Introduction:

Animals and plants and are believed to have undergone considerable modifications over a prolonged time period. Such changes are seen in the great similarities and differences amongst the animals and plants existing on earth. New species of organisms constantly grow from prior ancestral species by way of structural and behavioral adaptations to cope with modifications in the environment. Some of the species of organisms as well perish and become extinct in the continual procedure of change taking place in nature. The concept of evolution describes the whole ongoing growth that takes place in an orderly and sequential manner in living organisms.

Evolution is stated as the growth of more complicated organisms from an already existing simpler organism(s) over a long time period; often million(s) of years. A number of scientists had made precious contributions to the theory of evolution.

Pre-Darwin theories of Evolution:

For a much long time, people had for all time wondered about the origin or beginning of life. Such speculations are grouped into three main theories as follows:

1) Theory of Eternity of the Present Condition:

This theory recommends that there is no starting or end to the universe. According to this theory, the living organisms found on earth nowadays have existed for some millions of years and would remain unaffected all through eternity.

2) Theory of Special Creation:

This theory was stated by religious bodies as an outcome of the account of the creation of plants, animals and the whole thing in this world by God.

3) Theory of catastrophism:

This theory is acknowledged global catastrophes which brought about the death of animals in the past. The death of such old animals on a massive scale gave increase to a new set of animals which occurred over millions of years due to modifications in atmospheric conditions.

Lamarck's Theory of Evolution:

Lamarck (1744 - 1829) was a French scientist. He put forward an evolutionary theory termed as Lamarckism or the theory of use and disuse. The theory proposed that features acquired by organisms due to the ecological changes can be inherited. Description of Lamarckism is as described:

a) Modifications in the environment create some requirements in the organisms.

b) To satisfy the needs, organisms build up some organs.

c) Continuous utilization of the organs builds up them more in size.

d) Organs which were not employed degenerate

e) The newly build up organs acquired by the organisms are inherited by their offspring.

Lamarck described that the long neck of the giraffe was due to generations of neck stretching to browse the tops of trees and shrubs in the absence of lower grasses. The main objection to this theory of use and disuse is that acquired characters are not inheritable. Or else, a man who lost one of his hands in a main accident would continue to generate one-handed children!

Darwin's Theory of Evolution:

The theory of evolution regarding natural selection was put forward by an English scientist, Charles Darwin (1809 - 1882) and it became the most broadly accepted of the evolutionary theories. Charles Darwin not just supported the fact of evolution with powerful proof, however as well described the method of evolution.

Darwin's five theories were:

1) Evolution: The species come and go through time, whereas they exist they change.

2) Common descent: The organisms are descended (that is, go down) from one or some common ancestors and have expanded from the original stock.

3) Species multiply: The diversification of life includes populations of one species diverging till they become two separate species; this has most likely taken billions of times on earth.

4) Gradualism: The evolutionary change takes place via incremental small changes in populations; new species are not made all of a sudden.

5) Natural selection: The evolutionary change takes place via variation among individuals; some variants give the individual an additional survival probability.

Evidence in Support of Evolution

Darwin postulated that evolutionary procedure was a very slow procedure which couldn't be directly observed. Though, scientists from many regions of study have collected a lot of indirect proof to support the theory of evolution. A few of the evidence in support of evolution came from the given sources: fossil records, molecular records, biochemical similarities, Geographical distribution, comparative anatomy and embryology.

Evolution Evidence from Fossil Records:

Fossils are the remains of animals and plants which are preserved either in the form of whole organisms, hard skeletal materials (bones, shells and teeth) in rocks or as the mould cast. The study of fossils is termed as Paleontology.

If fossils are arranged according to their age, the oldest rocks have very few fossilized organisms with simple structures while the younger and more modern rocks include many fossilized organisms in great variety having complex structures. This exhibits an evolutionary trend from very simple organisms in the far-away past to a more complicated and more new forms. Fossil evidence alone is not enough to support the evolutionary concepts.

The fossils which are mostly discovered grouped with the living forms however, can't be confirmed when they in reality symbolize the ancestors of the living species.

Evolution Evidence from Biochemical Similarities:

Various similarities have been observed in the chemical constitution and biochemical procedures of all organisms. Each and every organism comprises of molecules of carbohydrates, proteins, lipids and nucleic acids. The source of energy for the metabolic procedures in the body of each and every living organism is ATP (Adenosine triphosphate). The storage, expression and transmission of genetic information in the living organisms are mainly of the similar materials and procedures. All such biochemical similarities point to a general ancestry for all the organisms.

Evolution Evidence from Molecular Records:

All cells on the Earth, from our white blood cells, to simple bacteria, to cells in the leaves of trees, are competent of reading any piece of DNA from any life form on Earth. This is very strong proof for a common ancestor from which all life goes down.

Evolution Evidence from Geographical Distribution of Animals

The other clue to patterns of past evolution is found in the natural geographic distribution of associated species. It is clear that main isolated land regions and island groups frequently evolved their own dissimilar plant and animal communities. 

Evolution Evidence from Comparative Anatomy:

The comparative anatomy study predates the modern study of the evolution. Early evolutionary scientists such as Lamarck and Buffon employed comparative anatomy to find out relationships among species. Organisms having similar structures, they argued, should have acquired these behaviors from a common ancestor. At present, comparative anatomy can serve up as the first line of reasoning in finding out the relatedness of species.

Evolution Evidence from Embryology:

It is the study of the formation and growth of organism in its earliest phases of development. In most of the cases, the embryo of an organism looks similar to that of its ancestor at certain phases of development. The human embryo for example, consists of gill slits at a phase as in fish, later it develops tail. The human embryo is much identical to the young embryo of other vertebrates which as well pass via the gill-slit phase. This exhibits that all the vertebrates share a common ancestry. Vertebrate development has changed by modifying and expanding on existing genetic materials.

Modern views on Evolution:

Evolutionary change according to the Darwin is mainly caused by natural selection acting via environmental pressures on individuals in a population. Darwin didn't know how inheritance works and as such he could not describe the source of variation and how individual variations occur. The modem vision of evolution that is termed to as Neo-Darwinism is a grouping of Darwin's theory with the present-thy knowledge of genetics and molecular biology. Presently, the procedure of evolution is explained in line with the knowledge of genes and chromosomes that accounts for genetic variation on which the natural selection acts. Except for natural selection, scientists have come to recognize mutation, gene flow and genetic drift as extra natural forces, acting either alone or in combination, to find out the rate and course of evolutionary change. Evolution is now observed in a distinct perspective from the survival or death of individuals however in the perpetuation or removal of the genes which they carry. Individual organisms don't evolve as they retain the similar genetic make-up all through their lives and pass on their genes to the subsequent generation. Evolution only takes place with populations of a species and this occurs if the genetic content of the gene pool modifies.

Genes:

A gene is a section of a DNA (Deoxyribonucleic acid) molecule which is accountable for the inheritance of features. Genes are situated on chromosomes that are found within the nuclei of cells. All the information for the inheritable feature of an individual is contained in genes. Throughout cell division, genes are duplicated all along with chromosomes to make sure that each of the daughter cells has similar set of genes as the parent cell. The genes of parents are passed to their offspring via the gametes all through sexual reproduction.

Mutations:

These are uncontrolled modifications occurring in DNA resultant in the modification of genetic information in particular organisms. Though a very rare occurrence, mutation is the only procedure that modifies genes and once it takes place, it introduces new features into the population of organisms. Mutation might take place in body cells in which case it is termed as somatic mutation and can't be inherited by the offspring. Mutation may also take place during gametes (sex cells) formation, in which case it is termed as germ line mutation that can be inherited. Such a germ line mutation gives the raw material for evolution, as it leads to the inheritance of new features which might or might not be obvious in the parental organism.

Formation of New Species:

The new species of organisms can just be formed via the modification of existing species. This occurs in two ways as shown:

a) Transformation of one species into the other species. In this situation, the ancestral species becomes extinct and is substituted by a new species.

b) New species develop as branches from an existing species. This is the procedure of speciation and is accountable for the diversity of organisms.

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