:
 DISADVANTAGES OF SELECTIVE BREEDING/
ARTIFICIAL SELECTION

• Can cause inbreeding – that increases the occurrence of

certain diseases
• Reduced variation
• Can cause inherited defects
• Can lead to local extinction of certain varieties

Inbreeding is the production of offspring

from the breeding of organisms that are
closely related genetically.
 NATURAL SELECTION ARTIFICIAL SELECTION

It is the process where organisms It is the process by which organisms

better adapted to their environment are chosen by the breeder to
tend to survive and reproduce more produce desirable and inheritable
offspring. characters in the successive
generations.
Organisms will be better adapted Organisms may not be well adapted

Desirable features are not chosen by Humans select organism with

humans desirable feature
Nature-made selection process Man-made selection process

Slow process Fast process

Leads to evolution Does not facilitate evolution

Causes variation / more diversity Less variation / reduced diversity

Does not lead to extinction Can lead to extinction of local

varieties