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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.
UNDERSTANDING 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. |
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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.
SMALL PHYLOGENY GLOSSARY 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. |
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