Life Science: Session 6
Building a Tree of Life
What is a tree of life?
The theory of evolution by natural selection makes a powerful supposition: All species, past and present, trace their ancestry to a single common ancestor. If this is valid, and if evidence of common ancestry can be found, scientists should be able to trace the evolution of life on Earth. A tree of life is a powerful graphic model that is used to display this information.
To interpret a tree of life is simple. Every node represents a common ancestor, from which two different lineages branch. Branches that occur closer to the top of the tree represent lineages that arose later in evolutionary time. The tips of branches represent living species. The closer two tips are within one branch, the more closely they’re related.
How are trees of life built?
Scientists constructing early trees of life used a variety of data to hypothesize relatedness of modern species – mostly external and internal features. This was supplemented with other types of evidence, including the fossil record. It was (and is) particularly exciting for scientists to find transitional forms, which provide evidence of common ancestry. One notable example is Archaeopteryx, which, preserved as a fossil imprint, has both reptile and bird features, and is used to argue that birds evolved from reptiles.
Today, molecular evidence in the form of DNA sequencing is used to provide data to build a tree of life. The assumption with this type of data is based on mutation — changes in DNA sequences due to errors in replication. Mutation provides the variation upon which natural selection acts in order to cause evolution. As two species evolve from a common ancestor, their DNA will continue to mutate and will become less and less similar. Eventually, differences in DNA become a genetic reflection of the differences between species. DNA sequencing, therefore, is allowing scientists to build a tree of life that they consider to be much more accurate.
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