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Powerpoint Presentation: Punctuated Equilibrium

Evolution Index

The origins of life
Evolution and Fixity
Natural selection
Lamarkian Evolution
Antibiotic Resistance
Industrial melanism
Palaeontology : The study of fossils
The C-14 Decay Curve
In Search of Deep Time
Evolution of the Horse
The classification of living organisms : Taxonomy
Kingdoms
Humans: Neotonous, bipedal African apes
The Hominids
The Changing Trees of Human Evolution
Genetic verses Cultural Evolution
Phenylketonuria (PKU) Fact Sheet
Cystic Fibrocis (CF) Fact Sheet

Topic Chapters Index

 

Neo-Darwinism

Neo-Darwinism states that evolutionary change is both slow and gradual resulting from the accumulation of many small genetic changes favoured by natural selection, with other effects occasionally making small contributions (gradualism). On this view, new species can be formed in two ways:

1. Phyletic Transformation

Gradual accumulation of small genetic variations preserved by natural selection, cause a whole population imperceptibly to evolve in to a new species.

It would be impossible to draw a clear line between the end of the first species and the beginning of its descendant species.

There would be a long period during which forms intermediate between the two species would be present.

Allopatric Speciation

Geographical or reproductive isolation of a part of the population would allow it to evolve in a different direction, or possibly more rapidly than the main population.

If the isolated population is small, it might be very difficult to find fossils of the intermediate stages.

 

Neo-Darwinism

 

EVOLUTION

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Punctuated Equilibrium

The Punctuated Equilibrium Model

Originally proposed by Stephen Jay Gould and Niles Eldredge (1972).

They observed that the fossil record gives a different picture for the evolution.

They claim that there were long periods of stasis (4-10 million years) involving little evolutionary change with occasional rapid formation of new species (as little as 5,000 - 50,000 years).

During stasis the species resists evolutionary change. Gould and Eldredge suggest that a species would move to a new area where it can find its habitat than adapt to a new one. They call this developmental homeostasis. If a small population of a species should get isolated in an area where its habitat does not exist then a rapid change takes place to bring the population back to equilibrium (stasis). But it is no longer the same species.

This could also occur on a global scale if there was a sufficiently large catastrophe.

 

Punctuated Equilibrium

 

Four mechanisms have been proposed for punctuated equilibrium pattern of speciation:

  1. Rapid natural selection in isolated populations

  2. Genetic drift in small isolated populations.

    Both these mechanisms are compatible with neo-Darwinism.

     

  3. Goldschmidt's "hopeful monsters" A small genetic change at a crucial stage in development could produce an individual very different in form from its parents - a "monster". Occasionally a monster would be viable and able to survive well enough for natural selection to make any minor adjustments. As Richard Dawkins points out, this is unlikely. An individual that is born so different from its parents that it is considered a different species, is not likely to be able to breed (i.e. find a mate).

  4. Sudden change due to the breakdown of developmental homeostasis. The development of the organism is controlled in such a way that it produces very similar individuals until sufficient genetic changes accumulate for the control to breakdown and allow the development of different forms.

    The last possibility is particularly favoured by Gould andEldredge, as it provides the best explanation of the long periods of stasis or equilibrium. Many neo-Darwinists reject nos. 3 and 4.

 

Criticisms

What is a species?

The definition of a species from fossils alone is not easy. Though the fossil remains (e.g. bones, teeth or shells) may indicate no change during period of stasis there may be significant changes in soft tissues or biochemistry.

How rapid is rapid?

Even though punctuated equilibrium requires rapid change, this does not mean instantaneous. If a species has a minimum duration of 4 million years and the rapid evolution of this theory represents 1% of this time. That means macroevolution of a new species could occur in as little as 40 000 years. Is this a short time?

The incomplete fossil record

Because the changes proposed by punctuated equilibrium are so rapid (on a geological scale), and the fossil record is not complete the transition from one species to another is often missing. Most of the evidence comes from recent geological periods where the fossil record is more complete.

 

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