Question

Compare the following (with definitions and phylogenetic trees): monophyletic, paraphyletic, and polyphyletic taxonomic groups.

Answer 1

Defination:-

Monophyletic Taxa	Paraphyletic Taxa	Polyphyletic Taxa
A monophyletic group of species shares a single common ancestor and also includes all of the descendants of that common ancestor.	A paraphyletic group includes a single ancestor and some of its descendants; it is similar to a monophyletic group, but some descendants are excluded.	A polyphyletic group is a group that is not defined by a single common ancestors.

  Define Phylogenetic trees?   
i) A phylogenetic tree also known as evolutionary tree is a diagrammatic representation of the evolutionary relationship among various taxa.  
ii) The terms evolutionary tree, phylogenetic tree, and cladogramare often used interchangeably to mean the same thing—that is, the evolutionary relationships among taxa.  
Explain with the help of diagram:-             
i) Those terms which are defined above in a table are used to describe groupings of organisms.
ii) It  indicate the extent to which they can be considered as "natural groups". iii) They can be explained using examples, so consider the following Phylogenetic-tree diagram:
                      Aves
                        /
                       /
          Crocodilia  /
Mammalia    \    Dinosauria
     \       \      /
      \       \    /
       \       \  /
   Synapsida  Reptilia
         \      /
          \    /
           \  /
         Amniota

• Consider the group consisting of all the animals in this diagram - that is, Amniota. This group is monophyletic because it consists of a single animal together with all of its descendants. The Dinosauria, including the modern birds, is another monophyletic group, sometimes defined as the most recent common ancestor of Igunanodon and Megalosaurus together with all its descendants.

  Monophyletic groups are also called clades, and are generally considered as the only "natural'' kind of group. They are very important in phylogenetic classification.

• Now consider the group consisting of the non-avian dinosaurs (which is what people usually mean by the informal term "dinosaurs''). This is a paraphyletic group, because it can't be defined simply as "this animal plus all its descendants'', but must be described as one clade minus another: in this case, Dinosauria minus Aves.

  The "non-avian dinosaurs'' make up a singly paraphyletic group because only one clade need be omitted from its base definition. Groups may also be doubly paraphyletic, thrice paraphyletic, etc., depending on how many sub-clades they omit.

• Finally, consider the group of "warm-blooded animals", which consists of Mammalia and Aves. This is a polyphyletic group - a totally unnatural assemblage - because it can't even be expressed as a paraphyletic group, that is, a clade minus one or more of its subclades. Such groups are not used at all in phylogenetic work since they are a purely artificial construct. In terms of common descent, a "warm-blooded animals" grouping makes no more sense than a Synapsida-plus-Crocodilia group - though this is not to say the notion of a warm-blooded group may not be useful in some informal discussions.