Unit 3
Topic 4 Adaptation, natural selection and the evolution of species
Pupil Notes
Mutation
A mutation is a sudden, random change in the structure of a gene or chromosome (genetic
material). Mutations are the only way in which new alleles can occur in a population. Since
genes control characteristics, new alleles arising by mutation can lead to the appearance of
new characteristics. These new characteristics may:
•
cause no obvious change – e.g. new blood types
•
result in a disadvantage e.g. the gene for haemophilia
•
result in an advantage e.g. mutations causing antibiotic resistance in bacteria
Mutations are usually of low frequency but the rate of mutation can be increased by
mutagenic agents such as:•
Radiation – gamma rays , X-rays and UV light

Chemicals – Colchicine, nicotine and mustard gas
Variation
Variation describes differences between members of the same species. These differences are caused
by individuals having different genes. It is these differences that allow species to change and evolve
when environmental conditions change since the new conditions will suit some characteristics more
than others and these individuals will survive to pass on their genes.
Natural selection
Organisms produce more offspring than the environment can support. Only those individuals
which have characteristics making them best adapted to their environment survive to
reproduce and pass on the genes that gave them a selective advantage to the next
generation.
This is the process of natural selection
Mutations and Natural Selection - Peppered moth: An example of natural selection
There are two forms of the moth, a speckled form and a dark form.
The original speckled form of the
moth is well adapted to its
environment since its speckled
colour gives it camouflage when
resting on tree trunks covered in
lichens preventing it being seen
by predatory birds – the speckled
form has a selective advantage
A random mutation appeared
giving a few individuals a
new allele which leads to the
colour being dark
The new mutation does not normally give a selective advantage as individuals who have a
dark coat are easily spotted by predator birds and so do not survive in large numbers.
A change in the environment caused by factories giving out soot turn the trees black.
Suddenly any individuals carrying the dark mutation have a selective advantage as the dark
colour helps them camouflage themselves against the predator birds so individuals carrying
the mutation survive, reproduce and pass the new feature to their offspring.
Light and dark moth resting on a sootcovered tree trunk in a polluted area
Light and dark moth resting on a lichencovered tree trunk in an unpolluted area
Speciation
Speciation means the development of new species of plants or animals.
New species can only be formed from existing species.
This is how a new species forms
1. Isolation
The first step in forming a new species is when a population gets separated from the rest of
the species, e.g. if the isolated population is on an island, the sea acts as a barrier preventing
the two groups from mixing and breeding with each other so they no longer share genes.
2. Mutations and natural selection
Different mutations occur in the two groups so that their genes and characteristics become
different.
Since the two groups don’t have the same environment, the selection pressures on each of
them are different – these means that different characteristics will be favoured to survive
and be passed on to offspring.
After a long time the two groups have changed so much that they have become different
species
This process has led to the development of many species on the Galapagos Islands that are
found nowhere else. For example there are many different types of finch on the islands that
evolved from ancestor finches that were cut off by the sea from other finches on the
mainland. The beaks of these birds have become adapted to eat different foods:
Ancestral finch – eats small seeds
Three types of
Galapagos finch
Galapagos finch – beak adapted to
eat small insects
Galapagos finch – beak adapted to
eat cactus flowers, fruit and nectar
Galapagos finch – beak adapted to
eat large seeds