5.2 Applications and skills
5.2.5 Drawing and analysing cladograms
Cladograms are testable hypotheses about evolutionary relationships between organisms. They predict:
- how similar organisms are to each other, based on the number or percentage of shared genes
- which organisms share a common ancestry
- the sequence of events in the evolutionary history of an organism.
What they do not predict is:
- when the key mutations occurred
- how long the changes lasted.
Drawing cladograms using external characteristics
By examining external features, and grouping organisms accordingly, as taxonomists do, we can easily distinguish five classes of vertebrates, as follows:
|Comparison of vertebrate groups|
|Name||Body plan||Reproduction||Distinguishing feature|
|Reptiles||4 legs for walking on land||Internal fertilisation; lay eggs on land||Double eyelid|
|Birds||2 wings and 2 legs||Internal fertilisation; lay eggs on land||Feathers|
|Amphibians||4 legs||External fertilisation; lay eggs in water||Absorb oxygen through moist skin|
|Mammals||2 arms and 2 legs||Internal fertilisation; live birth||Hairy skin, extended parental care|
|Fish||Streamlined for aquatic environment||External fertilisation; lay eggs in water||Fins|
We might also attempt to use those external features to construct a cladogram, which would look like the image below:
Figure 5.2.5a – A simple cladogram
At each node there is a hypothetical common ancestor. The traits in common are shown on lines between the nodes.
Monophyletic vs polyphyletic groups
The problem with using external characteristics to construct cladograms is that, in general, we cannot be sure which of the characteristics is a more important evolutionary adaptation. For example, in Fig 5.2.5a, both mammals and birds are homeothermic (warm-blooded), but reptiles are not. From this cladogram, it looks as if warm-bloodedness in mammals and birds evolved independently, or that reptiles lost the ability to regulate body temperature. Neither of those explanations is logical.
Phylogenetic analysis shows that all of the organisms we put into the class Reptilia do not belong to the same clade. Clades are monophyletic – that is, they have one lineage, coming from the same common ancestry. Reptiles and some other groups are polyphyletic.
Activity: Construct a cladogram
|Comparison of vertebrate groups|
|Dorsal nerve||Nerve cord along the back of the animal|
|Vertebrae||Made of ossified bone, not cartilage|
|Paired legs||Having 2 or 4 legs for walking|
|Amniotic sac||Embryo matures in a fluid-filled environment|
|Mammary glands||Young suckled from milk-producing glands|
|Placental sac||Embryo develops nourished by a placenta|
|Foramen magnum||Foramen magnum is a hole at the base of the skull associated with bipedalism|
Step 1: Download the worksheet and fill in the following table with + to indicate that the animal has the characteristic and – to indicate that it does not. You may need to do some research. See Figures 5.2.5c-g.
Step 2: Arrange the organisms in order of lowest to highest number of derived characters.
Step 3: Draw a branching diagram, separating all of the organisms into two groups based on a single character at a time.
Step 5: Check your answers
- Are external traits a reliable source of information on evolutionary relationships?
- What steps would you take to deduce evolutionary relationships from a cladogram?
Nature of Science
The Law of Parsimony: When two competing theories can explain the phenomenon equally, the simpler explanation is preferred. The idea that warm-bloodedness evolved independently in two different groups of animals is not favoured by this rule. This is often referred to as Occam’s razor: a theory should be as simple as possible with maximum explanatory power.
Carl Woese (1928–2012)- Figure 5.2.5b - proposed the three-domain classification system in the 1970s. Many prominent scientists did not agree with the change. To what extent does this story mirror Thomas Kuhn’s ideas on ‘revolutionary science’? In what other areas of the natural sciences are there paradigm shifts in progress?
Figure 5.2.5b – Carl Woese
Figure 5.2.5c – Lamprey
Figure 5.2.5d – Monkey
Figure 5.2.5e – Frog
Figure 5.2.5f – Turtle
Figure 5.2.5g – Tuna