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Mechanisms of Evolution

Beyond Darwin and Neo-Darwinism

Punctuated equilibria

Examination of the fossil record (aside from natural incompleteness) reveals a patchiness not predicted by “phyletic gradualism”, as described by Eldredge and Gould. Species with widespread distribution exhibit relatively stable morphology, while new species with different morphologies arise relatively abruptly.

Features of the Theory of Punctuated Equilibria:
1. Interpretation of paleontology ought to be based on the study of living or recently living organisms (neontology).
2. Large, widespread species usually change slowly, if at all, during their time of residence.
3. Sampling of the fossil record will reveal a pattern of stasis in most species, and the abrupt appearance of newly derived species is a consequence of ecological succession and dispersion.
4. Most speciation proceeds independently from a single ancestral line (cladogenesis) rather than by in toto replacement by a morphogenetically distinct population (anagenesis).
5. Adaptive change in lineages occurs mostly during periods of speciation (during cladogenesis).
6. Trends in adaptation occur mostly through the mechanism of species selection.
7. Daughter species usually develop during a time that is short in comparison to the residence time of the species (across limited strata).
8. Most speciation results from isolation of a small, reproductively isolated, geographically peripheral sub-population (parapatric or peripatric speciation, or allopatric speciation of peripheral isolates). That is, daughter species usually develop in a geographically limited region.

A passage in Darwin’s Origin of Species indicates that Darwin viewed the cumulative changes that lead to speciation as acting slowly. However, Darwin indicates that “free intercrossing” retards speciation, which he describes as intermittent and affecting, “only a very few of the inhabitants of the same region at the same time.” [see Charles Darwin, Origin of Species 1st Edition 1859, p.153]

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

Therefore, it might reasonably be suggested that Eldredge and Gould described “phyletic gradualism” for the purpose of contrast with their own theory of punctuated equilibrium. (adapted from here.)


Anonymous Anonymous said...

Neontology is a branch of biology that emphasizes the study of modern biota (living or recent organisms) rather than fossilized organisms (paleontology).

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.

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.

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.

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.

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.

4:23 PM  

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