18.1.

1 Variation

Types of Variation

 Variation is defined as differences between individuals of the same

species
 Phenotypic variation is the difference in features between individuals of
the same species
 Some of these differences are caused by differences in genes, which is
genetic variation
 Phenotypic variation can be divided into two types depending on how
you are able to group the measurements:
o Continuous Variation is when there are very many small degrees
of difference for a particular characteristic between individuals and
they are arranged in order and can usually be measured on a scale
o Examples include height, mass, finger length etc. where there can
be many ‘in between’ groups
o Discontinuous Variation is when there are distinct differences for
a characteristic
o For example, people are either blood group A, B, AB or O; are
either male or female; can either roll their tongue or not - there are
no ‘in betweens’
 When graphs of these data are plotted, continuous variation gives smooth
bell curves (a result of all the small degrees of difference), whereas
discontinuous gives a ‘step – like’ shape
 H
eight is an example of continuous variation which gives rise to a smooth bell-
shaped curve when plotted as a graph

Blood group is an example of discontinuous variation which gives rise to a

step-shaped graph

Phenotypic Variation
  Phenotypic variation can be caused in two main ways:
o It can be genetic - controlled entirely by genes
o Or it can be environmental - caused entirely by the environment in
which the organism lives

Genetic Variation

 Examples of genetic variation in humans include:

o blood group
o eye colour
o gender
o ability to roll tongue
o whether ear lobes are free or fixed

Whether earlobes are attached (lobeless) or free (lobed) is an example of

genetic variation

Environmental Variation

 Characteristics of all species can be affected by environmental factors

such as climate, diet, accidents, culture and lifestyle
 In this instance ‘environmental’ simply means ‘outside of the organism’
and so can include factors like climate, diet, culture, lifestyle and
accidents during lifetime
 Examples include:
o An accident may lead to scarring on the body
o Eating too much and not leading an active lifestyle will cause
weight gain
 o Being raised in a certain country will cause you to speak a certain
language with a certain accent
o A plant in the shade of a big tree will grow taller to reach more
light

Genetic and Environmental Causes

 Discontinuous variation is usually caused by genetic variation alone

 Continuous features often vary because of a combination of genetic and
environmental causes, for example:
o tall parents will pass genes to their children for height
o their children have the genetic potential to also be tall
o however, if their diet is poor then they will not grow very well
o therefore, their environment also has an impact on their height
 Another way of looking at this is that although genes decide what
characteristics we inherit, the surrounding environment will affect how
these inherited characteristics develop

Mutation

 Mutations are genetic changes

 Most mutations have no effect on the phenotype as the protein that a
mutated gene produces may work just as well as the protein from the non
- mutated gene
 Rarely, mutations lead to the development of new alleles and so new
phenotypes and if they do, most have a small effect on the organism
 Occasionally, the new allele gives the individual a survival advantage
over other members of the species
 For example:
o A bird develops a mutation leading to a change in feather colours
o This makes it more attractive to birds of the opposite sex
o Which causes the bird to breed more frequently and have more
chances of passing on the mutated phenotype to the next generation
 Mutations can also lead to harmful changes that can have dramatic
effects on the organism - for example, sickle cell anaemia in humans
 Mutations happen spontaneously and continuously but their frequency
can be increased by exposure to the following:
o Gamma rays, x - rays and ultraviolet rays - all types of ionising
radiation which can damage bonds and cause changes in base
sequences
o Certain types of chemicals - for example chemicals such as tar in
tobacco
 Increased rates of mutation can cause cells to become cancerous, which
is why the above are linked to increased incidence of different types of
cancer.
Mutation: Extended

 Mutations are random genetic changes to the base sequence of DNA

 Most mutations have no effect on the phenotype as the protein that a
mutated gene produces may work just as well as the protein from the non
- mutated gene
 Rarely, mutations lead to the development of new alleles and so new
phenotypes and if they do, most have a small effect on the organism
 Occasionally, the new allele gives the individual a survival advantage
over other members of the species
 For example:
o A bird develops a mutation leading to a change in feather colours
o This makes it more attractive to birds of the opposite sex
o Which causes the bird to breed more frequently and have more
chances of passing on the mutated phenotype to the next generation
 Mutations can also lead to harmful changes that can have dramatic
effects on the organism - for example, sickle cell anaemia in humans
 Mutations happen spontaneously and continuously but their frequency
can be increased by exposure to the following:
o Gamma rays, x - rays and ultraviolet rays - all types of ionising
radiation which can damage bonds and cause changes in base
sequences
o Certain types of chemicals - for example chemicals such as tar in
tobacco
 Increased rates of mutation can cause cells to become cancerous, which
is why the above are linked to increased incidence of different types of
cancer

Summary of Sources of Genetic Variation in Populations

 Mutations
o New alleles form through random changes to DNA
 Meiosis
o New allele combinations form through segregation
 Random mating
o Which partnerships form for sexual reproduction
 Random fertilisation
o Which sperm and egg combinations occur during sexual
reproduction

18.1.2 Adaptive Features

Adaptations & Fitness

 Adaptive features are the inherited functional features of an organism

that increase its fitness
 Fitness is the probability of an organism surviving and reproducing in
the environment in which it is found
 You should be able to interpret images or other information about a
species in order to describe its adaptive features, for example:

A typical question here might be to explain how the leaf area and distribution
and density of stomata help different species of plant survive in their different
habitats

18.1.3 Adaptive Features: Extended

Hydrophytes & Xerophytes: Extended

Hydrophytes

 Plants adapted to live in extremely wet conditions

 Common adaptations include:
o Large air spaces in their leaves to keep them close to the surface
of the water where there is more light for photosynthesis
o Small roots as they can also extract nutrients from the surrounding
water through their tissues
 o Stomata usually open all the time and mainly found on the upper
epidermis of the leaf where they can exchange gases much more
easily with the air

Hydrophytes are adapted to live in wet conditions such as ponds

Xerophytes

 Plant adapted to live in extremely dry conditions

 Common adaptations include:
o Thick waxy cuticle - the cuticle cuts down water loss in two ways:
it acts as a barrier to evaporation and also the shiny surface reflects
heat and so lowers temperature
o Sunken stomata: stomata may be sunk in pits in the epidermis;
moist air trapped here lengthens the diffusion pathway and reduces
evaporation rate
o Leaf rolled with stomata inside and an inner surface covered in
hairs - traps moist air and prevents air movement across stomata
which reduces transpiration
 o Small leaves: many xerophytic plants have small, needle-shaped
leaves which reduce the surface area and therefore the evaporating
surface
o Extensive shallow roots allowing for the quick absorption of large
quantities of water when it rains
o Thickened leaves or stems which contain cells that store water

Xerophytes are adapted to live in extremely dry conditions such as deserts

18.1.4 Natural Selection

Natural Selection

 In any environment, the individuals that have the best adaptive features
are the ones most likely to survive and reproduce
 This results in natural selection:
 Individuals in a species show a range of variation caused by differences
in genes
 When organisms reproduce, they produce more offspring than the
environment is able to support
 This leads to competition for food and other resources which results in a
‘struggle for survival’
  Individuals with characteristics most suited to the environment have a
higher chance of survival and more chances to reproduce
 Therefore, the alleles resulting in these characteristics are passed to
their offspring at a higher rate than those with characteristics less suited
to survival
 This means that in the next generation, there will be a greater number
of individuals with the better adapted variations in characteristics
 This theory of natural selection was put forward by Charles Darwin and
became known as ‘survival of the fittest’

An example of natural selection

 Natural selection illustrated by snail shell colour

 Within the population of snails there is variation in shell colour

 Normal varieties of shell colours in this snail species is black or grey (as
evidenced by the first picture)
 Chance mutations lead to a small number of snails / one snail having a
white shell
 This ‘small number’ is shown in the second diagram where there are less
white shelled snails than black or grey shelled snails
 The white shelled snail(s) survive longer
 This is the ‘survival of the fittest’, a term used to explain why some
organisms succeed in the competitive struggle for survival against other
members of their population
 The reason the white shelled snail(s) survive longer is because they are
better camouflaged
 This means that they are less likely to be seen by predators and eaten
 As they survive longer they get more opportunities to reproduce
 And so the allele for white shells is passed onto offspring more
frequently than the alleles for black or grey shells
 Over generations, this is repeated until the majority of snails in the
population have white shells
 Another good example of natural selection is the evolution of the peppered
moths

Exam Tip

There are hundreds of thousands of examples of natural selection, and you

cannot possibly be familiar with all of them, however, they ALL follow the
same sequence described above:

 Based on the idea that within a species there is always variation and
chance mutations, some individuals will develop a phenotype
(characteristic) that gives them a survival advantage and therefore will:
o live longer
o breed more
o and be more likely to pass their genes on
 Repeated over generations, the ‘mutated’ phenotype will become the
norm
Remember, it is the concept you have to understand, not the specific example.

Adaptation by Natural Selection: Extended

 If the environment does not change, selection does not change

 This will favour individuals with the same characteristics as their parents
 If the environment changes, or a chance mutation produces a new
allele, selection might now favour individuals with different
characteristics or with the new allele
 So the individuals that survive and reproduce will have a different set of
alleles that they pass on to their offspring
 Over time, this will bring about a change in the characteristics of the
species - it will produce evolution
 Evolution is defined as the change in adaptive features of a population
over time as a result of natural selection
 Natural selection results in a process of adaptation, which means that,
over generations, those features that are better adapted to the
environment become more common
o This is how evolution occurs
 This means populations of organisms become better suited to their
environment
 A good example of this is the development of antibiotic resistance by
bacteria
18.1.5 Artificial Selection

Selective Breeding

 Selective breeding means to select individuals with desirable

characteristics and breed them together
 The process doesn’t stop there though because it’s likely that not all of
the offspring will show the characteristics you want so offspring that do
show the desired characteristics are selected and bred together
 This process has to be repeated for many successive generations before
you can definitely say you have a ‘new breed’ which will reliably show
those selected characteristics in all offspring
 An exmaple of selective breeding is dog breeders who select which dogs
can mate together to increase the likelihood of puppies displaying
desirable characteristics eg. coat colour
 This has given rise to the many recognisable dog breeds of today, all the
same species (Canis familiaris) and all descended from one breed
Natural vs Artificial Selection: Extended