Polyphyly

A polyphyletic group is a collection of living things that come from different evolutionary paths but do not include their most recent common ancestor. This means these organisms share similar traits even though they did not inherit them from the same ancestor. These shared traits are called homoplasies and happen because of something called convergent evolution, where similar features develop separately in different groups.

One clear example is the idea of warm-bloodedness. This ability to keep a steady body temperature evolved separately in the ancestors of mammals and the ancestors of birds. So, if we talk about "warm-blooded animals," we are talking about a polyphyletic group because these animals do not share a single recent ancestor who was warm-blooded.

Other examples of polyphyletic groups include slime molds, worms, algae, trees, C4 photosynthetic plants, and edentates. Many taxonomists, scientists who study how to group living things, try to avoid putting organisms into polyphyletic groups. They want to make sure groups share a common ancestor. However, some researchers focus more on how these organisms live and work together, and they might still study polyphyletic groups. For instance, scientists studying a group of fungi called Alternaria might treat it as a proper genus even though it is polyphyletic. These ideas help scientists find important genes for barcoding different species.

Etymology

The word "polyphyly" comes from ancient Greek words meaning "many" and "species." It describes groups of living things that come from many different ancestors instead of just one.

Other related words include "monophyly," which means all the living things come from one unique ancestor, and "paraphyly," which means some groups are left out from a common ancestor.

Avoidance

Many schools of taxonomy discourage using groups that are polyphyletic in their classifications. They believe that only monophyletic groups, also called clades, are valid because these groups are defined by shared traits that all members inherited from a common ancestor. Polyphyletic groups do not share a single common ancestor.

Using monophyletic groups helps scientists make better predictions. For example, if a new type of grass is found and placed in the monophyletic family Poaceae, scientists can guess many things about it, like its structure and how it grows. In contrast, older classifications, like putting plants with two stamens into a group, are not as useful because these traits evolved separately in different groups.

Polyphyletic species

Species are important in science because they help us understand and group living things in nature. Usually, we think of species as having a single family tree that goes back to one common ancestor. But sometimes, new species can form when two different species mix, especially in plants. This mixing can create species that come from more than one ancestor, which is called polyphyly. Some scientists don’t use the idea of "-phyly" when talking about single species.