Biology Reference
In-Depth Information
Fig. 4.9
quickly when they encounter favourable growth conditions, and so outcompete other
organisms. In contrast, most eukaryotes do not compete at the level of growth and
cell division, but by making more elaborate internal compartments that allow the
cells to become larger and more structurally sophisticated than prokaryotic cells.
This internal specialisation allows eukaryotic cells to form multicellular organisms
where different cells have different functions. All the organisms visible to the naked
eye are eukaryotes.
Both the Archaea and the Bacteria are made of prokaryotic cells, while all the
Eukarya are made of eukaryotic cells. Confusion has arisen between these terms
because, while the terms “prokaryote” and “ eukaryote” were first used to describe
the main structural difference between the two types of cell, they were then assumed
by some biologists to also reflect their evolutionary relationships. But according to
the three-domain model presented in Fig. 4.8, the prokaryotic Archaea are more
closely related to the eukaryotic Eukarya than they are to the prokaryotic Bacteria.
This tree is based on the analysis of one gene - the gene that encodes the rRNA
found in the small ribosomal subunit, so we need to ask the question as to whether
trees based on similarities in other genes look the same. You will also note that
this tree is based on the assumption that different species are related by vertical
descent, that is, genes are transferred from parent cells to daughter cells only by the
processes of cell division and sexual reproduction. But suppose that there are other
ways than vertical descent by which genes transferred between cells, what happens
