Definition of Evolution
Evolution is the theory according to which complex forms are considered to have been derived from simpler ones. The word ‘evolution’ has been derived from the Latin word ‘evolvere’ which means to unroll or unfold i.e., the development of something by slow natural processes from rudimentary to more highly organised condition.
Evolution, therefore, may be defined as the gradual and slow development or change of something by natural processes. This change may include the change or development of something like the origin and evolution on the earth; of the land masses on the earth; of seas, mountains etc; of the living organisms like plants and animals on the earth, etc. The last type i.e., the evolution of living organisms e.g., plants and animals comes under the organic evolution.
So, organic evolution means the origin and appearance of new living organisms (both plants and animals) on the earth, as a result of slow and steady natural processes of continuous change.
The term organic evolution may also be defined more precisely as “cumulative change in the characteristics of organisms, occurring in successive generations related by descent” or “as the organization of species of animals and plants as conceived by those who attribute it to a process of development from earlier forms and not as a process of special creation” or “as the doctrine according to which higher forms of life have gradually arisen out of lower” Charles Darwin (1859) has defined evolution simply as “descent with modification”.
Regarding the mode of evolution, there are two main trends e.g., (a) progressive evolution i.e., the upward direction of the evolution, which takes place gradually from simple predecessors to complex types resulting thereby in more complexity and elaboration in structure, and (b) regressive or retrogressive evolution i.e., the downward direction of the evolution, where evolution takes place from complex to simpler types due to reduction and suppression of organs resulting thereby in decline or degeneration in structure.
It is now assumed that present living organisms, both plants and animals have evolved as a result of modification and changes from simplest types of existing forms. The derivation or origin of different and diverse types of organisms did not take place in a year or two, but gradually through the ages comprising millions of years.
No form of life either plant or animal existed in the earth during the first phase of creation of the earth for several hundreds of millions of years. The earth itself evolved out of gaseous burning mass which gradually cooled down.
Consequently, its surface solidified giving rise to lands, oceans and atmosphere. The origin of life began in water in an exceedingly simple form as a minute protoplasmic speck from disintegrated inorganic matters owing to chemical and physical changes which were accidental and whim of nature.
This simplest living being could carry on all life processes e.g., growth, reproduction and other metabolic activities. At the time of appearance of the simplest first living being, there was no difference between plants and animals.
These simple life-forms passed through millions of generations without appreciable change, until with the change of earth’s condition—this slowly gave rise to variations and modification which were better adapted to newer conditions.
The two phenomena of variation and adaptation are the prime factors in causing evolution of new forms of life. Evolution of life then took place into two phyla viz, (a) in the development of animals and (b) in the development of plants. Plant and animal-forms underwent repeated changes as a consequence of adaptation and environments.
Life in the form of simplest organisms came into existence first. From such simplest organisms higher order of plants and animals gradually evolved.
The modifications and changes of simpler organisms (like viruses and bacteria) leading to the origin of higher forms gradually took place through ages comprising millions of years or so.
Whatever the origin of viruses and bacteria may have been, the different divisions of algae have evolved from bacterial ancestors, probably from the members of autotrophic bacteria. But it is not clearly understood from any available evidence whether the different algal divisions arose independently or from a common stock.
The blue-green algae (Cyanophyta) are the earliest plants known from the fossil record—they are extraordinarily primitive in their lack of a distinct nucleus or plastids but they contain chlorophyll. From this fact it is easily understandable that the Cyanophyta may be ancestral to some other algal divisions including Chlorophyta, although the actual evidence of this concept is still meager.
The origin of two groups of molds, viz, Myxomycophyta and Eumycophyta is also doubtful, their origin may be, from bacterial ancestor, or from algae due to loss of chlorophyll, or even from protozoa.
However, much controversial the problem of the origin of those lower groups of plants may be, it appears clearly that the land plants with two major divisions e.g., Bryophyta and Tracheophyta (vascular plants) arose from the green algae independently.
The Bryophytes further have become differentiated into three major groups viz, liverworts, hornworts and mosses, but these group have not produced further any progressive types of plants.
The earliest and primitive tracheophytes (i.e., Psilophytes of Pteridophyta) have succeeded in colonizing the land. This division quickly gave rise to three major subdivisions viz, the Lycopsida (Lepidophyta), the Sphenopsida (Calamophyta) and the Pteropsida (Pterophyta).
For the time being, the first two were dominant groups, but were reduced to minor groups at the end of Paleozoic Era. At first, the pteropsida line was represented by the ferns only, but this line ultimately gave rise to the gymnosperms which in turn seem to have given rise to the angiosperms. From the more primitive sub-class of angiosperms i.e., dicots, much more specialised sub-class, monocots have arisen in course of time.
But great problem arose in ascertaining how, when and under what conditions did all those living organisms come into existence. Various theories, since the time of Greek philosophers, have been put forward to explain the origin of living organisms—these are :
(1) Theory of Eternity
The ancient people believed that all the varied forms of present-day plants and animals have been there from the beginning of the earth and they will continue to live for ever without any change till eternity.
(2) Theory of Special Creation
According to this idea, each and every life-form was created by some supernatural power as a separate and fixed entity. Each life-form was unable to change itself or to give rise to other types of life-forms and was, therefore, not related by descent to any other.
This view was first advocated by a Spanish priest Suarez (1548-1677). According to him “everything was made in six days.” On the first day, earth was made; on the third days, ancestors of plants and animals suddenly appeared; on the fifth and sixth days, ancestors of all present-day living beings appeared.
Theory of special creation was believed previously by many faithful and religious minded people of the west for many centuries although the idea of this theory still persists in the minds of modern people.
(3) Theory of Catastrophism
This theory was proposed by a French palaeontologist Cuvier (1769-1832) from the discovery and study of fossils. It was discovered that all the plants and animals lived in a particular area of the earth for a definite period, after which again a fresh lot of living organisms reappeared.
It was also studied that plants and animals which lived in a particular period were different from those of earlier or later periods. This theory explained the above conditions on the basis of the fact that “the earth was visited periodically by catastrophics which destroyed all life-forms, time to time, each catastrophe being followed by new and specially created forms of life.”
(4) Theory of Spontaneous Origin or Theory of Abiogenesis
Some believed that various types of living organisms originated spontaneously from non-living substances (like mud, rocks, remains of plants and animals, decaying matters etc) automatically in course of time.
Thus, Aristotle believed that mosquitos and fleas arose from decaying matter. Tadpoles, worms and many other micro-organisms were supposed to arise from mud. Flies were supposed to be formed from putrefying flesh.
This theory has been discarded by many scientists like Redi (an Italian physician), Spallanzani (Italian priest) and others on the basis of the fact that life can not be formed de novo, instead life must come from some pre-existing one e.g., formation of new bacteria and fungi due to the multiplication of pre-existing bacteria and spores of fungi.
(5) Theory of Organic Evolution or The Doctrine of Descent
This is the most modern concept of evolution. According to this theory, the plants and animals that have inhabited the earth have been changing through millions of years in the different geological periods of the earth’s history.
This change is very slow, gradual and in orderly form, as a result of this type of change more complex and varied forms of plants and animals have been derived from the simpler types that existed in the beginning of the earth.
This theory was first put forward by earlier Greek philosophers like Democritus, Aristotle and others. But the real proof of the organic evolution came from the contribution of later workers like Linnaeus (1707-1778), Buffon (1702-1788), Erasmus Darwin (1731-1802), Lamarck (1744-1829), St.-Hilaire (1772-1844), Charles Darwin (1809-1882), Weisemann (1834-1914), Hugo de Vries (1848-1935) and others.