A4.1.2 Evidence for evolution from base sequences in DNA or RNA and amino acid sequences Notes

1. Evolution involves heritable changes in populations over time, which arise from mutations in DNA.
2. Since DNA encodes proteins, evolutionary changes are recorded in:
   1. Base sequences of DNA and RNA
   2. Amino acid sequences of proteins
3. By comparing these sequences between species, scientists can infer how closely related they are and reconstruct their evolutionary history.

DNA Sequence Comparisons

1. DNA sequencing allows scientists to compare the order of nucleotides (A, T, C, G) in specific genes.
2. Closely related species (e.g., humans and chimpanzees) have very similar DNA sequences, while distantly related species (e.g., humans and fish) have more differences.
3. Conserved genes (genes essential for survival, such as those involved in respiration) change very slowly and are especially useful for long-term comparisons.

RNA Sequence Comparisons

1. RNA, especially ribosomal RNA (rRNA), provides another line of molecular evidence.
2. rRNA is highly conserved across all life, meaning its sequences change very slowly.
3. Small differences in rRNA base sequences reveal deep evolutionary relationships, such as the separation of Bacteria, Archaea, and Eukaryotes.
4. rRNA studies were the foundation of the three-domain classification system.

Protein Sequence Comparisons

1. DNA mutations may alter the amino acid sequence of proteins.
2. Comparing amino acid sequences provides another line of evidence for evolutionary relationships.
3. The more differences in protein sequence, the more time since divergence.
4. Highly conserved proteins (such as cytochrome c or hemoglobin) are often used.

Gene Families as Evidence of Evolution

1. Hox Genes control body plan development and are found in diverse animals, from fruit flies to humans.
2. Their similarity across species suggests they originated in a common ancestor and diversified over millions of years.
3. These conserved genes demonstrate how evolution modifies existing genetic blueprints to create diversity.

Case study

Evolution of SARS-CoV-2 (COVID-19)

• The coronavirus pandemic (2020) provided a real-time demonstration of evolution.
• Mutations in the viral genome produced variants (e.g., Alpha, Delta, Omicron).
• Differences in base sequences of the spike protein gene altered infectivity and transmissibility.
• By sequencing and comparing viral genomes, scientists traced the spread of new variants and monitored their evolution.

Cladograms and Sequence Data

1. Sequence comparisons are often used to build cladograms (evolutionary trees).
2. Closely related species cluster together, while more distant relatives branch off earlier.
3. Cladograms based on sequence data often match morphological classifications, but they are more reliable because they reflect actual genetic relationships.