Discovery Science: Earth – Classification Of Living Things – Evolutionary Trees

Earth Science: Classification Of Living Things – Evolutionary Trees

An evolutionary tree is a schematic representation of natural relationships between organisms. Evolutionary trees are always binary, that is each node results in only two daughter branches.

Evolutionary trees are comparable to relationship diagrams or cladograms whose purpose it is to show the results of phylogenetic systematics in a graphic way. They are changing all the time and often contain knowledge gaps. Therefore, they are not to be seen as a final product of classification but as an approximation of the natural system of organisms.

Nucleotide sequences of the 16S ribosomal RNA (16S rRNA) are often analyzed in order to construct modern evolutionary trees. It is thought to be especially valuable for this purpose, because the rRNA is assumed to be structurally highly conserved, as proteins are synthesized in the ribosomes, which are present in all organ-isms. Even minor mutations usually result in some functional loss of the ribosomes.

Assuming that the number of mutations is proportional to time, differences in sequence would be a measure of the evolutionary distance between species. Evolutionary trees produced in this way are thought to be very reliable, especially because their validity has been confirmed by studies using other methods. In order to create a phylogenetic tree, the 16S rRNA sequence data of an organism are compared to the data of other species by using computer programs.

In the mid-1970s, several prokaryotes were examined using this method. The result was that the prokaryotic organisms actually did not belong to one homogeneous group but they split into two separate groups very early in evolutionary time. Therefore, archaea, single-celled microorganisms, are now considered as a phylogenetic group by itself, distinct from bacteria and the eukaryote group consisting of plants, animals, fungi, and protists.

With the aid of this classification method, scientists were also able to prove that the kingdom of fungi are not part of the plant kingdom. Instead, they constitute a eukaryotic sister group to animals and plants.


The archaea are an evolutionary very old group that has survived even the harshest environmental conditions

Some species, for example, live in tern peratures above 212°F (100°C) or in extremely saline, acid, or alkaline environments. In certain cases they can sustain pressures of up to 1,000 bar.


PHYLOGENETIC METHODS to com pare species include gene sequence anal- ysis, the comparison of nucleotide sequences of 16S rRNA, and DNA-DNA hybridization.