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Mechanisms of Evolution

Beyond Darwin and Neo-Darwinism

Beyond Darwin and Neo-Darwinism

Charles Darwin accurately predicted that his theories would evoke skepticism from scientists and ridicule from the religious. Happily, scientists came to accept his ideas, and his breakthrough insights have proved highly influential. Unhappily, those biblical literalists who insist that "Genesis" is an accurate description of life's origins, continue to unjustly vilify Darwin's theories.

Summary of Darwin's observations and his Theory of Evolution by Natural Selection:

1. Most animals have such high fertility rates that their population size would increase exponentially if all individuals were to reproduce.
2. Yet, except for seasonal fluctuations, populations remain relatively stable in size.
3. Because environmental resources are limited, individuals compete for resources, limiting survival and reproduction.
4. Individual characteristics vary within populations and those members of a population that are better adapted for survival in the face of competition are more likely to pass their characteristics on to the next generation.

5. Thus, species gradually accumulate inherited adaptations that best suit them for their environment, passing these on to progeny. Speciation involves gradually accumulated differentiation of characteristics.

Darwin was not aware of the existence of DNA, nor of the mechanisms that alter genotype. Darwin focussed on the inheritance of adaptive individual characteristics that had ensured reproductive success, and the resultant slow accumulation of adaptive phenotypic change. Darwin did not say that all species are gradually evolving (cf. quote.)

Subsequent evolutionary theorists first disputed Darwin's concept of gradual evolution. Gould and Eldredge introduced the concept "phyletic gradualism " which they discredited through the concept of punctuated equilibria. The Theory of Punctuated Equilibria was proposed in order to explain patchiness in the fossil record and the the localized adaptive radiation of species observed following extinction events. This stage of thinking about evolutionary mechanisms has been termed "Neo Darwinism".

Modern advances in molecular genetics, coupled with studies of population genetics have led to the "Modern Synthesis" of understanding concerning mechanisms of evolution. Current understanding incorporates knowledge of genetic drift, gene flow, mutation, recombination, and natural selection mechanisms.

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

Creationists and defenders of "intelligent design" theory commonly attack a "strawman" depiction of Darwinism or Neo-Darwinism as representing current thinking in their attempt to discredit evolutionary science. It is important for any person wishing to defend evolution-as-fact and modern evolutionary theories to attain a thorough understanding of modern evolutionary theory as well as fallacious creationist arguments.


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8:17 AM  
Blogger qtr 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.

4:50 PM  

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