Mutations & Natural Selection (College Board AP® Biology): Study Guide

Mutations & natural selection

• The process of natural selection is closely linked to mutation

• Mutations lead to changes in genotype that may creates new alleles and affect the phenotype of the individual

• These changes are then subject to natural selection

• New alleles can benefit the organism or individuals they are present in

  • Others have no effect and are considered neutral

  • Some lead to detrimental effects on the organism

• Advantageous alleles allow individuals to outcompete others in the population e.g. for food, habitat occupancy or for a mate

  • This ensures that the affected individual is more likely to pass on the beneficial allele(s) to future offspring

  • Therefore increasing the evolutionary fitness of the organism as the basis of natural selection

• Natural selection favors any changes to DNA sequences which may increase survival and reproductive chances through several methods:

  • Horizontal acquisition of genetic material from prokaryotic cells

  • Recombination of genetic information in viruses within a host cell

  • Reproduction processes increase genetic variation are evolutionarily conserved and are shared by various organisms

    • This means that these processes are shared across species from bacteria to humans.

    • E.g. sexual reproduction which favors genetic variation within species through meiotic processes crossing over and independent assortment

Horizontal transmission of genetic material

• DNA, often in the form of plasmids, is frequently transferred between prokaryotes (even from one species to another)

• The processes of horizontal gene transfer ensure that genetic variation is increased

• This occurs during:

  • transformation - foreign naked DNA in the environment is taken up by a cell and incorporated into its DNA

  • transduction - DNA is transferred from one bacterial cell to another via viral particles (bacteriophages)

  • conjugation - a thin tube, or pillus, forms between two bacteria to allow the exchange of DNA

  • transposition - the movement of a segment of DNA or genes between and within the same chromosome or between different DNA molecules (e.g., plasmids and chromosomal DNA); sometimes referred to as 'jumping genes'

Antibiotic resistance

• A bacterium containing a gene for antibiotic resistance, could transfer this gene on to other bacterial cells through horizontal gene transfer

  • This is how ‘superbugs’ with multiple resistance have developed (e.g., methicillin resistant Staphylococcus aureus – MRSA)

Pesticide resistance

• Pesticide resistance organisms, such as insects and bacteria, acquire genetic changes that allow them to survive exposure to pesticides

• These resistance phenotypes can spread through a population via mutations and horizontal gene transfer which enables adaptation of the organisms to pesticide environments

• Resistance to a pesticide may arise at random due to a mutation

  • A single nucleotide change in a gene can alter the target site of a pesticide, reducing its effectiveness

• Horizontal gene transfer allows resistant organisms to transfer resistance genes to others, (even across species)

  • This speeds up the spread of resistance without needed new mutations in each individual organism

Viral recombination

• Sometimes, two virus strains co-infect the same host cell

• The genetic material from the two strains interact with each other during viral replication, using the host cell's replication processes

• This is called viral recombination

• Recombination generally occurs between members of the same virus type (e.g., between two retroviruses)

• Virus progeny acquire genes from both strains

• This increases genetic variation with virus strains and the ability of the virus to withstand pressures from natural selection

Heterozygous advantage

• Sickle cell anemia is a disease where genetic variation influences health and survival

• The disease is caused by a mutation in the hemoglobin gene (HBB) which leads to the production of abnormal hemoglobin (HbS)

• When the oxygen levels are low, mutated hemoglobin forms rigid structures, causing red blood cells to become sickle-shaped and block blood flow

• The inheritance of sickle cell anemia disease is through individuals who are homozygous for the sickle cell allele (Hb^s)

• There is a heterozygous advantageous to individuals who inherit one normal allele and one sickle cell allele

  • These individuals 'carry' the sickle cell trait but do not show symptoms, however they have shown to be resistant to malaria, offering a survival advantage in areas with high malaria prevalence

• Heterozygote advantage is an example of how genetic variation can be maintained in a population

  • The advantages of having a partial sickle cell trait leads to malaria resistance while avoiding the severe consequences of full sickle cell disease

  • The advantageous trait can be maintained through sexual reproduction