11.1.3 Molecular Methods for Classifying Organisms

Question 1

Molecular Methods for Classifying Organisms

Answer 1

• DNA sequences can now be used to provide great insight into evolutionary relationships between organisms.
• If you have a good reference point in evolutionary history, you can develop a molecular clock, which you can use to infer times of speciation events between organisms.
• Molecules that are commonly used to develop cladograms are cytochrome c, rRNA, and mitochondrial DNA. DNA hybridization is commonly used to build cladograms of closely related species.

Question 2

note

Answer 2

• Until recently, cladograms have been based on anatomical, developmental, and biochemical characteristics. Data collected from DNA sequences can now be used to provide great insight into evolutionary relationships among organisms.
• Different types of genes and gene products are used as molecular clocks and to build cladograms. Depending on their function within the organism different molecules
  successfully mutate at different rates. Molecules that mutate more quickly are useful for comparing organisms that are closely related. Molecules that mutate slowly are appropriate for comparing divergent organisms.
• An example of a molecule that is used to build cladograms is the respiratory pigment cytochrome c. Because all eukaryotes have cytochrome c and it is very slow to mutate, you can build a cladogram based on its amino acid sequence, which includes species as diverse as sesame seeds and humans.
• Mitochondrial rRNA was used to construct the cladogram of flightless birds on the left. Although both the kiwi and the moa are both native to New Zealand, the cladogram based on molecular data shows that they are rather distantly related.
• Although molecular methods of classifying organisms do provide good insight into relationships among species, the results should be treated with some caution. For example, a langur and a cow are quite distantly related, but the amino acid sequence of lysozyme in both of these animals varies from the lysozyme sequence of humans by the same 5 amino acids. If you were to build a cladogram based on the amino acid sequence of lysozyme, langurs and cows would appear to be much more closely related than they actually are. The fact that their lysozyme sequences are so similar is an example of convergent evolution.

Question 3

molecular clock

Answer 3

• used to infer the times of speciation events between organisms. In order to use a molecular clock, you need to have good knowledge of the timing of a speciation event. In this example, organisms X and Y diverged 1,000,000 years ago. Assume that there are 10 differences in the amino acid sequence of a protein between these two organisms. You can use this protein as a molecular clock that predicts one amino acid change every 100,000
  years.

Question 4

Proteins can be used as ____________________, which are useful for determining how long ago two species diverged.

Answer 4

• molecular clocks

Question 5

Mitochondrial DNA would be most useful for studying the relationship between

Answer 5

• Homo erectus and Homo sapiens

Question 6

Given the fact that the fossil record provides information about evolutionary relationships among organisms, why are molecules also used to construct phylogenies?

Answer 6

• The more lines of evidence there are, the greater likelihood of an accurate determination

Question 7

The lysozyme sequence in langurs and cows varies from the lysozyme sequence in humans by the same five amino acids. The relevance is that

Answer 7

• lysozyme sequences are not useful for determining the phylogenetic relationship between humans, langurs, and cows.

Question 8

The concept behind using a molecular clock is that

Answer 8

• certain molecules mutate at a relatively constant rate, so they can be calibrated to time speciation events.

Question 9

Ribosomal RNA sequences can be used to examine the evolutionary relationship between widely divergent lineages because

Answer 9

• most changes in a ribosomal RNA sequence would prevent it from functioning properly.
• all organisms possess it.