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ASK SOMEONE walking down the street to give a short explanation of how she or he understands biological evolution and you will probably get an answer that refers to "survival of the fittest". This is not surprising. Darwin's discovery of natural selection is the central focus of high school textbook chapters on evolution. Natural selection is an important process of evolution and Darwin's eloquent explanation of it is largely responsible for the widespread acceptance of evolution. But, evolution is much more than natural selection. Perhaps natural selection is not the best concept to serve as a basis for understanding evolution.

We think it would be a good idea to base the instruction of evolution on something more fundamental. Evolution could more easily be comprehended if its primary product, phylogeny -- the ancestor to descendant relationships that connect organisms -- was more clearly presented at an early stage in teaching biology. At present, phylogeny is usually taught as the most confusing of a few types of classification schemes that biologists employ. But, it provides the big picture view or the backdrop for learning about the many processes of evolution. We present what we hope is a relatively straightforward bare-bones explanation of phylogeny without reference to classification schemes or any of the methods for deducing evolutionary relationships.

Every organism alive today has a history that can be traced back to when life originated, more than three and a half billion years ago. All living things (bacteria under a microscope, trees in your backyard, and pets in your lap) are the current representatives of lineages, or continuous lines of descent, that began more than three billion years ago when the first cells evolved. Even YOUR history can be traced back to this time when all life shared a single common ancestor. To share a common ancestor is to be related. This means that all living things share some degree of relatedness.

You can take any two organisms and trace their histories back to a point in time when they shared a most recent common ancestor. Consider a bird and a jellyfish (Figure 1). For most of life's history, many hundreds of million years, birds and jellyfishes shared the same lineage, or had a common history. At some point in time, the lineage split into two lineages. At this point of splitting, the two lineages shared their most recent common ancestor. After this time, the two lineages had separate evolutionary histories.

Similarly, you can take any three organisms and trace their histories back to where all three shared a most recent common ancestor. Let's add a fern to our example with the bird and the jellyfish. (Figure 1). Even ferns share a common history with birds and jellyfishes. But, birds and jellyfish share more common history with each other than either does with ferns. The birds and jellyfishes share a more recent common ancestor than they do with ferns; therefore, they are more closely related to each other than either is to ferns. Put another way, birds and jellyfishes are both equally related to ferns since they both share the same portion of their lineages with ferns.


common ancestor - any organism that is part of a lineage that two other organisms (or groups) share is a common ancestor of the two organisms.
more recent common ancestor - a common ancestor of two or more organisms (or groups) that occurs later in time than another common ancestor.
most recent common ancestor - the last ancestor in the lineage that two organisms shared, represented by the point at which the common lineage of the two split.

common history - two organisms (or groups) that share a common lineage have a common history; all living things share some common history.

lineage - a continuous line of descent connected by reproduction from parent to offspring.

phylogeny - the set of ancestor to descendant relationships that connect all things that have ever lived.

related - to be related is to share a common ancestor; all living things are related to some degree.

equally related - two organisms (or groups) are equally related to a third if the two share a more recent common ancestor with each other than either does with the third.
more closely related - two organisms (or groups) are more closely related to each other than to a third if the two share a more recent common ancestor than either does with the third.
That's it, phylogeny in a nutshell. These are concepts that can be used to read any branching diagram of organisms. With these ideas, one can better understand and explore the biodiversity of today and long ago. Understanding how the history of life has produced today's' biodiversity allows us to discuss and learn about how organisms and their features have changed through time. It helps us appreciate our place in the world and to make informed decisions that affect the well being of our natural resources and of ourselves.

The material we have presented here, is just the tip of the iceberg. Phylogeny is an exciting and active field of study. Figuring out things that happened in the past is hard, but rewarding, work. Biologists use many different approaches, methods, and types of data for reconstructing phylogeny. If you want to learn more, there are a number of Internet sources for information pertaining to phylogeny. A good place to start investigating is the List of Phylogenetic Resources by the University of California Museum of Paleontology.

©   Updated: June 2003   Contact: jen-AT-paleobio or allen-AT-paleobio