Synapomorphy and apomorphy

derivedsynapomorphiessynapomorphyapomorphyapomorphicapomorphiessynapomorphicsynapomorphderived traitcharacteristics
In phylogenetics, apomorphy and synapomorphy refer to derived characters of a clade: characters or traits that are derived from ancestral characters over evolutionary history.wikipedia
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Monophyly

monophyleticmonophyletic groupnon-monophyletic
In other words, it is an apomorphy shared by members of a monophyletic group, and thus assumed to be present in their most recent common ancestor.
Monophyletic groups are typically characterised by shared derived characteristics (synapomorphies), which distinguish organisms in the clade from other organisms.

Elephant

elephantstrunkelephant tusk
However, in the Afrotheria clade, which includes elephant shrews, golden moles and elephants, there is an increase in the number of thoracolumbar vertebrae.
Early proboscideans developed longer mandibles and smaller craniums while more derived ones developed shorter mandibles, which shifted the head's centre of gravity.

Autapomorphy

autapomorphiesautapomorphautapomorphic
So the concept can be understood as well in terms of "a character newer than" (autapomorphy) and "a character older than" (plesiomorphy) the apomorphy: mammary glands are evolutionarily newer than vertebral column, so mammary glands are an autapomorphy if vertebral column is an apomorphy, but if mammary glands are the apomorphy being considered then vertebral column is a plesiomorphy.
In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon.

Tetrapod

tetrapodsTetrapodaland vertebrates
For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a symplesiomorphy for mammals in relation to one another—rodents and primates, for example.
The first tetrapods (from a traditional, apomorphy-based perspective) appeared by the late Devonian, 367.5 million years ago; the specific aquatic ancestors of the tetrapods, and the process by which they colonized Earth's land after emerging from water remains unclear.

Homology (biology)

homologoushomologyhomolog
As implied in this definition, many cladists consider secondary homology to be synonymous with synapomorphy, a shared derived character or trait state that distinguishes a clade from other organisms.

Cladistics

cladisticcladistic analysisclade
So the concept can be understood as well in terms of "a character newer than" (autapomorphy) and "a character older than" (plesiomorphy) the apomorphy: mammary glands are evolutionarily newer than vertebral column, so mammary glands are an autapomorphy if vertebral column is an apomorphy, but if mammary glands are the apomorphy being considered then vertebral column is a plesiomorphy.
Hypothesized relationships are typically based on shared derived characteristics (synapomorphies) that can be traced to the most recent common ancestor and are not present in more distant groups and ancestors.

Plesiomorphy and symplesiomorphy

plesiomorphicplesiomorphsymplesiomorphy
So the concept can be understood as well in terms of "a character newer than" (autapomorphy) and "a character older than" (plesiomorphy) the apomorphy: mammary glands are evolutionarily newer than vertebral column, so mammary glands are an autapomorphy if vertebral column is an apomorphy, but if mammary glands are the apomorphy being considered then vertebral column is a plesiomorphy. For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a symplesiomorphy for mammals in relation to one another—rodents and primates, for example. What counts as a synapomorphy for one clade may well be a primitive character or plesiomorphy at a less inclusive or nested clade.
A plesiomorphy or a plesiomorphic character can be seen as the opposite of an apomorphy, which is a derived character, an “evolutionairy novelty”.

Relative apparent synapomorphy analysis

RASA
A new method of measuring phylogenetic characteristics is the use of Relative Apparent Synapomorphy Analysis (RASA).
A synapomorphy is a shared trait found among two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait.

Cladogram

cladogramscladistic analysiscladistic interpretation
Cladograms are diagrams that depict evolutionary relationships within groups of taxa.
Researchers must decide which character states are "ancestral" (plesiomorphies) and which are derived (synapomorphies), because only synapomorphic character states provide evidence of grouping.

Phylogenetics

phylogeneticphylogenetic analysisphylogenetically
In phylogenetics, apomorphy and synapomorphy refer to derived characters of a clade: characters or traits that are derived from ancestral characters over evolutionary history.

Clade

cladesgroupcladistic
In phylogenetics, apomorphy and synapomorphy refer to derived characters of a clade: characters or traits that are derived from ancestral characters over evolutionary history.

Phenotypic trait

traittraitscharacters
In phylogenetics, apomorphy and synapomorphy refer to derived characters of a clade: characters or traits that are derived from ancestral characters over evolutionary history.

Most recent common ancestor

last common ancestorcommon ancestorrecent common ancestor
In other words, it is an apomorphy shared by members of a monophyletic group, and thus assumed to be present in their most recent common ancestor.

Afrotheria

afrothereafrotherianafrotheres
However, in the Afrotheria clade, which includes elephant shrews, golden moles and elephants, there is an increase in the number of thoracolumbar vertebrae.

Elephant shrew

MacroscelididaeMacroscelideasengi
However, in the Afrotheria clade, which includes elephant shrews, golden moles and elephants, there is an increase in the number of thoracolumbar vertebrae.

Golden mole

ChrysochloridaeAmblysominaeChrysochlorinae
However, in the Afrotheria clade, which includes elephant shrews, golden moles and elephants, there is an increase in the number of thoracolumbar vertebrae.

Willi Hennig

HennigE. H. W. HennigEmil Hans Willi Hennig
The word synapomorphy—coined by German entomologist Willi Hennig—is derived from the Greek words σύν, syn = shared; ἀπό, apo = away from; and μορφή, morphe = shape.

Greek language

GreekAncient GreekModern Greek
The word synapomorphy—coined by German entomologist Willi Hennig—is derived from the Greek words σύν, syn = shared; ἀπό, apo = away from; and μορφή, morphe = shape.

Convergent evolution

convergentconvergenceanalogous
The objective of analysis is to determine if a given characteristic is common between taxa as a result of either shared ancestors or the process of convergence.

Mammary gland

mammaemammary glandsmammary
For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a symplesiomorphy for mammals in relation to one another—rodents and primates, for example.

Mammal

mammalsMammaliamammalian
For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a symplesiomorphy for mammals in relation to one another—rodents and primates, for example.

Evolution of tetrapods

tetrapodsevolvedevolving into the amphibian tetrapods
Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.

List of fossil bird genera

List of fossil birdsFossil birdsbirds
It is not certain that the pygostyles found in birds are indeed synapomorphies.

Moraceae

mulberry familyMoraceae sp.mulberry
The only synapomorphy within the Moraceae is presence of laticifers and milky sap in all parenchymatous tissues, but generally useful field characters include two carpels sometimes with one reduced, compound inconspicuous flowers, and compound fruits.