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4.5.4Genetics, Variation and Interdependence

Evolutionary Relationships — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 31 of 46 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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4.5.4 Evolutionary Relationships — AQA A Level Biology 7402 notes: Explain how comparing DNA, mRNA and protein sequences shows how closely related species are, and interpret data from such comparisons.When two species descend from a common ancestor, they inherit the same genes.Af…

  • more similar
  • more recently
  • DNA base sequences
  • DNA hybridisation
  • higher temperature
  • sequence of amino acids

Section 1 of 9Why molecules show relationships

When two species descend from a common ancestor, they inherit the same genes.

Mutations
After they separate, the DNA of each line gathers different mutations over time.
The longer ago the lines split, the more differences have built up.
More similar
So the more similar the DNA (or the proteins coded for) of two species, the more recently they shared a common ancestor and the more closely related they are.
Molecular comparison is objective, can be put into numbers and can be used on organisms with no obvious anatomical features.

Section 2 of 9Comparing DNA

DNA base sequences. The sequence of bases in a gene (or genome) can be determined by sequencing and compared directly. The percentage of bases that are identical, aligned for the same gene, indicates relatedness. Humans and chimpanzees show a very high percentage of similarity; humans and distant species show much less.

DNA hybridisation. This is used when sequence data are not available.

1
DNA is extracted from two species and each sample is cut into short fragments.
2
The DNA is heated to separate the double strands into single strands.
3
Single strands from the two species are mixed and cooled, so that hybrid double strands form wherever bases are complementary.
4
The hybrid DNA is heated again, and the temperature at which it separates is measured.

The more similar the sequences, the more hydrogen bonds hold the hybrid together, so a higher temperature is needed to separate them. A higher separation temperature means more closely related species.

Section 3 of 9Comparing mRNA and proteins

The sequence of amino acids in a protein is determined by the DNA base sequence through mRNA, so the amino acid sequence of the same protein in different species shows relationships.

Cytochrome c
A commonly used example is cytochrome c, a respiratory protein found in all aerobic organisms.
Its sequence is quite conserved, since it must keep its shape to work, so differences accumulate slowly and show ancient relationships.

Points to remember:

Degenerate
The genetic code is degenerate, so two species could have different DNA base sequences for a gene and still make the same protein.
  • DNA comparison is therefore more sensitive than protein comparison.
The more amino acid differences between two species' protein, the longer ago they diverged.
Immunological methods
The proteins of closely related species can be compared by immunological methods: antibodies against the serum albumin of one species react more strongly with the albumin of a close relative than with a distant one.

Section 4 of 9Using the data

Differences are turned into a table or a tree.

If the number of differences in a protein is, for example, 1 between species A and B, 4 between A and C, and 4 between B and C,
  • then A and B are closest and C is the outgroup.
Percentage
The count of differences can be turned into a percentage: differences divided by the length of the sequence, multiplied by 100.
  • The rate of change may be used as a molecular clock to estimate when two lines separated, assuming the rate of mutation is roughly constant,
  • which is an approximation.

Section 5 of 9Other evidence and its use

Molecular data are combined with anatomy, embryology, courtship and fossils. Molecular comparisons have changed classifications, for example by showing that giant pandas are bears and that whales are most closely related to hoofed mammals (such as hippopotamuses). They are also used to identify which populations of an endangered species are genetically distinct, which guides conservation.

Section 6 of 9Worked data example

Suppose the percentage of DNA hybridisation, compared with human DNA, is 98% for chimpanzee, 93% for baboon and 70% for a lemur.

Worked data example
The order of relatedness is chimpanzee, then baboon, then lemur.
The data also suggest that the common ancestor of humans and chimpanzees lived more recently than the ancestor shared with the baboon.
When the same type of data is shown for different proteins, always check that the same protein or gene was used for each species,
  • since different genes change at different rates.

Remember also that molecular data help with classification problems. Where two species look the same (cryptic species), their DNA can show that they have been separated for a long time; where unrelated species look alike through convergent evolution, DNA shows that they are distant. The data are quantitative, so they allow comparison by statistics.

Section 7 of 9Exam-style questions

  1. The table gives the number of amino acid differences in cytochrome c compared with humans: chimpanzee 0, rhesus monkey 1, dog 10, tuna 21. Explain what the data show about the evolutionary relationships. [3]3 marks

  2. Describe how DNA hybridisation is used to compare two species. [4]4 marks

  3. Explain why DNA comparison can show more differences between two species than a comparison of the amino acid sequence of a protein coded by the same gene. [2]2 marks

Section 8 of 9Key terms

TermMeaning
DNA hybridisationForming double strands from single strands of two species to measure how complementary the DNA is.
Cytochrome cA respiratory electron-carrier protein whose sequence is compared between species.
Molecular clockUsing the rate of mutation to estimate how long ago species diverged.
Degenerate codeSeveral codons can code for the same amino acid.
Immunological comparisonComparing the reaction of antibodies with the same protein from different species.
SequenceThe order of bases in DNA or amino acids in a protein.
ConservedLittle changed over evolutionary time.

Section 9 of 9Exam technique

1
Saying that more similar DNA means a higher chance of interbreeding. It indicates a more recent ancestor, not necessarily fertile hybrids.
2
Forgetting that a higher separation temperature in DNA hybridisation means closer relationship, not a more distant one.
3
Writing that species with the same protein have identical DNA. The code is degenerate, so DNA may still differ.
4
Saying that the species evolved from each other. Species share a common ancestor.

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