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I want to know the correct definitions for the terms apomorphy and plesiomorphy. Unfortunately, there seems to be a variety of ways to define these terms depending on the language one uses. Thus, it has been hard to determine the correct definitions. I have settled on definitions that seem correct to me and also have examples. I would like confirmation from someone if I am understanding the definitions correctly.

  • Apomorphy -- any character state or trait novel to a species and its descendants. An apomorphy occurs when a taxon is selected to have a particular trait. Example: within the class reptilia, the suborder serpentes (snakes) has an apomorphy because its members have no legs.
  • Plesiomorphy -- an evolutionary trait or character state that is homologous within a particular taxon but is not unique to members of that group and therefore cannot be used as a diagnostic character for the group. Example: within the class reptilia, legs are a plesiomorphy for its members.

My Question:

Are these definitions and examples accurate?

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That sounds pretty good. On this kind of things wikipaedia is very reliable. You can also look at this very synthetic page:

https://www.enotes.com/homework-help/distinguish-between-synapomorphy-apomorphy-304128

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  • $\begingroup$ The link has expired. $\endgroup$ – Tyto alba May 16 '17 at 19:42
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Apomorphy can simply be defined as derived trait or character state that is distinct to a particular species or group in a phylogenetic tree down to its descendants. The example you cited is fine.

Plesiomorphy is an ancestral or primitive traits that are homologous to a certain group of organisms but not unique to the other members of the group, hence cannot be used as a diagnostic character for the group.

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  • $\begingroup$ Please try adding citation or scientific references to your answer. $\endgroup$ – Imtiaz Raqib Oct 16 '17 at 2:13

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