Substitution
Substitution mutations involve the replacement of one or more nucleotides with an equal number of different nucleotides – most substitutions involve a change in only one nucleotide (point mutations, single nucleotide polymorphisms).
Transition mutations involve the substitution of a pyrimidine nucleotide for a pyrimidine, or a purine nucleotide for a purine. Transversion mutations involve switching between a pyrimidine nucleotide and a purine.
Substitution mutations generate silent, missense, or nonsense mutations depending upon the consequences of the substitutions. Silent mutations code for the same amino acid in the protein; missense for different amino acids, which may or may not alter the activity of a protein; and, nonsense for a stop codon, which alters the code to that for a truncated protein. Accordingly, substitution mutations can be neutral, beneficial, or deleterious.
Missense point mutations can transform the Ras proto-oncogene into the Ras oncogene. Mutations of Ras proto-oncogenes are common → H-RAS, N-RAS, and K-RAS oncogenes. Inappropriate activation of the Ras gene plays a key role in signal transduction, proliferation, and malignant transformation. Mutagenic mutation alters the Ras protein so that the GTPase reaction can no longer be stimulated by GAP, increasing the half life of active Ras-GTP mutants and ensuring that this molecular switch remains 'on'. Unchecked proliferation occurs because mutations that prevent GTP hydrolysis favor constitutive activation as RAS-GTP. The commonest neoplastic mutations are at the 12 (G to T transversion, Gly→Val) → GAP insensitive, and the 61 positions → stabilizing against GTP hydrolysis.
Table Mechanisms of Biological Evolution : Gene Regulation in E.coli :
Transition mutations involve the substitution of a pyrimidine nucleotide for a pyrimidine, or a purine nucleotide for a purine. Transversion mutations involve switching between a pyrimidine nucleotide and a purine.
Substitution mutations generate silent, missense, or nonsense mutations depending upon the consequences of the substitutions. Silent mutations code for the same amino acid in the protein; missense for different amino acids, which may or may not alter the activity of a protein; and, nonsense for a stop codon, which alters the code to that for a truncated protein. Accordingly, substitution mutations can be neutral, beneficial, or deleterious.
Missense point mutations can transform the Ras proto-oncogene into the Ras oncogene. Mutations of Ras proto-oncogenes are common → H-RAS, N-RAS, and K-RAS oncogenes. Inappropriate activation of the Ras gene plays a key role in signal transduction, proliferation, and malignant transformation. Mutagenic mutation alters the Ras protein so that the GTPase reaction can no longer be stimulated by GAP, increasing the half life of active Ras-GTP mutants and ensuring that this molecular switch remains 'on'. Unchecked proliferation occurs because mutations that prevent GTP hydrolysis favor constitutive activation as RAS-GTP. The commonest neoplastic mutations are at the 12 (G to T transversion, Gly→Val) → GAP insensitive, and the 61 positions → stabilizing against GTP hydrolysis.
Table Mechanisms of Biological Evolution : Gene Regulation in E.coli :
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Allopatric speciation occurs when a geographical barrier sub-divides a parent species, resulting in geographic and reproductive isolation such that the descendent species can no longer interbreed upon removal of the barrier.
Anagenesis differs from cladogenesis in that one species progressively transforms into a replacement species when sufficient gene mutations fix in the descendant population. At this point, the ancestral species has become extinct. This mechanism is distinct from the increase in numbers of species generated by cladogenetic branching events.
Cladogenesis is the mechanism of speciation in which one or more lineages (clades) arise from an ancestral line. Such speciation events increase the variety of plants or animals through branching of the phylogenetic tree. Cladogenesis is differentiated from anagenesis, which is the in toto replacement of one species by an anatomically distinct species.
Monophyletic taxon or clade: an accurate grouping of only (opp. polyphyletic) and all (opp. paraphyletic) descendents of a shared common ancestor. A monopyletic group is genetically homogeneous and reflects evolutionary relationships.
Paraphyletic taxon or clade: a monophyletic group that excludes one or more discrete groups descended from the most recent common ancestral species of the entire group. Other descendent species of the most recent common ancestor have been excluded from the paraphyletic taxon, usually because of morphologic distinctiveness.
Phenetic system: groupings of organisms based on mutual similarity of phenotypic (physical and chemical) characteristics. Phenetic groupings may or may not correlate with evolutionary relationships.
Phylogenetic system: groups organisms based on shared evolutionary heritage. DNA and RNA sequencing techniques are considered to give the most meaningful phylogenies.
Phylogenetic separation into evolutionary relationships (clades), based on comparison of genomes is likely to supplant phenotypical (phenetic) taxonomies of the prokaryotes.
Peripatry (paripatry) is a subset of allopatry in which an isolated group has a smaller population than the parent group. Ernst Mayr introduced the term. Peripatric speciation occurs when the smaller sub-group of a species enters a novel niche within the range of the parent species, becoming geographically and reproductively isolated. Peripatric speciation (paripatric) is distinguished from allopatric speciation by the smaller size of the isolate group, and from sympatric speciation, which involves no barrier to breeding.
Polyphyletic taxon: opposite to monophyletic taxon: A polyphyletic group is mistakenly or improperly erected on the basis of homoplasy.—characteristics that have arisen despite not sharing a common ancestor. Homoplasy arises because of convergent evolution, parallelism, evolutionary reversals, horizontal gene transfer, or gene duplications. Polyphyletic taxa are genetically heterogeneous because members do not share a common ancestor.
Neontology is a branch of biology that emphasizes the study of modern biota (living or recent organisms) rather than fossilized organisms (paleontology).
Numerical Taxonomies are a common approach to phenetic taxonomy that employ a number of phenotypic characteristics to generate similarity coefficients that may be mapped in dendrograms. Groupings based on numerical taxonomy may or may not correlate with evolutionary relationships.
Taxonomies aim to group organisms according to shared characteristics against the background of biological diversity.
Sympatry involves no geographical separation of sub-populations of individuals. Sympatric speciation events occur most often in plants by the mechanism of polyploidy in which the number of chromosomes is doubled or tripled. John Maynard Smith proposed a model called disruptive speciation, in which homozygotes might have greater fitness than heterozygotes under some environmental conditions.