When is a taxon valid?
A taxon is a group, such as an order or domain, into which animals are classified. Apparently, you have to know this question for the exam. But when can something like this be invalid?
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The classification of living beings (not only animals) in taxa must be carried out according to certain rules, and if these rules were not observed, an invalid taxon comes out.
Modern taxonomy follows the rules of cladicals, ie valid taxa must be based on relationship and include all descendants of a common ancestor. Such groups are called caches or monopyhletic groups.
In addition, there are two types of invalid taxa: first paraphyletic groups which do not include all descendants of an ancestor (eg "fish", which excludes the terrestrial vertebrates, or "reptias", which excludes the birds) and second polyphyletic groups that throw together species that are not related to each other (eg "worms", which are actually completely different).
Thank you.
Thanks for the star!
You've written paraphysically twice.
Improved, thank you!
You're welcome.