Did You Know?
The term 'symplesiomorphy' was first used in the year 1950 by a German biologist named Willi Hennig.
Classification is a method in biology which helps us divide organisms into different groups, in such a way that the organisms in each group have similar characteristics. This Linnaean method for taxonomy was widely popular till the 1960s, but it says little about how organisms have evolved. This has led to the rise of a new system called 'cladistics', which groups those organisms together who show similar ancestral characteristics.
In this system, each group is called a 'clade', and contains an ancestor who first evolved a characteristic, along with its descendants. Depending on whether a characteristic is simply transmitted to the descendants or evolved further, and how primitive it is, these traits are also categorized into different types under the cladistics system. One of these types is a symplesiomorphy, which is explained in the following sections.
A symplesiomorphy is a property shown by different groups of organisms as a result of a common ancestry between them. The ancestor who evolved this trait is a primitive, rather than a recent one. Thus, symplesiomorphies can be described as 'shared and primitive' characters, rather than a derived character (evolved only in a recent ancestor). Such properties are homologous, which means that they are similar in structure and position, though they may have different roles.
Symplesiomorphies are of little use in deciding how different species or groups are related to each other. This is because, while two groups may show a common trait, this doesn't mean that they evolved it for the first time; it only indicates a common ancestry. This means that, these groups are not the only ones showing this trait. Other groups not included in the study may show them too. For this reason, only synapomorphies can be used to establish evolutionary relationships, because they group organisms based on a property which is solely seen in them, along with a recent ancestor who evolved it first.
All mammals have four limbs. However, this characteristic is not unique to this group, as all members of the 'tetrapods' category, which includes reptiles, amphibians, and birds, along with mammals, possess four limbs. Since these groups evolved long before mammals did, the trait of having four limbs is a symplesiomorphy for mammals. In the same way, having an amniotic egg (embryo develops inside membrane) and a vertebra is a symplesiomorphy for mammals, since these properties were first evolved by reptiles and birds.
Liverworts (small, flowerless plants) were the first to evolve among land plants, followed by conifers, and finally angiosperms (flowering plants). All these groups show the stage of a multicellular sporophyte (producing spores) in their lives, but since it was first evolved in liverworts, this property is a symplesiomorphy for conifers and angiosperms, as a primitive ancestor (liverworts) first evolved it. Moreover, it is not restricted to angiosperms and conifers alone.
Most angiosperms show a simple (undivided) leaf structure. But this property does not indicate that all groups of plants in this category are closely related simply because they show this property. It developed in a primitive ancestor of angiosperms, and thus, it only indicates that all groups within it share a common ancestry.
Both, bony (skeleton of bone) and cartilaginous (skeleton of cartilage) fish breathe by their gills. It has been established that land vertebrates (with backbone) like mammals have evolved from a group of bony fish, which respired by lungs, or through their skin. But simply because most bony fish breathe by their gills like cartilaginous fish, one cannot say that they are more closely related to the latter than to terrestrial vertebrates.
The cells of both, plants and animals show a structure called 'mitochondria'. But this doesn't indicate anything about their relation to each other, because plants and animals, along with a number of other groups, are included within the category of 'eukaryotes'. All eukaryotes show a mitochondrion within their cells. Therefore, this property is a symplesiomorphy for plants and animals, because their more primitive ancestors (bacteria) evolved it first.
One similarity among primates (mammals with hands) and non-primate mammals is that most of them have 5 digits on their forelimbs and hind-limbs. But this should not be taken as a sign of their evolutionary relationship, as many primitive ancestors of mammals, including dinosaurs and amphibians, showed the same property.
In the category of 'tetrapods', egg-laying is a symplesiomorphy found in birds, reptiles, and some mammals (called 'monotremes'), but absent in other mammals. This characteristic cannot be used to construct relationships, because it was first evolved in primitive amphibians and reptiles, and not in recent ancestors.
Deciding whether a property is a symplesiomorphy or a synapomorphy, depends on the groups involved, and their position on the evolutionary tree. For this reason, symplesiomorphies can be synapomorphies, and vice versa. For instance, in the above example about mitochondria, if the whole group of eukaryotes is considered, this property is a synapomorphy, since it first evolved in and is restricted to this group alone. However, because plants and animals evolved relatively late, it is a symplesiomorphy for them.