The
theory of symbiosis is a theory of evolution that seeks to show how the
prokaryotic organisms developed from eukaryotic cells. It proposes that this development
took place following the symbiosis of the organelles that distinguished the
eukaryote cells (Martin, Garg & Zimorski, 2015). This theory further states
that mitochondria, plastids, and other organelles that seem to have undergone
endosymbiosis while they were essentially prokaryotes that essentially were
free-living. A result of this situation is that mitochondria seem to be related
to Rickettsialles proteobacteria, and plastids such as chloroplasts seem to be
related to nitrogen-fixing filamentous cyanobacteria. One of the biggest
supporting evidences concerning the theory of symbiosis is that new
mitochondria tend to form only through binary fission, and this is especially
considering that cells cannot create new plastids. Moreover, the theory suggests
that porins, which are essentially transport proteins, can be found in the
outer membranes of bacterial cell membranes, mitochondria, and chloroplasts;
showing a common origin of these organisms. Finally, evidence shows that some
mitochondria and plastids have certain DNA molecules that are extremely similar
to the DNA contained in bacteria; further advancing the theory of symbiosis
(Morelli & Rosano, 2016).
Weaknesses
One
of the most significant weaknesses of this theory is the claim stating that circular
mitochondria DNA is similar to the binary fusion found in bacteria. An
important difference is that the most pertinent components tend to be more
eukaryotic in form than prokaryotic, which is in addition to the fact that
replication is different between mitochondrial DNA and bacterial DNA. Moreover,
despite their being enclosed in a double membrane, the structures of bacteria
and mitochondria are essentially different and this is to such an extent that
when antibiotics are administered, they end up in a situation where they are
better able to distinguish between the mitochondrial and bacterial membranes
(Harish & Kurland, 2017). This is an important factor because it shows that
membranes are structurally different, meaning that the theory of symbiosis is
disproved. One of the biggest arguments made by proponents of the symbiotic
theory is that mitochondria and bacteria share similar characteristics,
especially when it comes to size and shape. However, findings show that
mitochondria have more of a bacillus shape and are not spherical as initially
assumed.
Strengths
The
biggest strength of the theory of symbiosis is that it advances the theory of
evolution. This is especially considering that it advances the theory, proposed
by Charles Darwin, of natural selection. The symbiotic relationship between
cells and organisms seems to have played a role in the evolution of diverse
organisms because many of them ended up coming together to advance certain
characteristics. The theory of symbiosis also shows that organisms, which
developed as a result of the symbiotic relationship, ended up in a situation
where they not only grew apart, but in some instances came together in order to
ensure their own survival (Booth, 2014). Only the strongest were able to make sure
that they passed on their DNA to future generations, while at the same time
advancing a situation where they either out-competed other species, or were
able to avoid extinction.