Homology: Definition & Examples

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

Homologous organisms are everywhere. This lesson defines homology and offers different examples of homology. Take the quiz that follows to demonstrate your understanding of this topic.


Have you ever thought about how similar or different living organisms are to each other? For instance, maybe you thought that the wings of birds and bat or the fins on sharks and dolphins really look similar to one another. This thought process falls under the umbrella of homology. Homology is the study of similarities between organisms to determine common ancestors based on genes, physiology or development. The structures or genes that fall under homology are referred to as being homologous.

Examples of Homology

Let's look at the wings that I just mentioned along with the front limbs of some other animals with which you may not have associated those wings. The front limbs on bats, birds, humans and lizards are all homologous because they are composed of the same types of bones. The basic structure of these limbs are made up of the radius, ulna and humerus. This provides some proof to the possibility that we and the other animals mentioned all came from a common ancestor years ago. I bet you are thinking that it is hard to believe that you and the bat that you never want to encounter could have a common ancestor, but homology says otherwise. Homology is not just found in physical structures or just in animals, so let's look at some other examples.

The humerus, ulna and radius
Picture showing the bones of the forelimbs

An example of homology that has really been interesting to discover has to do with the genes that control the development of the eyes. Almost all animals - whether we are talking about dogs, humans, flies or birds - have the same type of gene that codes for the creation of our eyes. Relatively recently, geneticists even took a version of the gene that codes for eyes in humans and inserted it into flies, and it resulted in the development of eyes in the flies. This has been repeated between many other animals as well now. The fact that this is even possible means that the gene had to have come from an ancestor that is shared by any animal in which the gene works.

The eyes of animals are an example of homology
Clipart of eyes

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