Mutations & Natural Selection

Natural selection is dealt with in detail in Unit 7, although the phenomenon of natural selection is closely linked to mutation
Mutation creates new alleles
Some new alleles give a benefit on the organism concerned
- Whilst others have no effect
- And some confer detrimental effects on the organism
In the case of a beneficial allele, this allows owners of that allele to outcompete others in the population eg. for food, habitat occupancy or for a mate
- This ensures that the affected individual is more likely to pass on its (beneficial) allele(s)
- This is the basis of natural selection

Natural selection favors any changes to DNA sequences which may increase survival and reproductive chances
For example:
- Horizontal acquisition of DNA from other cells to include:
  - Transformation
  - Transduction
  - Conjugation
  - Transposition
- Recombination of genetic information in viruses within a host cell
- Sexual reproduction favors genetic variation within species

Horizontal transmission

DNA, often in the form of plasmids, are frequently transferred between bacteria (even from one species to another)
This occurs during
- Conjugation - a thin tube, or pillus, forms between two bacteria to allow the exchange of DNA) – DNA from the bacterial chromosome can also be transferred in this way
- Transduction - DNA is transferred from one organism to another through viral particles
- Transformation - foreign naked DNA is taken up by a cell
- Transposition - the movement of DNA or genes between chromosomes
In this way, variation is increased
A bacterium containing a mutant gene e.g. one that gives it antibiotic resistance, could pass this gene on to other bacterial cells (even those from a different species).
- This is how ‘superbugs’ with multiple resistance have developed (e.g. methicillin resistant Staphylococcus aureus – MRSA)

Horizontal and Vertical Gene Transmission in Prokaryotes Diagram

Horizontal gene transmission can exchange DNA quickly and confer benefits eg antibiotic resistance

Sometimes, two virus strains coinfect the same host cell
The genetic material from the two strains interact with each other during viral replication, using the host cell's replication machinery
This is called viral recombination
Virus progeny 'inherit' genes from both strains
Recombination generally occurs between members of the same virus type (eg. between two retroviruses).
This increases variety and the ability of the virus to withstand pressures from natural selection

Mutations & Natural Selection (College Board AP Biology)