Brief Essay on Organic Evolution

The term evolution is derived from Latin word “evolve” which means unfolding or unrolling, which implies the process of gradual and orderly chage from one condition to another through succession or a series of changing events.

By organic evolution it is understood that the gradual and orderly changes are taking place in all life forms on the earth’s crust, since the origin of first life.

According to the doctrine of organic evolution as propounded by Charles Darwin (1859) the plants and animals of present day form are the descendants of the plants and animals of the past and the progenitors of plants and animals of the future. The chief postulates of doctrine of organic evolution are:

i. The first or primitive living organisms were very simple in their form and structure.

ii. All plants and animals which have appeared on the earth are the ultimate descendants of the simple, primordial organisms.

iii. During the course of evolutionary change there occurred an ever increasing complexity of structure and diversifications.

iv. Evolution proceeds either on progressive or on retrogressive lines. Both these processes are proceeding simultaneously.

v. Evolution, through which new species of plants and animals are generated, advances slowly which requires millions of years.

Evolution in plants in independent of animal evolution. During prehistoric period of life the divergence of plants from animals plants show adaptive responses and specialization just as animals but line of evolution differs. Organic evolution in plants is a truthful event and a fact. It is supported by following evidences.

1. Evidences from palaeobotany:

The story of evolution of plants is written in rocks in the alphabets of fossils. Thus the fossil history of plants shows the following procession of plant’s life through the ages.

i. Algae are the primitive group of plants originated in aquatic hantat as the age of fossil algae stromatolites was traced back to pre Cambrian period.

ii. Nematophyton are the first true vascular plants invading land evolved from algae in Paleozoic rock.

iii. Psilophyales are common in Devonian rock fossil lycopods are the next steps of evolution.

iv. Proto lepidodendron, follows the lepidodendron are the huge tree like fern ancestors of gynnosperms and angiosperm.

v. Pteridosperms of carboniferous rock provided evidence for first seed plant with frond like leaves and seeds borne in sporophyll.

vi. A good number of fossil conifers are discovered from palaeozoic and mexozoic rocks related to present day forms with modification in them.

vii. Nymphaceae is the angiospermic family which has close relation with fossil ancestors of Jurassic rock.

viii. The wood fossil of homoxylon from rajmahal hills of India is Jurassic fossil resemble the xylem with gymnosperms and pollen grain resemble nelumbo of angiosperns.

2. Evidences from comparative morphology:

There is marked morphological resemblances in body form of different plant taxo shows evolutionary tendencies.

i. Comparative leaf morphology :

The leaves are good photosynthesizing machines which show selective advantage in the variation in leaf structure in different group of plants and modified in relation to its utility. Simple leaves are primitive over compound leaves; pinnate compound leaves are advanced over palmate type. Parallel venation is advanced over netted veins.

Leaves are evolutionary diverged under different adaptive types in the sequence of bud scales of woody plants, spines of cacti scale leaves of onion, tendrils of pea, pitcher, and vensfly trap, succulent leaves etc show evolutionary plasticity.

ii. Stem morphology :

Stems whether erect creeping or climbing does same function. From transport and mechanical support herbaceous stem is ancestors of woody stems; erect stems are advanced over prostate or creeping type.

iii. Floral morphology :

Morphology flowers shows striking instances of evolutions in relation to pollination and fertllization or vernalization. Floral colour, number and nature of petals and sepal, nectarines show evolutionary corresponding. Manoliacous flower is primitive over orchidian flowers.

iv. Homology and analogy :

Organs which similar in structure and differs in the development are homologous but the organs which are differs embryologically in development and similar in function but differs in morphology are analogous organs.

The phylloclade of ruscus and cladodes of asparagus are similar in function and origin and homologous in nature. But both phylloclade and cladodes are analogous to the normal leaves flowers are homologous to vegetative shoot.

Homology and analogy shows strong evidences to the line of evolution.

v. Vestigeal organs :

The structures in the body of plants which are reduced in structure and less used are called vestigeal. In course of evolution these organs reduce retrogressively and become vesitgel.

Staminodes in flowers, specialization of stamens into nectaries, rudimentary leaflets in apricot, reduction of leaflets in to glands in apricot are the signs & evolution.

3. Evidences from comparative anatomy:

i. Cambium comparatively less active in herbaceous stem than the woody stem.

ii. Limited secondary growth in herbaceous plants than woody plants.

iii. Absence of heart wood in herbs and more heart wood in woody stems.

iv. Lack of definite arrangement of vascular bundles in monocots and in ring in dicots.

v. Polyrch stele of monocot is advanced over diarch or triach stele of dicot.

4. Evidences from comparative physiology:

Photosynthesis is the fundamental natural process in green plants; respiration is the common feature of all life forms. Physiological similarity among simplest plants to complex form indicates their common origin from chlorella to higher trees.

5. Evidences from comparative biochemistry:

Presence of characteristic chemical substance like aromatic oils, alkaloids, carbohydrates, viz, armatic in labiatal, alkaloids in solanaceae, oily seeds of eupherbiaceai, resins in pinaceae, establisher relationship among different group of plants.

6. Evidences from developmental biology:

According to biogenetic law or theory of recapitulation by haeckel, developmental pattern of different group of plants establish the phylogenetic relations.

i. Protonema stage of moss resembles algae indicating the evolution of moss from algae.

ii. Filamentous prothallus of ferns found close relation with algae.

iii. Cilliate sperm in modern cycads shows pteridophytean anestry and recalled bryophytes.

7. Evidence from plant geography:

Similar species occupying different geographical area likes to develop divergent lines of evolution longer period of isolation of one region from other the greater is the degree of difference which brought about evolution of new character.

Thus flora of one geographic region differs from other even they are indigenous to a same scope of land.

8. Evidences from ecology:

From ecological succession studies it is evident that, hydric or hydrophytic habit is primitive over the mesophytic and mesophytic is primitive over xerophytic habit.

The plants originated on rock surface like lichens are pioneers of mesophytic forest in aeithoseral community.