Molecular Evidences

A) Universality of genetic code and Protein synthesis machinary :

Apart from the other evidences for origin of life and it’s relatedness among different organisms based on their anatomy and morphology, molecular evidences produces striking feature for relatedness among the organisms started from the small creature to giant one.

It is worth noting that the same genetic code having the triplet codon is found from viruses to man. For each and every living creatures, for each amino acids the codon is similar ( Source -Organic Evolution, V.B Rastogi ).

During the process of translation (protein synthesis) these codon are arranged in a specific pattern by proof reading of tRNA in mRNA . Though the translation process is slightly varies in different organism but the machinary which required for the protein synthesis is equal in nature such as subunits of ribosomes, mRNA,tRNA etc. Therefore it can be established as a genetic evidences for relatedness.

B) Three domain of life :

According to the Woese, Kandler and Wheelis in 1990 the life can be of tripartite scheme instead of bipartite and is based on three domains; the Bacteria (formerly eubacteria), Archaea (formerly archaebacteria) and Eukarya (formerly eukaryotes; this term is still more often used). This scheme has put forwarded due to the supportive growing body of biochemical , genomic and phylogenetic evidences where it suggested that the archaebacteria should be elevated to a taxonomic status equal to that of eukaryotes and eubacteria. The two i.e. eubacteria and archaebacteria are the single-celled organism having no true nucleus.

Carl Woese’s three domain tree

The Eukaryotes represented by Australian green tree frog, Bacteria by staphylococcus aureus and Archaea by sulfolobus.

The hierarchy of biological classification has eight major taxonomic ranks. Life is divided into domains; which are further subdivided into groups (the intermediate minor ranking generally not shown).

Domain also called as Superkingdom or Empire regarded as highest taxonomic rank of organism in which the three domain system of taxonomy designed by Carl Woese an American microbiologist and biophysicist . According to him all life forms that has a nucleus and membrane bound organelles, and multicellular organisms is included in Eukarya. Stefan Luketa in 2012 proposed a five ‘ dominion’ system adding with two more to the three domain of life.

Domain is a synonyms for the category of dominion introduced by Moore in 1974. Stefan Luketa only uses the term ‘dominion’. He placed prion and virus under the two domains separately. Each of these three domains of life recognised by biologist today contain unique rRNA. This fact itself forms the basis of three domain system. While presence of nuclear membrane differentiates the Eukarya domain from Archaea and Bacteria domains, beacuse both of them lack a nuclear membrane whereas  distinct biochemical and RNA markers differentiate the Archaea and Bacteria domains from each other.

I) Archaea : Archaea are prokaryotes,typically characterised by lipid membrane which constitutes of branched hydrocarbon chains attached to glycerol by ether linkage. The presence of these ether linkage in Archaea adds to their ability or withstand in extreme temperature and highly acidic conditions, but many Archaea living in mild environments. Halophiles; that thrives in highly salty environments and Hyperthermophiles  are the organisms which thrives in extremely hot environments, are some examples of Archaea. Archaea evolved into many cell sizes but all are relatively small. Thier size ranges from 0.1ⲙm to 15ⲙm diameter and upto 200ⲙm long. They are about the size of bacteria or similar to the size of mitochondrion of eukaryotes. Members of the genus Thermoplasma are the smallest of the Archaea.

II) Bacteria : Though the bacteria are prokaryotic in nature just like Archaea, their membranes are made up of unbranched fatty acids chains attached to glycerol by ester linkages. Cyanobacteria and Mycoplasmas are two examples of bacteria, they characteristically do not have their linkage like Archaea and they are grouped into a different category hence there is a great deal of diversity in this domain and between that there is another interesting phenomena called ‘horizontal gene transfer‘ aslo seen in this domain. So it is next to impossible for determining how many species of bacteria exists on the planet.

III) Eukarya : Members of the domain Eukarya are the eukaryotes, having true nucleus, presence of membrane bound organelles (including a nucleus containing genetic material) and are represented by four kingdoms : Protista, Fungi, Plantae and Animalia.

C) Neutral theory of molecular evolution :

The neutral theory of evolution is an influential theory introduced by Motoo Kimura. He suggested that a large fraction of new mutations do not have an effect on evolutionary fitness, so natural selection would neither favour disfavour them.

D) Molecular clock :

Molecular clock is the measure of rates of molecular evolution and amounts of genetic variation. Rather this variation can be estimated from the amino acids sequences of protein or nucleotide sequence of a region of DNA in two or more species.

The molecular clock is a technique in molecular evolution to relate the time at which the two species diverged to the number of molecular differences measured through their DNA sequences or proteins. It is some times called gene clock or evolutionary clock.

The concept of molecular clock is based on the hypothesis that DNA and protein sequences evolve at a rate that is relatively constant over time among different organisms. This constancy is used to estimate the length of time and that various organism have been diverging from one another by measuring the degrees of difference between the two sequences.

The molecular clock hypothesis was originally proposed by researcher Emile Zucker Kandl and Linus Pauling. On the basis of empirical observations, bit it soon received theoritical backing when biologist Motoo Kimura developed the ‘natural theory of molecular evolution’ in 1968.

E) Example of globin gene family, cytochrome c :

I) Globin gene : Haemoglobin is taken as as an example of globin protein molecules which produces by the expression of specific globin gene. We know that haemoglobin is respiratory pigment, it is present almost in all vertebrates, yeast, roots of leguminous plants, annelids, crustaceans and in molluscs.

It is a complex molecule having 2 alpha (𝜶) chains and 2 beta (β) chains, a iron ring called heme at the centre of the molecule. These two polypeptides chain constituting of amino acids sequences which is responsible for reconfigured into various types with respect to evolutionary forces. Nevertheless the amino acids sequences is unique for a particular species and varied from species to species. It is assumed that more is the differences in amino acids sequences between two species then it could be rightly said that their evolutionary distance also more. In simple sense we can say that they have a wide gap of connecting link in between them.

By considering the 𝜶-chains; it was seen that there is only one difference between the chains of man and gorilla whereas 22 differences between the chain of man and horse (source – www.personal.umd.umich.edu). In case of β- chains; here also only one difference for man and gorilla whereas about 30 points difference between man and horse. The haemoglobin of all vertebrates including fishes, amphibians, reptiles, birds and mammals having a four chains structure but the cyclostome (Division – Agnatha, Class – Cyclostomata) has only one chain in its haemoglobin molecule. Therefore it is assumed that a single-stranded stage may developed into a four-stranded one in the evolutionary process.

II) Cytochrome c : It is a protein molecule utilised in oxidation of food during cellular respiration. It contains a 100 amino acids based  polypeptide chain. Here also the sequence of amino acids in polypeptide varies from species to species. If more is the variation or differences between specieses more will be the distance in phylogenetic hierarchy.

There is a single point difference in the sequence of amino acid between man and rhesus monkey, man also have 12 points difference with horse and around 43-48 points with yeast. So we can access that the cytochrome c structure is homologous from yeast to man with basic amino acids sequences although the phylogenetic differences is more.