Classification Systems: Classical Taxonomy, Phenetics & Cladistics

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  • 0:00 Classification Systems
  • 1:09 Classical Taxonomy
  • 2:22 Phenetics
  • 3:59 Cladistics
  • 5:39 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

How do we begin to sort out the millions of species on Earth? In this lesson, we will explore three of the most common classification systems used by modern science. Then, you will be able to test your understanding with a brief quiz.

Classification Systems

This is the world. Within the world, there are a lot of things. Amongst all of those things, quite a few of them are alive. In fact, there are an estimated 8,700,00 species of living things on this planet. Even more astonishing is the fact that researchers estimate that this number is only around 1% of the total number of species that ever lived throughout Earth's 4 billion year history...meaning that 99% of the species that ever lived are now extinct. So, we're talking about a lot of living things.

For scientists, the ways that these things are related to each other are important. Systems of classification attempt to organize the millions of living things on this planet, alive and dead, into understandable categories that we can use to study how things change and where they come from. But, how do we even begin to sort this much information? Well, let's check out a few common solutions.

Classical Taxonomy

Let's start with one of the earliest forms of scientific classifications, in which organisms were categorized first into larger, then more specific groups, which we call classical taxonomy. All living things are grouped into domains, kingdoms, phylum, class, order, family, genus, and finally species, the most specific classification. So, for example, humans belong to the domain of Eukaryotes, which we share with everything from flowers to sharks to algae, but humans, and humans alone, are the species Homo sapiens. See how that works?

Now, classical taxonomy creates these rankings by looking at how things relate to each other genetically, meaning through shared ancestry and appearance. This means using both morphology, the form and structure of organisms, and phylogeny, evolutionary history, to classify living things. While this was accepted for a long, long time, classical taxonomy is no longer the most accepted system due to the fact that it relies on the subjective judgment of the researcher more than pure, scientific facts.


The basic ideas of classical taxonomy, the classification of things to understand how they are related, influenced how biologists thought about the world. But, this system was not perfect, and so researchers looked for alternatives. Another major classification system is phenetics, in which organisms are classified into hierarchies of similarity, based solely on morphology. Basically, phenetics compared and categorized things based on their overall similarity of appearance. Here's how it works. Two species are compared, and shared characteristics are counted.

Now, it's important to note that phenetics places no emphasis on certain traits over others; all shared characteristics are treated equally. A computer then processes this data through a numerical algorithm to create a similarity coefficient, the mathematical degree of similarity, with zero being no similarity and one being completely identical. With this you can create phenograms, creating clusters of things that are morphologically similar and, therefore most related. Phenograms are criticized because, although not subjective, they do not account for the fact that things may evolve similar physical traits without being related. For example, dolphins look like sharks but really are pretty distantly related to each other. So, for many researchers, this is a system of convenience more than accuracy.

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