theories of evolution

Lamarckism

Organic evolution
The doctrine of organic evolution states that the present day organisms are modified but lineal descendants of species that lived in former geological times and that more complex and highly differentiated forms have evolved from simpler ones by gradual modifications. It is still in operation.

One of the first attempts to explain the organic evolution was made by French Biologist Jean Baptiste Lamarck n 1809. he outlined his ideas in his book Philosophie Zoologique published in 1809.

This theory is based on observation that the changes in the environment lead to the changed pattern in the species.

This theory consists of four postulates or facts
Tendency to grow - the internal forces of life tends to increase in size of parts of an organism on the whole up to a certain limit. The limit is determined by life itself.
Formation of new organs - new organ or a part in the body is formed due to long felt need or want. For example webbed toes in duck.
Effects of use and disuse - the more frequently and continuously and animal used an organ; the more it was strengthened, developed and enlarged. If an organ wan not used, it gradually weakened and finally disappeared. For example legs of snake.
Inheritances of acquired character -- all changes which organism acquire during life time are acquired characters. These characters are inherited in the generations. The acquire characters go on accumulating through generations and finally in long course of time produce organism entirely different from the pre existing ones. So, this theory is also known as Theory of acquired characters. For example, Giraffe developed long neck and long legs by continuous effort and stretching to reach the higher level of trees. The longer neck and legs produced, passed on to the next generations. Over long period of time after successive generations, the present day Giraffe with long neck and leg were evolved.


Draw backs

Lamarck’s theory is rejected because acquired characters are never inherited. Acquired characters can bring changes in body or somatic cells, but not in the germ cells.

August Weismann proved that acquired characters affect the body and are somatic; they can not be inherited since they do not affect germ cells. Any variation must be present in the germ cells if they are to be inherited. In one experiment, he cut tail of mice for 20 generations, mouse produced after that have same length of tail. No decrease in length at all.

This theory could not produce any example from plants.

Darwinism

Charles Darwin and Alfred Russell Wallace jointly contributed a paper in 1858 putting forward the view that species had evolved by process of natural selection of those races most fitted to survive in particular surroundings. According to this theory, living organisms have common origin and through a series of small changes in the organism generation after generation new species are formed. This theory is known as theory of Natural Selection or Darwinism.

The theory of Natural Selection can be explained under following observations and consequences.

over production of offspring
Every lining organism tends to increase in a geometrical manner. Such reproductive
Potential of different species may be easily observed in nature. A single pair of English sparrow would be the ancestor of over 275 billion individuals in 10 years if they and their descendants could reproduce at their natural rate without any check. In case of slowest breeding animals like elephants, one pair of elephants in 800 years could have 29 million descendants in the absence of any check.

Total number of survivors remain nearly constant
In spite of their capacity to increase, population remains nearly constant. In case of fishes, they can lay large no of eggs, all the eggs do not hatch. Large number of fingerlings dies or killed before they reach reproductive age. Even in case of human beings, the mortality of infants is much higher. Besides, these are different checks by which population can not increase so high.

struggle for existence
the available food and other resources can support only a limited number of organisms . So animals and plants have to struggle for their existence.
Intraspecific struggle
Intraspecific struggle or competition occurs among the individuals of same species. This type of completion is fierce due to similar requirements. This may be for food, shelter and mate.
Interspecific struggle
This type of competition is found between two individuals of different species. The very common object for this type of competition is food.
Predation
There are different carnivore animals which kill and feed on the herbivores. These animals are said to be predators. Due to predation also no o animals are kept within limit.
Environmental struggle
Living organisms struggle with adverse environmental conditions like drought, heat cold, lightening, earthquake, volcanic eruption heavy rainfall flood etc. Besides out break of diseases, war etc can kill large no. of animals and plants.

variations and heredity
Individuals always try to become better adopted to utilize the resources and to survive successfully. For this they may have certain modifications or changes in their bodies. These changes are called variations. Variations are of two types. Some variations are harmful which make the animal unfit but some variations are useful or favorable. Useful variations are quite significant and make organisms fit in their environment. The same variation may be harmful at one environment but useful at other environment. Variations are inherited in the progeny and therefore progeny have better chances of survival.

Survival of fittest or Natural Selection
Those individuals having favorable variations would survive and propagate their kind. They can increase their population. Herbert Spencer called it survival of the fittest. Those which are fit can survive. According to Darwin, it is Natural Selection. Only those animals having favorable variations are selected. The animals with harmful variations will decline and one day they will become extinct.

Origin of species
Useful variations are accumulated generation after generation for thousands of years. After long time, the generations produced will be quite different from parents. This is the emergence of new subspecies. These new subspecies later on form different species. In this way new species in originated. Some process of variations with heredity accompanied by natural selection was sufficient over the vast geological time to produce all the living animals of today.

Natural Selection is admitted today as the most probable explanation of evolution. With the scanty of knowledge of the nature of variations and heredity before 1900, little could be added to what Darwin knew of the phenomena.

Criticism

A. Mutation is the process in which sudden large variations occur in the animal’s body. It is now proved that mutation has powerful role in evolution. Darwin ignored mutation.

B. Darwin could not identify the heritable variation and non heritable variation. Because only the heritable variations are counted.

C. Natural selection is not a creating or initiating force but only directing or controlling factor. Natural selection operated only after variations have appeared. It accounts for preservation rather that origin of favorable variation.

D. Darwin’s theory does not show how natural selection is brought about nor does it explain the origin of non useful variations.

E. This theory explained the survival of the fittest but not arrival of the fittest.

Neo Darwinism
On these and several other grounds it is now held by many scientists that the theory of natural selection is not only the factor in evolution. Darwin and his contemporaries were not aware of cytological and genetic information which became available later on. Since Darwin’s time interest has not lessened in the matter of how evolution has taken place. At present, the most widely accepted theory of evolution incorporates many of Darwin’s ideas into a frame work of genetics. Neo Darwinism is modification of Darwin’s theory taking into account the results of more recent work.

Scientist like Dobzhansky, RA Fischer, Julian Huxley, JBS Haldane Sewell Wright, Hugo de vries etc contributed to the Neo Darwinism.

It is based on following facts.

Mutation -- it is the sudden heritable change which occur in the chromosome.

Genetic recombination -- crossing over in the meiotic division brings changes in the genetic composition. It is responsible for the variations in the population.

Natural Selection -- useful variations are preserved by natural selection.

Genetic drift or Sewell Wright Effect -- variation in gene frequencies within populations can occur by chance than by natural selection. It is important in small or isolated population.

Reproductive isolation -- it can help to bring more changes in new environment. When not mixed, they become more and more different from one another.