Discussion of everything related to the Theory of Evolution.
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A phylum is a taxonomic category (we belong to the phylum Chordata) and a clade is a group of descendants of a common ancestor. So all your family members that are descended from your maternal grandmother form a clade (your mother, but not your father, your mother's sisters and brothers, their children, your sisters and brothers, and you). A phylum is also a clade.
thanks for the reply! so a clade is the same as a monophyletic group? Or do I have it all mixed up should I have really asked about some other phy- word? What I'm confused about is how a clade relates to what mono- para- and poly phyletic groups are about and the difference between cladistics and phylogenetics... sorry for the confusion... all the words that sound so similar have me confused... :/
A clade is a branch of the evolutionary tree comprising all the organisms that have evolved from one same common ancestral population. This is a monophylletic group.
Taxa and clades are not the same. Taxonomist make groups of organism (taxa) that share common traits. These traits can be morphollogical, embriological, molecular or even behavioural, etc. These similarities may reflect the sharing of a common ancestror , but they can have arised by convergent evolution too. In the first case we speak of plesiomorphy (homology), in the second term apomorphy (analogy) is called for.
Cladistics hypothesize that a taxon is a clade and then seek evidence. It often happens that a taxon excludes some of their natural members: this is a paraphylletic group. In other intances, cladistics reveal a taxon to be poliphylletic: it has members with not one but at least two ancestrors.
In the followink link:
you can see that:
- Reptiles are a paraphiletic group since it excludes birds, which are known to have evolved from archosaurs (the order of reptiles to which crocodiles and dinosaurs belong). Hence birds form a paraphilletic group too.
- Cladistics propose the clade Sauropsida including both birds and reptiles. Paleontological, anatomical, embriological, molecular and behavioural evidence support their common ancestry.
- Living homeotherms are a poliphyletic group as birds and mammals evolved separately.
Hope this helps and is not too obvious.
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