Biology Reference
In-Depth Information
Fig. 4.7
the ribosomes will not work well enough at making proteins. This constraint means
that the sequence of ribosomal RNA changes only very slowly during evolution, and
so can be used to determine how the major groups of organisms are related to one
another.
Figure 4.8 shows the three-domain classification scheme that is currently a pop-
ular model for describing the relations between the major groups of living organism
on the Earth. This model is derived by comparing the base sequence of the genes
encoding the RNA component of the small subunit of ribosomes in the different
organisms that exist today - these are called extant organisms. The more similar
these sequences, the closer together the organisms are placed. This scheme is often
described as a family or phylogenetic tree, that is, a diagram that shows how organ-
isms are genetically related to one another. The metaphor of a tree to describe the
relatedness of organisms was not invented by Charles Darwin but was promoted by
him. In Chapter 4 of
On the Origin of Species
he states:
The affinities of all beings of the same class have sometimes been represented by a great
tree
. The green and budding twigs may represent existing species: and those produced
in each former year may represent the long succession of extinct species. At each period of
growth all the growing twigs have tried to branch out on all sides, and to overtop and kill
the surrounding twigs and branches, in the same manner as species and groups of species
have tried to overmaster other species in the great battle of life.
...
You will note that all the branches in the tree shown in Fig. 4.8 are connected to
one another and all converge on the same point at the bottom. So this tree is saying
