Terminology for characters Flashcards
What are characters and character states?
A CHARACTER is an OBSERVABLE FEATURE of an organism, e.g. the number of eyes or the nature of the horny tips, to the digits in most tetrapods.
CHARACTER STATE is the CONDITION ACTUALLY OBSERVED, e.g. for the character “number of eyes” the states might be 2, 4, 6 etc.; the character “nature of the horny tips to the digits” might be claws, nails or hooves.
Explain binary and multistate characters.
A character that has only two states, e.g. presence or absence of wings. Coded as 0 or 1, vs. A character that has more than two states, e.g. whether the horny tips to the digits are claws, nails or hooves. Coded as 0, 1, 2, etc. depending on the total number of states.
Explain additive and non-additive characters
Non-additive characters are multistate characters for which any state can change into any other state as only one evolutionary step, e.g. 2 eyes can change to 4 eyes, 6 eyes or 8 eyes and it is only one evolutionary change.
Additive Characters are multistate characters for which only stepwise evolution is possible, e.g. 2 eyes can change to 6 eyes only through 4 eyes as an intermediate condition, thus, the change from 2 to 6 eyes is two evolutionary changes.
Homology?
Two attributes of two taxa are homologous if either:
1) They are the same as the character state found in the common ancestor of the two taxa, e.g. the forelimbs of mammals and lizards are homologous.
2) They are different character states that have an ancestor/descendant relationship, e.g the wings of birds and the forelimbs of lizards.
What is a homoplasy?
A homoplasy is a similarity in character state among taxa which is not homologous. Homoplasies result from parallel or convergent evolution or reversals. What appear as homologous at the beginning of an analysis may turn out to be homoplasious. For example, the repeated loss of limbs in tetrapods – independently in snakes, various groups of legless lizards and amphisbaenids.
What is an apomorphy?
A derived character state. Synapomorphies are shared derived character state. Cladistics seeks to discover synapomorphies and cladograms are synapomorphy schemes.
What is a plesiomorphy?
The ancestral character state. Apomorphy and plesiomorphy are relative terms depending upon the level of analysis being conducted. For example, among the Insecta as a whole, wings are an apomorphic condition whereas in the insect order Diptera, wings are a plesiomorphic condition (there are flies that can’t fly because they don’t have wings).
Symplesiomorphy?
An ancestral character state shared by two or more taxa. Symplesiomorphies are misleading and should not be used to classify organisms.
Define autapomorphy
An autapomorphy is a uniquely derived character state. Autapomorphies are relative to the level of analysis. For example, the presence of feathers is autapomorphic for the class Aves in a class level phylogeny of extant vertebrates but synapomorphic for all bird subtaxa.
Define polarity
The direction of evolutionary change among character states of a character, e.g. the polarity for nails and claws in mammals is from claws to nails, the latter are synapomorphic for primates.
