Mechanisms of Evolution: Natural Selection and Mutation

Natural Selection

• Mechanism of evolution where individuals with favorable alleles survive and reproduce, passing on their alleles
• Also known as "survival of the fittest"

Mutations

• Random and permanent change in DNA sequence
• Results in variation of a characteristic occurring suddenly and by chance in offspring with no family history of it
• Mutant is an organism that possesses a characteristic due to a mutation
• Mutation is the only source of a brand new allele appearing in a population
• Mutations can be beneficial in some environments, not always harmful

Mutagens

• Increase the rate at which mutations occur
• Examples include chemicals (e.g. Mustard gas, some antibiotics) and ionizing radiation sources (e.g. UV, X-rays, cosmic rays)

Types of Mutations

Gene Mutations

• Changes in a single gene, affecting the traits produced by that gene
• Occur during DNA replication before cell division
• Copied and passed on during each cell division
• Examples include Cystic Fibrosis

Effects of Gene Mutations

• Can lead to a change in DNA sequence or base sequence, affecting the protein produced
• Missense mutations: change amino acid, affecting protein produced
• Nonsense mutations: code to STOP, resulting in shorter protein
• Neutral mutations: change amino acid, but not enough to alter protein function
• Silent mutations: no change in amino acid, no effect on protein produced

Point Mutation

• Change in a single nucleotide (base)
• Can alter protein, have no effect, or prevent protein production
• Example: Cystic Fibrosis

Other Types of Point Mutations

• Substitution: exchanging one base for another
• Insertion: adding extra base pairs, resulting in frameshift
• Deletion: losing a section of DNA, resulting in frameshift
• Insertion or deletion results in a shift in reading frame
• Insertion has a bigger effect than substitution

Chromosomal Mutations

• Changes to all or part of a chromosome
• Affect several genes, as each chromosome contains many genes
• 5 types: deletions, duplications, inversions, translocations, non-disjunctions
• Most cause spontaneous abortion in early pregnancy

Causes of Mutations

• Spontaneous mutations: occur randomly due to error in mitosis or meiosis
• Induced mutations: caused by mutagens in the environment

Location of Mutations

• Hereditability:
  • Germline mutations: change in hereditary material in egg or sperm, passed on to offspring
  • Somatic mutations: change in non-reproductive cells, not passed on to offspring