Homology vs. Homoplasy in Evolutionary Science

Homology and Homoplasy of Bat and Bird Wings
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Two common terms used in the science of evolution are homology and homoplasy. While these terms sound similar (and indeed have a shared linguistic element), they are quite different in their scientific meanings. Both terms refer to sets of biological characteristics that are shared by two or more species (hence the prefix homo), but one term indicates that the shared characteristic came from a common ancestor species, while the other term refers to a shared characteristic that evolved independently in each species. 

Homology Defined

The term homology refers to biological structures or characteristics that are similar or the same found on two or more different species, when those characteristics can be traced to a common ancestor or the species. An example of homology is seen in the forelimbs of frogs, birds, rabbits and lizards. Although these limbs have a different appearance in each species, they all share the same set of bones. This same arrangement of bones has been identified in fossils of a very old extinct species, Eusthenopteron, which was inherited by frogs, birds, rabbits, and lizards. 

Homoplasy Defined

Homoplasy, on the other hand, describes a biological structure or characteristic that two or more different species have in common that was not inherited from a common ancestor. A homoplasy evolves independently, usually due to natural selection in similar environments or filling the same type of niche as the other species who also have that trait. A common example often cited is the eye, which developed independently in many different species. 

Divergent and Convergent Evolution

Homology is a product of divergent evolution. This means that a single ancestor species splits, or diverges, into two or more species at some time in its history. This occurs due to some type of natural selection or environmental isolation that separates the new species from the ancestor. The divergent species now begin to evolve separately, but they still retain some of the characteristics of the common ancestor. These shared ancestral characteristics are known as homologies.

Homoplasy, on the other hand, is due to convergent evolution. Here, different species develop, rather than inherit, similar traits. This may happen because the species are living in similar environments, filling similar niches, or through the process of natural selection. One example of convergent natural selection is when a species evolves to mimic the appearance of another, such as when a non-poisonous species develops similar markings to a highly venomous species. Such mimicry offers a distinct advantage by deterring potential predators. The similar markings shared by the scarlet king snake (a harmless species) and the deadly coral snake is an example of convergent evolution. 

Homology and Homoplasy in the Same Characteristic

Homology and homoplasy are often difficult to identify, since both may be present in the same physical characteristic. The wings of birds and bats is an example where both homology and homoplasy are present. The bones within the wings are homologous structures that are inherited from a common ancestor. All wings include a type of breastbone, a large upper arm bone, two forearm bones, and what would be hand bones. This basic bone structure is found in many species, including humans, leading to the correct conclusion that birds, bats, humans, and many other species share a common ancestor. 

But the wings themselves are homoplasies, since many of the species with this shared bone structure, including humans, do not have wings. From the shared ancestor with a certain bone structure, natural selection eventually led to the development of birds and bats with wings that allowed them to fill a niche and survive in a particular environment. Meanwhile, other divergent species eventually developed the fingers and thumbs necessary to occupy a different niche.