Theories of Organic Evolution

1. Lamarckism (Inheritance of acquired characters):

• The first theory of evolution was put forward by French naturalist Jean Baptiste de Lamarck (1744-1829). 

Facts of Lamarckism

i. Tendency to grow: Each and every organism has the natural tendency to grow. The increase in size of an organism is up to a certain limit which is determined by the life itself. 

ii. Formation of new organs: The formation of new organ of the body is the result of the need of which has arisen and continuous to be felt by the organism.

iii. Use and disuse of organ: An organ develops according to its use. Continuous use of organs makes them more efficient and leads to their better development while an organ that is no longer in use would degenerate and ultimately disappear.

iv. Inheritance of acquired characters:

• The characters acquired during the life time of an individual are transmitted to it’s offsprings.
• This new characters accumulate generation after generation, the offspring produced become totally different from the original parents.
• Thus after a number of generations, a new species is formed eg. Development of flightless birds like ostrich from flying ancestors due to continuous disuse of wings as they were found in well protected areas with plenty of food.

Examples

1. Giraffe: In support of his above views, Lamarck put forward the classical example of a giraffe. The long-necked and long limbed giraffe evolved from a short limbed giraffe like a goat. Due to shortage of grasses, the short-necked and short-limbed giraffe started feeding on the leaves of long trees. To reach the leaves available from long trees, the giraffe started stretching its neck and limb. The continuous use of neck and limb gradually made the giraffe long-necked and long-limbed giraffe of today The long neck character acquired by giraffe was transmitted to the offspring, thus establishing long-necked generations. The length was gradually incorporated for generations extending over a period of years and the present long neck giraffe was developed.
2.  Snakes: The absence of feet in snakes is an example of the disuse of an organ. The snakes were originally four-limbed animals. But they adopted to creeping habits and lived in the burrows. Due to the difficulty and obstruction in movement, they lost their limbs resulting into present-day limbless snakes.

Criticism/ Objection to lamarckism

• If the limb or finger is cut, it does not disappear in offspring.
• Eyes that are being used continuously. Instead of being improved as suggested by the theory of use and disuse of organs, eyes are still the weakest organ.
• Boring of ears and nose in women has been continued as a tradition from centuries but their progeny do not show trace of holes in ears and nose.

2. Darwinism (theory of natural selection)

• It is based on a mass of accurate observation. This theory of natural selection is based on the following factors.

i.Excessive rate of multiplication or over population: Every organisms produces much more offspring that can actually survive. In the absence of environmental disaster, living organisms reproduce and increase their number in a geometrical manner. 

For eg. A salmon produces 28 million eggs in one season. If all of them survived and continued to reproduce, a single salmon species would occupy all of the sea after a few generations.

ii. Struggle for existence: The individual multiply geometrically but food resources multiply arthimatically. The available resource can support only a limited number of organism. Darwin observes 3 types of the struggle for existence:  

• Intraspecific struggle: It is the competition between the individuals of same species. 
• Interspecific struggle: It is the competition between the individuals of different species. 
• Environmental struggle: It is the struggle with natural calamities. and environmental disasters such as excessive heat or cold, excess moisture or drought, floods, earthquakes, volcanoes, etc.

iii. Survival of the fittest :

• During the struggle for existence, only the successful variants survive and reproduce further while the rest are eliminated.
• This process is known as the survival of the fittest. In other words, nature selects, the fittest those individuals which are best adapted to the condition of life and eliminates those that are poorly developed.

iv. Variations and heredity

• Every plants and animals shows variations and therefore, no two individuals of same species are ever exactly alike.
• Darwin noticed 2 types of variations- useful variations and useless variations.
• He believed that the individual possessing useful variations would have a better chance of survival than those without them.
• According to him these useful variations are preserved and transmitted to the offspring.

v. Origin of species

Due to continuous variation organisms become better adapted to their environment. The variation gets inherited by its progeny. Progeny also preserves and accumulates these changes or adaptations. After successive generations, the organism becomes different from its parents. As a result new species was established which was quite different from its ancestor. 

Objections to the theory of natural selection

• He did not consider the possibility of the origin of new hereditary variations, which are responsible for the origin of species.
• He does not account for the beginning of organs.
• According to natural selection, only useful organs are selected in the struggle for existence. The vestigial organs serve no functions, yet they are being preserved generation after generation.
• Darwin’s theory does not describe mutations that appear suddenly creating new characters in organisms.
• It does not explain the effects of the use and disuse of organs.

3.  Neo-Darwinism

• It is a modified form of Darwinism. It put forwarded many important ideas for explaining the inheritance of variation and evolution.

i. Mutation: 

• It states that new species arise by sudden change or steps called mutation rather than by gradual process.
• Mutation is defined as sudden heritable changes in the genetic materials of organisms.
• It is caused by changes in the structure of genes, structure of chromosomes or by change in the number of chromosome.

ii. Isolation: 

• It is the process of separation of organism from their original population. It results in the prevention of mating among inter breeding isolation occur due to the physical and biotic barrier. 

iii. Natural selection:

• It suggest that adaptation result from multiple factors and natural selection is only one of these many factors.
• Natural selection does not produce genetic change but once genetic change has occurred, it acts to encourage some genes over there.

iv. Genetic drift:

• The term genetic drift refers to the loss or elimination of the genes of certain traits when one population migrates or dies of natural calamity.
• It alters the gene frequency of the remaining population which causes variations.
• In other words, the random changes in gene frequencies in a population occurring by chance are called genetic drift.
• Random genetic drift is an important mechanism in evolutionary change in small or isolated populations.

v. Genetic recombination

• The rearrangement of genes that occurs at the time of gametes formation is known as recombination it is brought about by crossing over and by the assortment of chromosomes during meiosis cells division which may lead to new combination of genes and phenotypes.