VARIATION
★ Variation is the differences between individuals of the
same species
★ There are two types of variation
★ CONTINUOUS VARIATION
○ Continuous variation results in a range of
phenotypes between two extremes
○ Body length and body mass
○ Shown using a histogram and a line graph
★ DISCONTINUOUS VARIATION
○ Discontinuous variation results in a limited
number of phenotypes with no intermediates
○ ABO blood group, seed shape and colour in peas
○ Shown using a bar chart
Variation is caused by the difference in genotype and the
environment of the individuals
CAUSES OF GENETIC VARIATION
★ Mutations
○ A gene mutation is a random change in the base
sequence of the DNA
○ It is how new alleles are formed
○ They are random events
○ Ionising radiation like X-ray, gamma radiation,
beta and alpha increases the rate of mutations
○ Some chemicals mainly heavy metals like lead and
mercury also increase the risk
★ Meiosis
○ During sexual reproduction, gametes are formed
by meiosis
○ In meiosis pairs of homologous chromosomes
exchange genes and separate from one another
○ The gamete formed will contain different
combinations of alleles
★ Random mating and random fertilisation
○ In a population, any two males and females can
reproduce together
○ Any two male and female gametes can fuse at
fertilisation
○ Thus there is a large combination of alleles that
can be produced in the zygote
ADAPTIVE FEATURES
★ The adaptive feature is an inherited feature that helps
an organism to survive and reproduce in its
environment
★ Interpret images or other information about a species to
describe its adaptive features
XEROPHYTES
Closing stomata
Stomata open at night and closed at
midday when evaporation is highest
Waxy cuticle
The leaves of desert plants are often
covered with a very thick waxy cuticle
made by the cells in the epidermis
The wax makes the leaf waterproof
Hairy leaves
These hairs trap a layer of moist air
next to the leaf. The moist air reduces
the diffusion gradient of water vapour
from the leaf into the air so less water
diffuses out of the leaf
Sunken stomata When water vapour diffuses out
on the underside through the stomata it collects in the
of leaves
air trapped in the pits reducing the
diffusion gradient so less water
diffuses out of the leaf
Reducing the
surface area of
the leaves
Leaves reduced to spines with
minimum surface area for transpiration
Having deep or
spreading roots
They seek water very deep down in the
soil or across a wide area
HYDROPHYTES
Leaves are highly divided to create a large surface area
for absorption and photosynthesis
Lack of xylem tubes, no stomata underside of leaves
Stomata are on the upper surface, have a thick waxy layer
to repel water and keep the stomata open and clear
Roots are often reduced and root hairs are often absent
In plants like hydrilla, the stems and leaves have sir
pockets to help them keep afloat
SELECTION
★ Selection pressure is something in the environment
that affects the chance that individuals with different
features will survive and reproduce
★ NATURAL SELECTION:
○ A process in which individuals with advantageous
features are more likely to survive, reproduce and
pass on their alleles to the next generation
1. There is a genetic variation in the population caused
by having different alleles of genes with some affecting
the adaptive features of the organism.
2. The organism will then produce lots of offspring
3. There will be a struggle for survival including
competition for resources
4. A greater chance of reproduction by the individuals
that are better adapted to the environment than
others
5. These individuals pass on their alleles to the next
generation
➔ Adaptation is the process resulting from natural
selection by which populations become more suited to
their environments over many generations
➔ The development of strains of antibiotic resistance
bacteria is an example of natural selection, Eg: MRSA methicillin resistance staphylococcus aureus
★ ARTIFICIAL SELECTION / SELECTIVE BREEDING:
1. Choosing particular organisms with desired
characteristics to breed together and continuing
this over many generations
2. Selection by humans of individuals with desired
features
3. Crossing these individuals to produce the next
generation
4. Selection of offspring showing the desirable
feature
5. It is carried out over many generations to improve
crop plants and domesticated animals
NATURAL SELECTION ARTIFICIAL SELECTION
Environment decides which
individuals can survive and
breed
Humans determine which
individuals can survive and
breed
Random mating between
individuals
Humans determine which
individuals reproduce with
each other
The adaptive feature
increases the chances of an
individual surviving and
reproducing in its natural
environment
The features that are
selected are not adaptive
features but the features that
the breeder wants: it might
make it less likely for that
organism to survive in the
wild
The selective advantage of
a particular feature is quite
small, there is only a slight
chance that some
individuals survive rather
than others
Only the best organisms are
chosen to reproduce
Weak selection pressure ie
variation present is greater
With strong selection
pressure, ie variation is less
The speed of change in
population is usually slow
Large changes in features
can happen over quite short
periods