Genetic Relationships Between Organisms (AQA A Level Biology): Revision Note

Relationships between organisms

• Before recent advances in gene technology, which allow us to directly investigate DNA sequences, investigating genetic diversity used to occur through inferring differences in the DNA from measurable characteristics such as

  • size

  • mating processes

  • fruit production

  • observable characteristics

• The use of DNA sequencing technologies has allowed us to assess and track genetic diversity more accurately

Comparing DNA base sequences

• DNA found in the nucleus, mitochondria and chloroplasts of cells can be sequenced and used to show evolutionary relationships between species

• DNA base sequences can be compared directly between organisms to determine genetic similarity

• The more similar the base sequences, the more closely related the organisms are

• Differences in base sequences arise from mutations over time: the greater the difference, the more distant the common ancestor

• DNA sequencing allows precise comparisons at the molecular level, within or between species

• DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

Comparing amino acid sequences

• Differences in DNA codes for amino acid sequences may lead to differences in proteins

• Comparing amino acid sequences (e.g. in cytochrome C or haemoglobin) provides insight into evolutionary relationships

• Fewer differences in amino acid sequences mean a closer evolutionary relationship

• Amino acid comparisons are less precise than DNA because:

  • The genetic code is degenerate

  • Some mutations are silent (they don't change the amino acid)

Interpreting Sequence Data

• Sequence comparisons help determine evolutionary relationships between organisms by analysing DNA or protein similarities

• As a general rule, the fewer the differences, the more recent the common ancestor, whereas more differences may mean a longer time since divergence

How the data may be presented

• DNA or amino acid sequence alignments:

  • Lines of base or amino acid letters lined up to show matching positions

  • The number of differences or mutations can be counted

  • For example:

Species A: ATGCCCGAT

Species B: ATGCCTGAT

There is one difference

• Comparison tables:

  • A comparison table often shows the number of base or amino acid differences between pairs of species

  • This helps identify which species are most similar and therefore most closely related

• Phylogenetic trees (cladograms):

  • These are branching diagrams that represent evolutionary relationships

  • Organisms with fewer differences cluster closer together on the tree and can help identify closely related species

  • Branch length may reflect the amount of genetic change or time since divergence

From observable traits to molecular methods

• Older methods of assessing genetic diversity are based on

  • observable phenotypes (e.g. flower colour, leaf shape)

  • measurable traits (e.g. enzyme activity, protein structure)

  • protein structure – inferred from things like antibody–antigen reactions (e.g. in immunological comparisons)

• These methods assumed that observable differences were caused by genetic variation

• However, phenotypes are influenced by both genes and the environment, which can make results misleading:

  • For example, two genetically identical plants may look different if grown in different light or soil conditions.

• Modern methods directly examine the genetic material itself (DNA or amino acid sequences)

• Techniques include:

  • DNA sequencing, which determines the exact order of nucleotide bases

  • comparing base sequences of genes between individuals or species

  • comparing amino acid sequences in proteins encoded by those genes