Background Information

Humans are always attempting to classify things. We do this because information is much more manageable when it has some sort of order to it. Imagine searching through iTunes without the songs being classified alphabetically by name, artist or album title. Without the use of a search function, it would be nearly impossible to find the song you were looking for amongst thousands of others.

Like songs in iTunes, living organisms can also be classified in different ways. Whereas iTunes classifies songs alphabetically based on title, artist and album, ornithologists classify birds based on traits that include wing shape/size, beak shape/size, colour, etc. In both cases, a classification system is being used based on some characteristic that is important for organization.

Biology deals with all organisms. Recent estimates show there are approximately 8.7 million different species of organisms on earth. This means biologists have the task of developing classification systems that allow them to organize all these species into groups. It’s no wonder different forms of classification have “evolved over time”!

Cladistics is based on Darwin’s realization that organisms fall into natural groups because they are related to each other by common descent. For example, domestic cats are more similar to lions than dogs just as brothers and sisters are more similar to each other than their cousins; they share a more recent common ancestor. Rather than just looking for natural groups, cladistics tries to classify organisms based on what makes them fall into natural groups: their common ancestry or evolutionary history.

This evolutionary history (or phylogeny) is based on an analysis of shared characteristics. Since these characteristics are changing over time, cladists distinguish between recent, derived (or apomorphic) traits and older, primitive (or plesiomorphic) traits. Cladograms are branching diagrams used to display the sharing of derived traits, or synapomorphies, usually in the form of a “tree”. These concepts will be covered further in the following pages of this section.


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