Homology refers to two things that have an equivalent role or relationship. Homology is important in comparative biology since it makes it possible to determine if two different animals or plants share a common ancestor. Essentially, homology in this context means that there are two species that have related parts that do similar things, but that are not exactly the same.
Following are some examples of homology:
- The arm of a human, the wing of a bird or a bat, the leg of a dog and the flipper of a dolphin or whale are homologous structures. They are different and have a different purpose, but they are similar and share common traits. They are considered homologous structures because they have a similar underlying anatomy.
- The forelimbs of a frog, a bird, a rabbit and a lizard look very different because they have evolved differently to account for the specific lifestyles of each animal. However, they share a common ancestor and a common set of bones (the radius, ulna and humerus). The shared bones date back to a prehistoric fish that emerged onto land, eventually becoming an extinct transitional animal from which they evolved. These different structures are also homologous to the arms of people, the wings of the bat, and the other animal parts described above.
- The pelvis of a dog, of a cat and of a human and of a snake are homologous structures.
- The tailbone of a human being and the tail of a monkey are examples of homology. The tailbone is actually called the tailbone because of this shared lineage. Because a human doesn't actually have a tail but the tailbone is the last vestige or remains of where a tail would be, it is referred to as "vestigial."
- The leaves of a pitcher plant, a Venus fly trap, a cactus and a poinsettia are all examples of homology. They are homologous structures because, although they have different shapes and different functions today, they all share a common ancestor.
- The mouthparts and the antennae of different insects such as the grasshopper, the honeybee, the butterfly and the mosquito are used for different purposes. The grasshopper, for example, primarily bites and chews while the honeybee bites and the butterfly sucks pollen. Although different today, these different parts are examples of homology because they share the same basic structure which was just modified or enlarged as needed for the particular species.
- Non-identical chromosomes in DNA that can pair with each other and that are believed to share common ancestry are also examples of homology. They can also indicate that there is a common or shared function in the chromosomes.
Homologous structures can be traced back to the last common ancestor that the animal's shared. For example, if the wing of a bat and the forearm of a person are examples of homology, this means that the anatomical structure of the forearm and of the wing were both present in the last common ancestor along the evolutionary chain that was shared by the bird and bat.
Now you have lots of examples of homology and you have a better understanding of how homology works in the field of comparative biology.