A homoplasy is a shared character between two or more animals that did not arise from a common ancestor. A homoplasy is the opposite of a homology, where a common ancestor provided the genes that gave rise to the trait in two or more animals. Often, a homoplasy will occur when two very different groups of animals evolve to do the same thing. This is known as convergent evolution, or convergence. Sometimes, a homoplasy trait is called an analogous trait. The best way to gain an understanding of what is and is not a homoplasy is to go over some examples.
Homoplasy in Wings
The easiest homoplasy to understand is the trait of wings. Throughout the animal kingdom, wings have evolved in a number of various shapes and materials, but their fundamental function is the same: flight. Birds, bats, and many insects have evolved wings. In each case, the trait evolved independently of the other groups. The closest common ancestor of birds, bats and insects most certainly did not have wings. After the lineages diverged, or headed off in their own direction, a similar pressure of flight being advantageous caused all lineages to develop flight.
In each case, they also found their own way to develop wings. Bird wings are specially adapted forelimbs covered in feathers. The tarsals and metatarsals (hand and wrist bones) are formed in birds in such a way that they effectively have no fingers, but instead have an elongated limb that forms a strong leading edge for the wing. The feathers serve to give wing structure and, in this way, lift is generated, much like by the wings of an airplane.
Bats, like birds, also have modified wrist and finger bones. Unlike birds, bats do not have feathers, as this trait never evolved in bats. Because of this, bats support their wings with very long finger bones, or tarsals. Thus, in the same way as birds, bats create lift with their wings and are able to fly. Insects are another group of animals with the ability to fly, and their wings are even more complex.
Because of the complexity of the insect world, it is not certain whether insect wings are a homoplasy or a homology. Imagine butterfly wings. If you were to look up close, you would see that these enormous wings (compared to the insect) are covered in small scales, which make beautiful colors. The butterfly flaps them slowly and seems to glide through the air with ease. Compare these enormous, beautiful wings to those of a beetle. The beetle, to get his wings out, must open his hard outer covering and unfold or expand his much more fragile wings. They are thin, translucent (you can see through them), and they do not appear strong enough to be able to carry the beetle. Then, the beetle flaps them at an enormous rate and is quickly carried away by the lift they generate.
To determine whether beetle and butterfly wings are a homoplasy or a homology, scientists must look at the genetic lineage of beetles and butterflies and determine if their common ancestor is the reason they have wings. Although wings in insects were once thought as a completely homologous trait, more genetic evidence has begun to show that wings have evolved multiple times in insects.
Homoplasy in Beaks
While not an often cited homoplasy, a squid and a falcon share a trait. At the opening of their mouth is a large beak, often sharp and meant to tear their prey apart. However, it can be seen immediately from their forms, locations of living, and closest genetic relatives, that the octopus and the falcon did not get their beaks from a common ancestor. The beaks evolved through convergence, or in other words, a similar need to rip throat-sized chunks from a prey animal. While it might not be pretty, evolution does tend to produce similar results given similar circumstances.
Not a Homoplasy
Now that you have a decent understanding of what a homoplasy is, let’s go over what it is not. Any time the trait is passed from parent to offspring, the trait is not a homoplasy. If a parent passes the trait to their offspring, the trait is a homology. When the trait gets passed down a long line of ancestors, the descendants can start to vary from each other in many ways. However, if they both still possess the trait, it is still a homologous trait, and not a homoplasy.
For instance, we are all familiar with mammals. Scientist, through years of study of their defining traits, and more recently, confirmations provided by genetic testing, have shown that mammals are a definable group of animals. These animals, by definition, have mammary glands which they use to feed their young. Although the mammary glands of whales and cows look different, and function in different ways, they are evolved from a common ancestor that had a primitive form of mammary glands. Therefore, mammary glands in whales and cows are homologous, not homoplastic.