SYNAPOMORPHY: DEFINITION AND EXAMPLES - BIOLOGY - 2025

A synapomorphy is any character that is unique to a group of species and the common ancestor that defines them. The term comes from the Greek and means "based on the shared form."

Synapomorphies allow defining taxa in the field of evolutionary biology. Therefore, they have interpretive value only within the taxonomic level at which they are speaking. That is, they are relative.

Mammals

Synapomorphies are derived characters that define a point of divergence at which a taxon followed a different evolutionary path from a sister taxon. A synapomorphy is a homology between species of the same taxon that share it.

The mammary glands, for example, are a synapomorphy of mammals, which they define. It is a character shared by all members of the class Mammalia, which is supposed to be monophyletic. In other words, all its members share the same origin, and none is outside the taxon thus defined.

Synapomorphy is a term used by the cladistic school of systematic biology. According to this, all living beings can be classified based on their derived characteristics. In addition, from this analysis, the evolutionary history of the species and the kinship relationships between them can also be considered.

Utility of synapomorphies in evolutionary analysis

Only synapomorphies define the monophyly of a given taxon. Although some species appear not to show the presence of the character, there are two ways to interpret it.

Sometimes, in unique and group-specific evolutionary trajectories, character was lost in a secondary way. That is, the species or groups of species derive from ancestors who shared the character.

A classic case is that of cetaceans that, despite being mammals, do not have hair. Hairs are another synapomorphy of mammals.

A second reason is the appearance of an advanced stage of character change in a group that appears not to have it. That is, they present a modified synapomorphy. This is the case of the reduction of the hind wings transformed into halteres in the insects of the class Diptera.

Unique trajectory

In any case, synapomorphies are the characters used to define the evolutionary study groups in cladistics. To be considered as such, a synapomorphy must have resulted from a unique trajectory.

That is, the complex series of mutations (at all levels and of all kinds) that led to its appearance in the ancestor and his descendants occurred only once.

If another group seems to show the character, it can be analyzed if what is observed is not analogy instead of homology. That is, two different groups may have reached a similar character by different means. This is what in evolutionary biology is called homoplasia.

Ancestral characters

Finally, the simplesiomorphies represent the ancestral characters. That is, those that are shared by two related taxa by common ancestor. Synapomorphies obviously separate the two taxa and define them as such (that is, distinct).

Examples of synapomorphies

The examples that we will give later concern two large groups of living beings. However, synapomorphies can be found at any level of the hierarchical scale of classification of living beings.

That is, every taxon is defined that way precisely because there is at least one synapomorphy that defines it.

Chordates

Chordates are a group of animals (with phylum rank) that are characterized by presenting a notochord or dorsal cord at some point in their development.

They present numerous evolutionary advances and have been able to colonize basically every available habitat on the planet.

The largest group of chordates is that of the class Vertebrata. Chordates have unique or exclusive characters (synapomorphies) that define them, including:

- Presence of a dorsal cord between the digestive and nervous tubes.

- Presence of a dorsal neural tube.

- Segmental longitudinal muscles.

- Pharyngeal openings.

- Endostyle (tunicates, amphoxes, lamprey larvae): the advanced homologous character is the thyroid gland in vertebrates.

- Post-anal tail.

Many of these synapomorphies gave rise to unique evolutionary specializations within these groups of animals. The notochord, for example, gave rise to the vertebral column in vertebrates.

Spermatophytes

Spermatophytes represent the monophyletic group of vascular plants that includes all those that produce seeds.

Therefore, the synapomorphy that defines the group is the production of seeds, not the presence of a vascular system, since other seedless plants also have it. That is, every seedling plant is vascular, but not every vascular plant produces seeds.

It is the group of plants that has the greatest biological diversity, the most extensive geographic distribution, and the most successful ecological adaptations. Among the synapomorphies of seed plants we find:

- Seed production.

- Production of a "secondary" xylem, at least in an ancestral way.

- Axillary branching.

Spermatophytes, in turn, are divided into two large monophyletic groups: gymnosperms, and angiosperms or flowering plants. Each of them has synapomorphies common to the species that comprise them.

Molecular synapomorphy

It should not be understood that all synapomorphy is morphological, structural or functional. That is, not all kinship relationships are established through phenotypes. On the contrary, molecular systematics and molecular evolution have demonstrated the resolving power of the sequences of biological macromolecules.

This is particularly true thanks to advances in increasingly powerful and accessible DNA sequencing techniques. The analysis of DNA and protein sequences has completely revolutionized our view of kinship relationships between species. In fact, they have given a whole new topology to the tree of life itself.

If we compare the nucleotide sequence of a particular gene between different species, we can also find synapomorphies. The amino acid sequences of proteins can also provide this information.

These have proven to be very useful in studies of systematics, phylogeny and evolution. In fact, at present any proposal for a phylogenetic kinship relationship, description of species, evolutionary trajectory, etc., must be supported by molecular data.

This integrative and multidisciplinary vision has clarified many of the doubts that simple morphology and the fossil record did not allow to resolve in the past.

References

1. Hall, BK (2003) Descent with modification: the unity underlying homology and homoplasy as seen through an analysis of development and evolution. Biological Reviews of the Cambridge Philosophical Society, 78: 409-433.
2. Hall, BK (2007) Homoplasy and homology: dichotomy or continuum? Journal of Human Evolution, 52: 473-479.
3. Loconte, H., Stevenson, DW (1990) Cladistics of the Spermatophyta. Brittonia, 42: 197-211.
4. Page, RDM, Holmes, EC (1998). Molecular evolution: a phylogenetic approach. Blackwell Publishing Ltd.
5. Scotland, RW (2010) Deep homology: a view from systematics. BioEssays, 32: 438-449.