National 5 Biology | Unit 3 | Key Area 6: Evolution of Species

Mutation

A rare, random change to genetic material (DNA) of an organism. Mutations are the only source of new alleles.

Mutant

An organism with a mutation that produces a change in phenotype (physical appearance).

What are the two stages where mutation of a cell’s DNA can occur?

- DNA replication, where DNA may be copied incorrectly

- Cell division, where chromosomes/chromatids can travel in the wrong direction → loss or gain of whole chromosome(s)/chromatid(s).

What happens if mutation occurs to the DNA of gametes during meiosis?

The mutation will then be heritable and has a chance of being passed onto future generations, such as haemophilia.

Mutation is spontaneous and to survival may confer:

- No advantage or disadvantage, ie. neutral, eg. an alternative chemically similar amino acid used to build a protein → protein and organism unaffected

- An advantage that may be of immediate benefit, eg. polyploid crop plants like wheat grow faster → greater yields

- A disadvantage that is lethal or will ‘weaken’ the individual → weeded out by natural selection, eg. cystic fibrosis

Mutagenic agent (mutagens)

Factors that increase the chance of mutation occurring.

Give examples of physical mutagens:

- High temperatures

- X-rays (radiation)

- Ultraviolet light (radiation)

Give examples of chemical mutagens:

- Mustard gas

- Thalidomide

Adaptation

A characteristic (allele) that is inherited and makes an organism better suited to its environment.

How do adaptations arise?

- Adaptations are the result of mutations and may give an individual an increased chance of survival.

- The mutations which cause these adaptations are inherited by the offspring of the individuals with the best adaptations, as these survive better and therefore produce more offspring.

Why is genetic variation within a population so important?

Variation makes it possible for a population to evolve over time in response to changing environmental conditions, therefore surviving better.

What is the danger of a population having too little genetic variation?

- Extinction; a chance in the environment or the introduction of a new disease could affect all individuals equally → no individuals survive.

- Populations with more variation have more potential to cope with future changes → less danger of extinction.

Natural selection

The process through which populations of living organisms adapt and change.

Why does natural selection occur?

- When there are selection pressures → species produce more offspring than the environment can sustain.

- If the size of species populations remain fairly constant over long periods of time, there must be a struggle for survival.

- Certain individuals that are better adapted to their immediate environment will survive long enough to reproduce, passing on favourable alleles that confer advantages, ie. survival of the fittest.

- These alleles increase in frequency within the population.

What does natural selection favour?

The emergence of new forms since environmental conditions are constantly changing.

What has to occur in order for speciation to occur?

After part of a population becomes isolated by an isolation barrier.

Name the three types of isolation barriers and give examples:

- Geographical, eg. rivers, mountains, seas

- Ecological, eg. pH, salinity, temperature

- Behavioural, eg. different courtship display

How does speciation occur?

- A population is separated by an isolation barrier.

- Different mutations occur in each sub-population. Natural selection selects for different mutations in each group, due to different selection pressures.

- Each sub-population evolves until they become so genetically different that they cannot interbreed and produce fertile offspring.