Mutations & Natural Selection (College Board AP® Biology)

Mutations & Natural Selection

The Impact of Mutations on 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

Other Ways that DNA Sequences Affect 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 Gene Transmission in Prokaryotes Diagram

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

Viral Recombination

• 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