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:

simple cladogram

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

In this activity, you will draw a cladogram for a group of animals using the following derived structural homologies. The traits are arranged from the oldest to the newest genetic mutation.

Comparison of vertebrate groups
Trait Explanation
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.

Characteristics
  Lamprey Monkey Frog Human Turtle Tuna
Dorsal nerve            
Vertebrae            
Paired legs            
Amniotic sac            
Mammary gland            
Placental sac            
Foramen magnum            

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 4: Rewrite your branching diagram as a cladogram, making sure all of the species are written in a straight line at the top (or side) of your diagram.

Step 5: Check your answers

Key questions

  • 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.

TOK

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?

Carl WoeseFigure 5.2.5b – Carl Woese

lampreyFigure 5.2.5c – Lamprey

monkeyFigure 5.2.5d – Monkey

frogFigure 5.2.5e – Frog

turtleFigure 5.2.5f – Turtle

tunaFigure 5.2.5g – Tuna