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Mechanisms of Evolution

Beyond Darwin and Neo-Darwinism

Basic mechanisms of evolution

There are two basic types of mechanism involved in biological evolution. First are the genetic sources of alleles – altered genes at a locus – within the genotype of an individual. Regulatory mechanisms determine genetic expression, and hence the manifestation of genotype as phenotype. Selection acts upon phenotypes. So, second are those statistical, population-level mechanisms that determine the fate of an altered allele. Collectively, these are the mechanisms that alter biological variation by increasing or decreasing the frequency of alleles between generations.

Regulatory mechanisms that affect phenotype:
Constitutive gene regulation,
Alternative splicing,
Epigenetic mechanisms.

Mechanisms that add alleles:
Horizontal Gene Transfer,
Endosymbiotic Gene Transfer,
Gene flow,
Natural selection.

Mechanisms that remove alleles:
Natural selection,
Genetic drift,
Founder effect.

 Table Mechanisms of Biological Evolution :  Gene Regulation in E.coli :

Genetic mutations are of interest to molecular geneticists because they cause disease and because they are the basic currency of biological evolution. Mutations that affect regulatory sequences are of particular significance to evolution because of their widespread phenotypic influence: '“pleiotropic” genes - those with multiple on-switches that enable the expression of a single gene in different tissues or at different stages of development. . . this pleiotropy gives evolution an artistic freedom to play with the regulatory elements in specific regions without making mutations that would affect the gene throughout the body. . . More generally, these kinds of molecular studies are enabling new advances in understanding the machinery of evolution. “These techniques are enabling dramatic progress in understanding the deep mechanics of evolution in more and more detail,” he said. “Researchers are now finding the actual `smoking guns' of evolution by documenting specific evolutionary changes at the DNA level. “And studies of phenomena such as fruitfly wing spots show how evolution is not some one-off process. It repeats itself over and over. They show that there is more than one way to tinker with the same gene, and by extension, to independently evolve the same trait,” Carroll said. ”' [HHMI news] [Abstract below]

Chance caught on the wing: cis-regulatory evolution and the origin of pigment patterns in Drosophila.
The gain, loss or modification of morphological traits is generally associated with changes in gene regulation during development. However, the molecular bases underlying these evolutionary changes have remained elusive. Here we identify one of the molecular mechanisms that contributes to the evolutionary gain of a male-specific wing pigmentation spot in Drosophila biarmipes, a species closely related to Drosophila melanogaster. We show that the evolution of this spot involved modifications of an ancestral cis-regulatory element of the yellow pigmentation gene. This element has gained multiple binding sites for transcription factors that are deeply conserved components of the regulatory landscape controlling wing development, including the selector protein Engrailed. The evolutionary stability of components of regulatory landscapes, which can be co-opted by chance mutations in cis-regulatory elements, might explain the repeated evolution of similar morphological patterns, such as wing pigmentation patterns in flies.
Gompel N, Prud'homme B, Wittkopp PJ, Kassner VA, Carroll SB. Chance caught on the wing: cis-regulatory evolution and the origin of pigment patterns in Drosophila. Nature. 2005 Feb 3;433(7025):481-7. Comment in: Nature. 2005 Feb 3;433(7025):466-7.

Evolutionary developmental biology: how and why to spot fly wings. [Nature. 2005] PMID: 15690019
Repeated morphological evolution through cis-regulatory changes in a pleiotropic gene. [Nature. 2006] PMID: 16625197
Evolution of yellow gene regulation and pigmentation in Drosophila. [Curr Biol. 2002] PMID: 12372246
Direct regulation of knot gene expression by Ultrabithorax and the evolution of cis-regulatory elements in Drosophila. [Development. 2005] PMID: 15753212
Regulation of body pigmentation by the Abdominal-B Hox protein and its gain and loss in Drosophila evolution. [Cell. 2006] PMID: 16814723
See all Related Articles...

Creationists direct their attacks upon evolution at point mutations (SNPs) that affect coding for proteins, and in so doing they divert attention (either knowingly or out of ignorance*) toward mutations that are more likely to cause disease than to effect evolutionary changes.

 Table Mechanisms of Biological Evolution :  Gene Regulation in E.coli :

* in the case of science-educated, professional (paid) proponents of creationism (idists & fodis), the explanation for this fallacious attack is probably a deliberate (fallacious) straw man argument, while the average internet debator (proid) appears to be ignorant of biological sciences and to be parroting the misinformation that abounds in creationist books and on creationist websites.


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12:02 PM  
Anonymous Anonymous said...

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.

4:52 PM  

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