Apomorphy and Synapomorphy in Phylogenetic Analysis
In the study of evolutionary relationships, scientists use specific terminology to describe how traits change over time. At the heart of this analysis is the apomorphy, or derived trait. An apomorphy is a novel character or character state that has evolved from an ancestral form, known as a plesiomorphy. By identifying these changes, biologists can reconstruct the history of life and determine how different species are related.
When an apomorphy is shared by two or more taxa (groups of organisms), it is called a synapomorphy. In the field of cladistics—the method of classifying animals and plants according to the succession of newly evolved characteristics—a synapomorphy implies homology, meaning the trait was inherited from a common ancestor.
For example, mammals possess several derived traits that are not found in other vertebrates like reptiles or amphibians, such as mammary glands, fur, an erect gait, and three middle ear bones. Because these traits are shared by mammals but absent in their ancestors, they serve as synapomorphies that define the mammal group.
![Phylogenies showing the terminology used to describe different patterns of ancestral and derived character or trait states.[1]](/images/69/4d/694d0473b350d7129a2d9ae2a4916767d64fb433fb0265c1d973873ac857a48c.jpg)
Key Facts
- Apomorphy: A derived trait that has evolved from an ancestral state.
- Synapomorphy: A derived trait shared by two or more taxa, indicating a common ancestor.
- Plesiomorphy: An ancestral character state.
- Autapomorphy: A derived trait unique to a single taxon.
- Character Polarity: The determination of whether a trait is ancestral or derived.
- Homoplasy: A trait shared by species but not present in their common ancestor due to convergent evolution.
Determining Character Polarity
To avoid circular reasoning in genealogical classifications, researchers must determine character polarity—the distinction between which state is ancestral and which is derived—without relying on the classification they are trying to build.
This is often achieved by using common-sense presence/absence characters as a scaffold. For instance, feathers are unique to birds. Once the group "birds" is defined by the synapomorphy of feathers, scientists can evaluate other characters to identify larger clades (such as the diapsid skull for diapsids) or more specific clades (such as the syrinx for songbirds).
Clade Analysis and the Tree of Life
The identification of a synapomorphy depends entirely on the specific clade (a group consisting of an ancestor and all its descendants) being analyzed. These relationships are often visualized using cladograms, which are tree or ladder-like diagrams that serve as predictive devices in modern genetics.
A trait's status can change depending on the level of the hierarchy being discussed. For example, mammary glands are a synapomorphy when comparing mammals to other tetrapods. However, when comparing rodents to primates, mammary glands are a symplesiomorphy—an ancestral trait shared by both groups.
This highlights the relationship between different types of derived traits: an autapomorphy is a trait unique to one group, while a synapomorphy is shared. Evolutionarily, a trait is "newer" or "older" relative to the apomorphy under consideration.
Comparative Trait Summary
| Term | Definition | Relationship to Ancestry |
|---|---|---|
| Apomorphy | Derived trait | New/Evolved |
| Plesiomorphy | Ancestral trait | Original/Primitive |
| Synapomorphy | Shared derived trait | Inherited from common ancestor |
| Autapomorphy | Unique derived trait | Specific to one taxon |
| Symplesiomorphy | Shared ancestral trait | Inherited from distant ancestor |
Complex Evolutionary Patterns
Not all shared traits indicate a close common ancestor. Homoplasy occurs when a trait is gained or lost independently in separate lineages. This can happen through convergence (independent evolution of similar traits) or parallel homoplasy.
Other complex patterns include:
- Reversal: The loss of a derived trait and the re-establishment of an ancestral state.
- Pseudoplesiomorphy: A trait that appears ancestral but is actually a reversal.
- Underlying Synapomorphy: A shared derived trait that has been lost in many members of the clade.
- Hemiplasy: A trait that appears homoplastic on a species tree but has a single origin on a gene tree, often due to multispecies coalescent discordance.
Frequently Asked Questions
What is the difference between a synapomorphy and a symplesiomorphy?
A synapomorphy is a derived trait shared by a group that was not present in their distant ancestors, helping to define a specific clade. A symplesiomorphy is an ancestral trait shared by a group that was already present in their more distant ancestors and therefore does not help distinguish the group from other related clades.
How do scientists determine if a trait is an apomorphy or a plesiomorphy?
This process is called determining character polarity. Scientists use known unique features of certain taxa as a scaffold to infer whether other traits are ancestral (plesiomorphic) or derived (apomorphic) without relying on circular arguments.
What is the difference between homology and homoplasy?
Homology refers to traits shared because they were inherited from a common ancestor (such as the bone structure in mammal limbs). Homoplasy refers to traits that appear similar but evolved independently through convergence or reversal, rather than from a shared ancestor.
What is an autapomorphy?
An autapomorphy is a distinctive derived trait that is unique to a single taxon or group. Unlike a synapomorphy, it cannot be used to group multiple taxa together because it is only found in one.
What is a reversal in phylogenetics?
A reversal occurs when a lineage loses a derived trait that was present in its ancestor and returns to a state that resembles the original ancestral (plesiomorphic) trait.