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

Mutations & natural selection

• Mutations may create new alleles and affect the phenotype of an individual; these changes are then subject to natural selection

  • Advantageous alleles allow individuals to outcompete others in the population, meaning that the affected individual is more likely to pass on the beneficial allele(s) to future offspring

Example: sickle cell anemia

• Sickle cell anemia is caused by a mutation in the hemoglobin gene which leads to the production of abnormal hemoglobin

  • Individuals who are homozygous for the sickle cell allele (Hb^s) will show symptoms of the disorder

• 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 been shown to be resistant to malaria, offering a survival advantage in areas with high malaria prevalence

• The advantage gained by being heterozygous for the sickle cell trait means that the sickle cell allele is selected for in regions of the world where malaria is prevalent

Generating genetic variation

• Genetic variation is an essential driver of natural selection

• Reproductive processes that increase genetic variation—such as sexual reproduction and genetic recombination—are found across a wide range of organisms

  • This means that these mechanisms have been evolutionarily conserved over millions of years because they provide significant benefits for populations

• In addition to generating new alleles by mutation, organisms can also generate new genetic variants by recombining alleles in new ways, e.g. during:

  • horizontal acquisition of genetic material in prokaryotes

  • recombination of genetic information from viruses

Horizontal gene transfer in prokaryotes

• Horizontal acquisition of genetic material in prokaryotes is the transfer of DNA, often in the form of plasmids, between prokaryotes of the same generation (i.e. not from parent cell to daughter cell); this can be achieved by:

  • transformation - 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)

Recombination of viral genes

• Sometimes two viruses infect the same host cell at the same time

• The genetic material from the two strains interacts during viral replication, using the host cell's replication processes; this is called viral recombination

• Virus progeny acquire genes from both strains, increasing genetic variation