Homology

Main Points

  • Homology is the similarity due to inheritance from a common ancestor.
  • Homoplasy or convergence are similar features that evolved multiple times – not resulting from a shared ancestor.
  • It’s not always easy to understand homology but we will not be able to understand the tree of life without it!

Homology is defined as similar features shared between organisms due to inheritance from a common ancestor. To learn more about inheritance, travel to the Evolutionary Synthesis page by clicking here.

Examine the diagram below of different forearms. These animals are quite different, and the forearm morphology is reflective of the use of that forearm (flying, walking, digging, etc.). The forearms all contain the same bones: a humerus, ulna and radius, and digit bones (carpals, metacarpals, and phalanges). These arm structures are similar because we all share a common ancestor on the Tree of Life that originally had this forearm bone structure. All of that ancestor’s descendants have the same basic forearm structure.

Arm bones of a variety of animals with each bone colored: humerus is red; radius is yellow; ulna is green; carpels are blue; metacarpels are orange; and phalanges are purple. Notice that although these animals are all very different and live in a variety of environments, they all share common bone structures.

Conversely, there are certain features of animals that look similar but are not a result of inheritance from a common ancestor. These features are called convergent or homoplastic. A good example of homoplasy is thinking about wings of different animals. Let’s think about the wings of bats, pterosaurs, and birds. Each of these animals have wings and the bones that make up the wings are homologous but the way the wings are formed are very different. The result of this difference is from the feature ‘wings’ evolving multiple times on the Tree of Life.

As scientists, there are several ways to figure out if two characters are homologous or homoplastic. There are tests you can do before you run a mathematical evolutionary analysis. Tests run before these analyses are called a priori (Latin: from before) assessments!

One of the first tests scientists conduct to figure out if two characters are homologous or homoplastic is a test of similarity. The test of similarity, simply put, is if the feature in question in both organisms is similar in its fine details, then the hypothesis that they are homologous is not yet rejected as a possibility. If we were to compare bird wings and bat wings using the test of similarity, we would see that they are constructed entirely differently, and we would reject the hypothesis that they are homologous using this test. The second test is congruence. The test of congruence states that if two features in question are homologous, they can’t appear in the same organism at the same time. If the organism has both, then the structures cannot be homologous and the hypothesis is rejected.

There is an additional test (called conjunction) you can perform after (so a posteriori) using a mathematical framework to determine evolutionary relationships among the animals in question. This creates a tree that you can then analyze how the tree came to be. Several computer programs allow you to examine each character you input for the animal and see how it changes over a tree. If the character you are interested in (let’s say wings) clearly appears multiple times on a tree, you deem this character homoplastic and reject the hypothesis that the character the organisms have in common is homologous. If the character only appears once, then we support the hypothesis that it is homologous.

Issues with understanding homology

The examples stated above are easily understood and well-accepted in the scientific community. Other animals, such as blastoids or foraminifera are less studied to identify these homologous structures and it can be much more difficult to identify them.

Without a strong understanding of homology, building the evolutionary tree for a group can be difficult and often misinterpreted. The example below shows how if we were to think that the character “wings” was homologous we would end up with birds and pterosaurs as sister taxa (see Reading the Tree of Life for details) when, in fact, birds and dinosaurs are more closely related.

An example of incorrectly placed homologous elements in the top. Wings would place birds and pterosaurs together but in fact birds are more closely related to T. rex. This means that wings appear more than once and are not a result of a common ancestor.

Proceed to ‘Extinction’

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