Genetic Variation & Evolution - Quick Revision

Mutation

• Definition: a permanent change in the sequence of DNA nucleotides; ultimate source of genetic variation.
• Causes: high temperatures, mutagenic chemicals, viruses, and radiation.
• Germline mutations: passed on to offspring; accumulate over many generations.
• Mutation outcomes: lethal, disadvantageous, neutral, or beneficial.
• Mutation rate and scope:
  • Each gene mutates once every $10^{6}$ copies.
  • Estimated lifetime cell divisions: about $10^{16}$ per person, so many mutations occur.
  • Most mutations occur in non-coding DNA; introns are removed leaving exons that code for proteins.
  • Even mutations in genes may have no effect due to the degeneracy of the genetic code.
• Therefore, germline mutations are the source of new alleles and drive long-term variation.

Sources of genetic variation in sexually reproducing organisms

• Independent assortment
• Random fertilisation
• Crossing over

Gene pool

• Definition: the sum of all the alleles of all individuals in a population.
• Large gene pool ↔ greater genetic diversity and a higher likelihood of surviving selection pressures.
• Changes to the gene pool:
  • Mutations: permanent changes to DNA sequences; introduce new alleles.
  • Gene flow: movement of alleles between populations.
  • Genetic drift: random changes in allele frequencies; more pronounced in small populations.

Natural selection

• Definition: environmental or selective pressures act on phenotypes, leading to changes in allele frequencies; certain alleles are favored.
• How it drives evolution: increases frequency of advantageous alleles over generations.
• Biotic factors: predators, disease, competition, symbiosis, human activities.
• Abiotic factors: rainfall, temperature, nutrient levels, light, drugs like antibiotics, pesticides.
• Example: peppered moth variation due to industrial soot changing tree coloration and predation.

Genetic drift

• Definition: random fluctuations in allele frequencies from one generation to the next.
• Effects: more pronounced in small populations; can lead to fixation or loss of alleles by chance.
• Often contrasted with natural selection as a stochastic process.

Cystic Fibrosis (CF)

• Caused by a faulty recessive allele; normal allele is dominant over the recessive mutant.
• Genotypes and phenotypes:
  • CC = Normal
  • Cc = Normal (carrier for CF)
  • cc = Cystic Fibrosis

Sickle cell mutation

• Cause: single base change; results in a different amino acid in the beta chain of hemoglobin.
• Specifics: glu (glutamic acid) replaced by val (valine) at the sixth amino acid position due to an A to T substitution in the codon.

Osteoporosis and LRP5

• Gene involved: low-density lipoprotein receptor related protein 5 (LRP5).
• Mutations in LRP5 can lead to osteoporosis due to reduced bone density and leakage of minerals like calcium.

Neutral mutation

• Definition: mutations that have no observable effect on appearance or function.

Antibiotic resistance (natural selection in bacteria)

• Steps:
  1) Natural variation exists in bacterial populations.
  2) Population grows across generations.
  3) Antibiotics apply a selection pressure.
  4) Antibiotics kill most bacteria; survivors are less susceptible.
  5) Surviving bacteria pass on resistance genes to offspring.
  6) Future generations show greater frequency of resistance.
• Practical note: complete the full course of antibiotics to kill off all bacteria.

Peppered Moth (natural selection example)

• Industrial soot and acid rain changed tree coloration.
• Resulted in a shift in moth coloration frequencies to better match the environment, demonstrating natural selection in action.

Darwin's observations (summary)

• Variation among individuals within a population and among related species.
• Variation provides differential reproductive success; those better adapted leave more offspring.
• This leads to evolution through differential survival and reproduction.