Phylogenic Trees

PHYLOGENIC TREES

Another name for phylogenetic tree is the evolutionary tree. This is a tree or diagram that shows the relationships between species, also referred to as their phylogeny. They can be based on physical similarities or genetic characteristics. The idea is that all life on this planet has a common ancestry so that the tree” is rooted with a common ancestor. Figure 1 shows an example of a phylogenetic tree:

Figure 1.

This concept of a tree of life” is an ancient one, when the ancients referred to the Great Chain of Being”. Charles Darwin illustrated his concept of the tree of life in his book On the Origin of Species. Tree diagrams are still used today to understand the basics of evolution. In modern times, the classification of species is now more dynamic and less static.

There are different types of phylogenetic trees. In a rooted tree, there is a single node that represents the most recent common ancestor with descendants making up the leaves of a tree. In an unrooted tree, the relatedness of the different leaves is made clear but there isn t any assumption about a common ancestor. It helps to identify ancestry and some of them involve speculation about the time period between ancestral splits, which is based on the molecular clock hypothesis we talked about in chapter one.

Both the rooted tree and the unrooted tree can be bifurcated or multifurcated. A bifurcated tree has two descendants that arise from a specific interior node. A multifurcated tree has more than one descendant coming from an interior node. Another name for a phylogenetic tree is a dendrogram.

There are other types of trees that are sometimes used. A cladogram does not have ancestors on its internal nodes. A chronogram is a tree that indicates a timespan between the branches. A Romerogram is a representation of evolution as a whole but isn t an evolutionary tree. Trees themselves have their limitations so sometimes a phylogenetic network is used instead.

The problem with phylogenetic trees is that they aren t necessarily correct. There can be horizontal gene transfer, hybridization, or genetic recombination between species that give them genetic similarities that they wouldn t otherwise have.

So, how are phylogenetic trees built? The basic idea is that organisms descend from other organisms but with modifications made that make them unique from the ancestor. Scientists look at the patterns of these modifications in order to determine who the common ancestor is. They look at the different modifications that have occurred over time in order to see which modern or ancestral species was the original species.

The novel traits or the most recent traits that are different from the ancestor are called the derived traits, which are different from the ancestral traits. Features that are gained or lost can be derived traits; they do not have to be an addition to the ancestor. The goal is to determine which traits are derived and which traits are ancestral. These traits can be physical, biochemical, or behavioral.

Another goal is parsimony, which means that one looks at the simplest explanation or the simplest path from the ancestor to the descendants. The right tree is probably the one that has the fewest number of independent genetic events in the process of drawing the tree.

One problem in looking at the different features in a phylogenetic tree is that there can be homologous features that have a shared ancestry and analogous features that look similar but arose completely independently from each other. Evolution can also work backward so that a gained trait can just as easily be lost in the future.

DNA sequencing is just one more recent tool that can be used to build phylogenetic trees. Related genes in different organisms are compared to see how similar they are to one another and to see what pattern best reflects the common ancestor.