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Polyphyletic Group: Definition & Overview

Instructor: Sarah Phenix
In this lesson, we'll explore the polyphyletic category of systematics, including some common biological examples. We'll start with a brief review of systematics, and then learn how scientists use the 'tree of life' to understand the evolutionary relationships among organisms.

What is Systematics?

Systematics is the study of how organisms relate to each other to better understand the way in which they have evolved. Scientists use something called a 'tree of life', also known as a cladogram or phylogram, to illustrate the relationships among organisms and their common ancestors.

The Tree of Life Anatomy
Cladistics

Relationships among organisms of different groups are based on phenotypic traits or physical characteristics. Organisms that share the same characteristics, such as human beings, are grouped into a category, or taxon. According to the tree of life, members of a common ancestral group that stop interbreeding form separate stems or sister taxons. For example, chimpanzees and human beings are sister taxa.

Types of Phyletic Groupings
Phyletic Groupings

There are three main designations of phyletic, or race and tribe, groupings: monophyletic, paraphyletic, and polyphyletic. While monophyletic refers to organisms belonging to one tribe, and paraphyletic means 'contrary to' or 'near a tribe', polyphyletic refers to organisms descending from many tribes.

Polyphyletic Groups

Monophyletic groups consist of a recent common ancestor and its offspring, all of which share the same physical traits. Organisms in paraphyletic groups, like fish and reptiles, share a recent common ancestor, but not all of the monophyletic descendants are part of the group. By comparison, organisms in polyphyletic groups may have similar characteristics, but do not share a recent common ancestor. Polyphyletic groups may be the result of convergence, which we'll discuss later on in the lesson.

Polyphyletic Grouping
Polyphyletic

For example, birds and mammals in a polyphyletic group of warm-blooded animals may share a variety of biochemical and physiological characteristics. However, their most recent common ancestor was a cold-blooded species, which means birds and mammals developed their warm-blooded nature as independent groups. Elephants, hippopotamuses, and rhinoceroses are also part of polyphyletic groups in that each type of mammal originated from a different small species. Other types of polyphyletic groups include anteaters and protozoans.

Polyphyletic & Parsimony

As a phyletic category, the polyphyletic grouping is the most problematic because it confounds the guiding notion of parsimony. The principle of parsimony, also referred to as Occam's Razor, states that the simplest explanation that accounts for all of the scientific evidence and information in a problem must be correct. If we apply the principle of parsimony to polyphyletic groupings, then both groups must have experienced the same environmental influences or genetic mutations to acquire their warm-bloodedness trait. While this may seem 'unnatural', it isn't unheard of in evolutionary history.

The instance where a similar trait develops in unrelated organisms is known as convergent evolution, or the way in which unrelated species develop comparable characteristics under comparable conditions. However, this type of evolution requires many chance events or similarly felt selection pressures that result in a less simplistic, and, therefore, less parsimonious explanation for their generation.

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