Biological Evolution
Scientific theories of evolution seek to explain the mechanisms of the observable fact of biological evolution. Scientists observing biological evolution first sought to explain observed morphological changes over time – phenotypic evidence of changes in body structure found in the fossil record. Since the advent of modern molecular genetics, biological evolution has come to be understood as a change in genotype – a genetic alteration in the intergenerational frequency of alleles in populations.
However, morphologic changes may reflect alterations in the regulation of genetic expression without a major alteration in genotype – witness the considerable differences that selective breeding has wrought in size and configuration within one canine species. Similarly, the paramount importance of gene regulation probably explains much of the morphological difference between humans and chimps – two species who share 98% of their DNA. Recently, researchers have demonstrated that gene regulation has enabled rapid phenotypic speciation in sticklebacks. Along the same lines of modification of genetic expression, alternative splicing enables a single gene to give rise to multiple versions of a protein.
mutation → allele → pre-mRNA → constitutive pre-mRNA splicing and/or epigenetic alternative splicing → proteins → intergenerational fate of allele
There are two basic types of mechanism involved in biological evolution. First are the genetic sources of alteration of a gene within the genotype of an individual. Second are those statistical mechanisms that determine the fate of an altered allele. These are the mechanisms that increase or decrease frequency of an allele (a form of a gene at a locus) within a population.
Table Mechanisms of Biological Evolution : Gene Regulation in E.coli :
However, morphologic changes may reflect alterations in the regulation of genetic expression without a major alteration in genotype – witness the considerable differences that selective breeding has wrought in size and configuration within one canine species. Similarly, the paramount importance of gene regulation probably explains much of the morphological difference between humans and chimps – two species who share 98% of their DNA. Recently, researchers have demonstrated that gene regulation has enabled rapid phenotypic speciation in sticklebacks. Along the same lines of modification of genetic expression, alternative splicing enables a single gene to give rise to multiple versions of a protein.
mutation → allele → pre-mRNA → constitutive pre-mRNA splicing and/or epigenetic alternative splicing → proteins → intergenerational fate of allele
There are two basic types of mechanism involved in biological evolution. First are the genetic sources of alteration of a gene within the genotype of an individual. Second are those statistical mechanisms that determine the fate of an altered allele. These are the mechanisms that increase or decrease frequency of an allele (a form of a gene at a locus) within a population.
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.