A synapomorphy is a shared, derived character, common between an ancestor and its descendants. A character, or trait, is anything observable about the organism. It may be the size of the organism, the type of skin covering the organism has, or even things like eye color. A character may also be considered a specific sequence of DNA, which is how modern phylogenetic trees are constructed. As seen in the image below, a synapomorphy could be any characteristic shared by the descendants of a common ancestor.
In fact, the term synapomorphy comes from the Greek “syn” meaning shared, “apo” meaning away from, and “morphe” meaning form or shape. In other words, the animals have a shared form as they move away from their ancestors and related animals. A synapomorphy can help scientists determine which groups of animals are related, and which aren’t. Animals which share a synapomorphy likely share a common ancestor. If groups of organisms share more than one synapomorphy, there is even more evidence that they are related.
A synapomorphy is also known as a homology,
Synapomorphy vs Apomorphy
An apomorphy, as pictured in the image above, is a shared characteristic between two or more groups of organisms. An apomorphy becomes a synapomorphy when it is shown that the trait also belonged to a common ancestor. This last step must take place in the fossil record, and is often hypothetical because we can never truly know which animals reproduced to create the organisms we see today.
A synapomorphy can reveal the relatedness of two species through its very presence. If a trait exists in two organisms, and is present in their most recent common ancestor, the trait can signal a clade. A clade is a term used when describing phylogenetic relationships. A clade denotes that all the organisms within the clade are related to a single common ancestor. Clades often contain many synapomorphies because the animals are so closely related. However, as organisms become new species they can develop new and unique characteristics. A novel trait is considered an autapomorphy.
Synapomorphy vs Plesiomorphy
In contrast to a synapomorphy, a plesiomorphy is a shared character, shared by two groups who inherited it from different ancestors. In the image above, the plesiomorphy identifies a character shared by two groups. Because the character (grayness) is not present in the darker organisms (black circles), the trait cannot be considered a synapomorphy. A synapomorphy says more about the relatedness of two species, because it indicates that the two organisms shared a common ancestor.
Synapomorphy vs Homoplasy
A homoplasy is the opposite of a homology, or synapomorphy. A synapomorphy implies that a homologous trait, one that is the same in both organisms, was inherited from the same ancestor. A homoplasy, on the other hand, is simply a trait that appeared in different organisms. This happens often in evolution, as different species evolve to accomplish the same tasks. Wings, for instance, have evolved a number of times. However, if one were to say the wings of birds and the wings of insects were a synapomorphy, that statement would be incorrect. Wings in birds and insects are a homoplasy, a trait which is similar but not from a common ancestor. Likewise, wings in birds and bats represent a homoplasy, not a synapomorphy because they were not inherited from the same organisms. Wings have evolved a number of times throughout evolution because the open air is a desirable niche which organisms can occupy.
Examples of Synapomorphy
Mammals share a synapomorphy of being able to produce milk. Milk is a nutritive substance which is excreted from the body and fed to the babies. While some people consider mammals to be furry animals with a placenta which give live birth, this definition excludes several obvious groups of mammals. The Monotremes, such as the platypus, still lay eggs but they feed their young milk which they excrete from glands. While their other features might make them seem more like birds or reptiles, milk production is a clear synapomorphy with the other mammals.
The Marsupials represent another group of mammals which does not fully conform to other shared traits of mammals. The marsupials give live young, but raise their tiny, undeveloped offspring in a pouch until it is fully developed. More “typical” mammals have developed larger placentas and brown adipose tissue to sustain their babies and increase their development during gestation.
All vertebrate animals share a single trait, the vertebrae. Vertebrae exist only within the Vertebrates, and are a synapomorphy of the subphylum. While all vertebrate organisms share this trait with a common ancestor, they differ in many other ways. In fact, the synapomorphy of having a vertebrae is just one clue that the animals are related. Other, related characteristics can obscure this relationship. For instance, the size, shape, and number of vertebrae can change depending on the organism.
Some organisms, such as the terrestrial vertebrates, have more derived vertebrae which support limbs and the weight of the organism on land. The buoyancy of water alleviates the strain of gravity, which is why most fish vertebrae are made of cartilage or weak bones. Terrestrial vertebrates must have much more rigid bones to support the weight of gravity in air. This is one reason why marine animals tend to get much larger than terrestrial ones.