NEET Biology Notes Evolution Theories of Evolution
Theories of Evolution
A number of biologist proposed various theoriêS for replaining evolution. The most accepted * were Lamarck and Darwin’s theory.
Lamarckism (Lamarck’s Theory of Inheritance of Acquired Characters) the first theory of evolution was proposed by Jean Baptiste de Lamarck (1744-1829), a French biologist. His famous book Philosophie Zoologique was published in 1809, in which, he discussed his theory in detail.
Propositions of Lamarckism are
- Internalvital force
- Effect of environment
- New need
- Use and disuse of organs
- Inheritance of acquired character
Examples in Support of Lamarckism
- Aquatic birds
- Flat fishes
- Flightless birds
- Cave dwellers
Criticism of Lamarckism
Evidences against the inheritance of acquired characters:
- There is no vital force in organisms, which increases their body parts.
- The environment can affect the animals but it is doubtful that a need forms new structures.
- The use and disuse of organs is correct up to some extent.
- The inheritance of acquired characters is disputed.
It is a modification of the original theory of Lamarck in order to make it more suitable to modern knowledge.
- Neo-Lamarckism does not give any importance to the factors of Lamarckism.
- The theory stresses on the direct effect of changed environment on the organism.
- Normally only those modifications are transferred to the next generation, which influence germ cells or where somatic cells give rise to germ cells.
The most impressive study on evolution was made by Charles Darwin. His principal publication, The Origin of Species by means of natural selection or the preservation of favoured races in the struggle for life appeared in 1859.
Factors of the Darwinism
- Over production (rapid multiplication) All organisms possess enormous fertility. They multiply in geometric ratio differential reproduction
- Limited food and space Despite of rapid multiplication, food and space and otherresources remain limited.
- Struggle for existence .
It can be of following three types:
- Environmental struggle
- Variations Except the identical twins, no two individuals are similar and their requirement is different.
- Inheritance of useful variations The organisms after getting fitted to the surrounding transmit their useful variations to the next generations, while the non-usefulvariations are eliminated.
- Formation of new species According to Darwin, useful variations are transmitted to the offspring and appear more prominently in succeeding generations.
- Selection by nature The new species with favourable variations are selected by nature called natural selection.
The organism with favourable variations would survive because they are the fittest to face their surroundings, while unfits are destroyed. Originally, it was an idea of Herbert Spencer (1820-1903), who used the term the survival of the fittest, while, Darwin named it as natural selection. The following three types of natural selection have been categarised by population geneticists:
- Satabilising selection It is also called maintenance evolution. The Stabilising influence of natural selection in an environment changes little in space and time.
- Directional selection It produces a regular change within a population in one direction in respect to certain characteristics.
- Disruptive selection When natural selection simultaneously favours individuals at both extremes of the distribution, the disruptive selection operates
Criticism of the Natural Selection Theory
Many objections against the natural selection theory are follows.
- Inheritance of small variations
- Over specialisation of some organs
- Vestigial organs
- Arrival of the fittest
- Degeneration of organs
- Genetic Basis of Adaptation
It is the evolutionary process, in which, darker individuals come to predominate over lighter individuals since, the industrial revolution as a result of natural selection.
- Hypothesis of industrial melanism was proposed by JW Tutt. This hypothesis was tested by Bernard Kettlewell.
- Since, the enactment of clean air legislation in 1956, in UK, the proportion of non-melanic forms has increased too much higher levels again as the selection pressure on these forms has been reduced in industrial areas.
In the first decade of 20th century, Hugo de Vries based on his work on evening primrose brought forth the idea of mutations, which causes evolution and not the minor variations.
Hugo de Vries (1848-1935), a Dutch botanist, conducted an experiment on Oenothera lamarckiana (evening primrose) and found several abrrent types. He found four types of plants, i.e. progressive (had additional characters), retrogressive (loss of one or more characters), degressive (weak plants with.limited survival capacity) and inconstant (similar to parents and gave rise to variants).
The mutation theory states that evolution is a jerkey process, where new varieties and species are formed by mutation that function as raw material of evolution. Mutation theory explains both progressive and retrogressive evolution.