The below mentioned article provides an overview on the Evolution in Plants and Animals.
The term evolution means unrolling or unfolding (Latin word ‘evolve’ = unfolding). It is defined as the process of gradual and orderly change from one condition to another. Such gradual and orderly changes are taking place in all the living, as well as non-living things.
The evolution, therefore, may be of two types:
1. Inorganic evolution, and
2. Organic evolution.
1. Inorganic Evolution:
It explains the changes that are taking place in non-living objects. The planets, stars, the earth’s topography, chemical compounds and chemical elements which form the inorganic world have undergone several changes in the past and shall remain in changing state in the future also. This is inorganic evolution. It is important in the study of geology, astronomy, chemistry and physics.
2. Organic Evolution:
It is concerned with living world. The modem living world (organic world) including both plants and animals has evolved gradually and in orderly manner from primeval ancestors through a process which we speak as organic evolution. Organic evolution refers to the continuous process by which living beings have come to their present forms and functions.
According to doctrine of organic evolution, the present living objects on the earth have descended from the early simpler organisms by the process of gradual modifications and changes. These modifications and changes are brought about by interactions among organisms and their environments. Through gradual modifications, new individuals with better adaptive features appear. To this we call natural selection.
The process of evolution is continuous and it will never come to an end. There are many good proofs in nature to suggest that evolution is taking place and it is taking place on different patterns.
Evolution may speed in either progressive or retrogressive way. Progressive evolution leads the simple forms towards the complex structural and physiological organizations. There are, however, many examples among plants and animals in which evolutionary transformations lead them from high complexity towards decreased complexity.
Thus, structurally more complex organic forms produce simplified individuals. Such a type of evolution is called retrogressive evolution.
The possible derivation of fungi from algal ancestors as a result of loss of chlorophyll, the development of structurally simplified types of flowers from more elaborate and more complex flowers, change from autotrophic nature to parasitic mode of nutrition in parasitic Cuscuta due to loss of chlorophyll are some of the important examples of retrogressive evolution in the plant kingdom.
In the evolution of different groups of plants and animals, many changes occur frequently which follow a similar trend and culminate into a more or less similar morphological organization, although the plants which are subjected to evolutionary changes may be genetically only very distantly related. Such an evolution is known as parallel evolution.
Parallel evolution may run either in progressive direction or in retrogressive direction. Independent evolution of vessels among angiosperms, in some gymnosperms and some of the pteridophytes is one example of parallel evolution.
Parallel evolution proceeding in different plant groups in almost similar way and in nearly similar conditions sometimes produces forms which resemble morphologically one another so closely that it becomes difficult to distinguish them.
This striking condition is sometimes called convergence and this type of parallel evolution is called convergent evolution. The development of hydromorphic and xeromorphic characters in plants of very distantly related groups of angiosperms may be quoted her as examples of convergent evolution.
At this juncture one may believe that evolution is taking place in a continuous and orderly fashion and it produces a series of modifications in the individuals but one confusion may develop in mind and that is—why stages or steps of evolution have never been seen? The only satisfactory answer to this is that the processes of organic evolution are very slow and they require thousands and millions of years in completion.
Unfortunately, the age of man is so short that one cannot see the happenings of many million years in his life time. It is only by seeing the similarities and dissimilarities in the structural organizations and behaviour of organisms in plant and animal kingdoms, the possible trends of evolution are interpreted.
In brief, the modem concept of organic evolution can be summarized in the following points:
(i) The theory suggests that the living world has undergone vast modifications in the remote past, it is still changing, and presumably it will go on changing in the future also.
(ii) The evolution in the past has resulted different types of organisms among which many have become extinct and many of them are still existing.
(iii) Evolution which yields new groups of plants and animals advances very slowly and it requires millions of years to produce new taxa.
(iv) Evolution proceeds either on progressive or on retrogressive lines. Both these processes of evolution are proceeding simultaneously in nature.