The following points highlight the nine evidences for organic evolution of living organisms. The evidences are: 1. Bio-Geographical 2. Morphological 3. Embryological 4. Geological 5. Taxonomical 6. Comparative Physiology and Biochemistry 7. Cytogenetical 8. Domestication 9. Specific Adaptations.
Evidence # 1. Bio-Geographical:
The study of biogeography or the geographical distribution of animals and plants on the surface of the earth throws some light on the possibilities of the origin of species by the process of evolution.
Geographical distribution:
No animal species enjoys uniform distribution over the whole surface of the world, each has a restricted range or area of distribution. Every species of animals produces offspring in excess of the numbers that can survive. The rapid rate of multiplication of all animals causes population pressure by which individuals tend to expand their boundaries.
Many other factors (competition, enemies, disease, adverse seasonal weather, shortage of food, decrease in available shelter) also act to reduce the population. The distribution of animals on the surface of the earth is dynamic and always subject to change. The factors which limit distribution are designated as the barriers.
The barriers are:
(1) Physical barriers:
High and extensive mountain ranges, land mass, large bodies of water, etc.
(2) Climatic barriers:
Temperature, moisture, sunlight and others.
(3) Biological barriers:
Absence of appropriate food or presence of competitors, enemies or diseases, etc.
Each and every species of living organism has a Limit of tolerance to each factor in its environment. The tolerance may be maximum as well as minimum. Changes in a factor in an environment beyond the threshold of tolerance cause in migration or survival or death. The individuals which can survive in the altered conditions are more tolerant i.e., better suited.
The limitation in distribution of a species depends on the sum total of external influences which are mostly interdependent and is subjected to Liebigs Law of minimum (Limited by the presence of essential factor in least amount).
Factors regulating distribution of animals:
Environmental changes sometimes compel animal species to migrate or disperse into new territories because the “old home” is rendered unsuitable for their survival”. But there are barriers of various sorts which stand on their ways and the dispersal of animals is governed by these barriers.
Barriers to dispersal:
1. Physical barriers:
High and extensive mountain ranges act as barriers and limit the distribution of many terrestrial animals. The mountain ranges become an effective barrier if they are situated parallel with the equator as seen in Europe and Asia. When the mountain ranges extend north and south their influence upon the distribution is not marked.
The animal species occupying the northern part differ markedly from those inhabiting the southern part. The great Himalaya Range with its summits beyond the limits of perpetual snow is a notable physical barrier. The south of the barrier is the hot and moist plains of India with characteristic tropical animals resembling those of Africa in many respects.
On the north of the barrier, the conditions of temperature and moisture are entirely different and as a consequent the animals inhabiting the region are totally different from the southern counterparts, resembling nearly to those of Europe. The mountain ranges in the western part of United States exert indirect influence on animal distribution.
On the western side of the barrier, the winds from the Pacific ocean laden with moisture deflected higher and condensed in the form of rain. The winds become moisture-less after crossing the barrier and aridity prevails east of the mountain including dry plains to desert. The differential climatic condition on the two sides of the mountain range influence upon the growth of vegetation.
The nature of vegetation exerts direct influence upon herbivorous animals and indirectly upon the carnivores. Mountain ranges cause variation in rainfall and temperature which govern the distribution of animals to a large extent. Mexican Plateau in North America is an effective and important topographic barrier.
Large bodies of water, when not frozen, form barriers for terrestrial vertebrates except the forms having powers of sustained flight (birds, bats, flying lemur, etc.). Barriers of one group of animals become highways for others.
Extensive bodies of water are inseparable barriers for terrestrial vertebrates but highways for the primary and secondary adapted aquatic animals. Large bodies of salt water are effective barrier for the exclusively freshwater fishes.
Some salt-water fishes (Salmon, Sturgeon, Shad, etc.) migrate to fresh water annually (anadromous migration) for breeding. The reverse migration occurs in eels which pass from fresh water to salt water (catadromous migration) for the same purpose. Salt water constitutes the most effective barrier for the modern amphibians, especially for the larval forms.
For many reptiles, especially crocodiles and sea turtles, seas do not afford an obstacle for movement. Many serpents excepting sea-snakes are usually incapable of passing large bodies of salt water. The flightless birds (Ostrich, Rhea, Cassowary, etc.) are debarred from trans-oceanic migrations. Impurity and lack of salinity of sea water afford barrier to the dispersal of marine invertebrates.
Land masses, just as bodies of water, form effective barriers for the sea animals. Cape Cod which separates cold waters of Massachusetts Bay from the sea is a typical instance. Isthmus of Panama is another example of land barrier. Caspian, a relic sea, contains many marine vertebrates (seals, porpoises) which is not off from the sea by land barrier.
The forefathers of the marine vertebrates entered into it when there was open communication with the sea. Caspian became geographically isolated from the sea with the severance of communication. Lake Nicaragua contains a species of Shark whose forefather entered into it when there was communication with the sea.
2. Climatic barriers:
Degree of heat and moisture exert influence on the distribution of animals. Temperature acts as a limiting factor for the distribution of coldblooded animals. The amphibians and reptiles are quite abundant in tropical and temperate regions, gradually diminishing towards the poles.
Lack of moisture may lead to desert conditions which become barrier to animals excepting desert adapted forms. Excess of moisture renders a swampy condition which becomes impassable to animals which are not adapted to that condition of life.
3. Biological barriers:
Food supplies limit distribution of animals. The animals depend directly or indirectly on the vegetation for food. The growth of vegetation depends largely upon the climatic factors, like degrees of temperature, moisture, sunlight, etc.
Dense growth of forest renders large terrestrial animals incapable of penetrating it. Lack of trees limits the distribution of primates. Arboreal primates require trees for safety, food and progression. Besides food, presence of competitors, enemies and diseases are effective biological barriers limiting dispersal of animals.
Means of dispersal:
Despite effective barriers, there are numerous migratory routes for the dispersal of animals.
The means of dispersals are:
1. Land bridges:
Some continents have been connected by land bridges at times when admixture of faunas of different continents takes place. Panamanian bridge was a land connection between North and South America at least up to late Eocene period. When this connection existed free intermigration of faunas took place between the continents. The land bridge is marked by the position where the Isthmus of Panama now lies.
Natural rafts and drift wood. Many terrestrial animals, either intentionally or accidentally, took shelter upon drifting material which help them to migrate over-water journeys of considerable extent. Natural rafts are potent factor in dispersal of animals. If the factors of repression and expansion remain equal, the area occupied by each species will remain unchanged.
2. Discontinuous distribution:
Every region of the globe has its own quota of animals and plants. Diverse forms of organisms inhabit the different regions of the earth. But the concept of the doctrine of specific centres reveals that a particular species arose at a definite place and then migrated to different regions from the centre of origin. The scattering movements of the animals are due to overproduction and overcrowding in a particular area.
Geographical distribution shows that sometimes a particular species or closely similar species becomes widely separated by its migratory power into different regions of the earth. Once migrated to different regions, they are separated by barriers and owing to geological or climatic variations prevailing in these regions they become different.
Animals, once alike and sharing the same ancestry, become in course of years, decidedly unlike for living in two regions having quite different climatic or life conditions.
Representatives of a group may inhabit two widely different and separated places. A good example is the camel family. One branch of the group comprising of true camels is found in central Asia and Arabia; while the other branch of the family represented by the much smaller and graceful Llama and Alpaca inhabiting the South America. These two branches of the camel family are closely related forms.
This fact is tested by anatomical as well as serological studies. How did they become so diverse? Paleontological study reveals that camels appeared first in North America and then migrated to Asia across a land bridge which connected America with Asia in the prehistoric days. Elephants are found in India and Africa. The Indian elephants are slightly different from there.
African brethren. Great Britain and New Zealand have almost similar climatic conditions but the living forms inhabiting these two regions are quite unlike.
The distributional anomalies in space make it quite dear that the present-day animals have a common ancestry and the migratory power enables them to diverge from the original home. Once migrated to widely different regions, the animals are prevented to come back by physical or biological barriers.
Thus isolated, the animals underwent structural and functional modifications to cope with the prevailing environmental conditions and became adapted accordingly. The animals which were successful to overcome the obstacles, became completely changed and transform into new species.
3. Island fauna:
Island fauna also provide convincing evidence of such distributional effects. Animals inhabiting the islands show marked deviation from the nearest mainland and greater the distance between the places, higher is the degree of differences.
The Galapagos islands may be cited as an example. This group of islands is situated at about a distance of 500 miles apart from the coast of America. The animals present in these islands are either migrated from the mainland or have been blown there through air or could have been drifted there.
The older the islands are the greater are the degrees of differences. Taking all these facts into consideration that the ancestral forms of the animals have migrated to the islands from the mainland and after arriving at these isolated regions they became gradually transformed into new species.
The cumulative evidences of the geographical distribution of animals lead us to the plausible interpretation that a definite change in living organisms is observed which are nothing but the evolutionary changes.
4. Bio-geographical realms:
Besides the familiar political boundaries that separate nations from each other, a remarkable attempt has been put forward to divide the land masses of the globe into different life realms. Such life realms are characterised by the distribution and likeness or unlikeness of their contained fauna. Several plans have been furnished by many workers. These plans have many resemblances but differ in certain minute details.
Variations are due to the selection of animal inhabitants as determinants. First serious attempt to divide the earth into different zoological realms was based upon the distribution of birds. This scheme was confronted with serious objections because of the utilisation of vagrant and barrier-defying creatures like birds as the determinants.
The recent scheme of division of the surface of the earth into different zoological realms is based chiefly on the distribution of mammals. Use of mammals as the determinants has several advantages, because being warm-blooded, they are capable of occupying a wide range of habitats and lastly they are the most recently evolved group of animals and got comparatively less time to radiate from the center of origin.
According to most naturalists (Sclater, 1958; Huxley, 1868; Wallace, 1876), the land surface of the globe is divided into six primary realms (Fig. 1.1) which in turn have been subdivided into subrealms. The land surface of the globe is divided into Nearctic, Neotropical, Palearctic, Ethiopian, Oriental and Australian realms.
5. Nearctic realm:
This region includes the whole of North America to the edge of Maxican Plateau, all the islands of the North together with Greenland.
It has the following subrealms:
(1) Californian,
(2) Rocky Mountain,
(3) Alleghanian and
(4) Canadian.
Nearctic region has characteristic fauna, such as the Opossum, Racoon, Blue Jay, Turkey buzzards, Rattle snakes, Axolotl, Necturus, Amia calva, etc.
6. Neotropical realm:
This region consists of Central America, south of the Maxican Plateau, whole of South America and the Antilles or West Indian islands.
It has the following subrealms:
(1) Brazilian,
(2) Chilean,
(3) Maxican and
(4) Antillean.
Neotropical realms have the following characteristic fauna. Prehensile tailed monkeys (cebidae), Marmoset, Chinchillas, Llama, Rhea, Gigantic Tortoise, Lepidosiren, Vampire bats, etc.
7. Palearetic realm:
This region embraces the whole of Eurasian continent excepting the portion-lying south of the northern line of Afghanisthan and Persia, the Himalayan Mountain and the Nan- ling range in China. Africa, north of
Sahara, Iceland, Spitzbergen, the Arctic islands, north of Siberia are included in this realm.
This region has the following subrealms:
(1) European,
(2) Mediterranean,
(3) Siberian and
(4) Manchurian.
It has Moles, Sheep and Goats, Dormice, Rheasant, Robin, Magpie as the characteristic fauna.
8. Ethiopian realm:
This region includes Africa and Arabia, south of Tropic of Cancer, although some authorities extend it up to north to the Atlas Mountain. Madagascar and other small adjacent islands also come within this realm.
It has the following subrealms:
(1) West African,
(2) East African,
(3) South African and
(4) Malagasy.
The diagnostic fauna are Gorilla, Chimpanzee, Baboons, Zebra, Secretary bird, Protopterus, Giraffe, Lion and Hippopoto- mus.
9. Oriental realm:
This region includes the south doast of Asia, east of Persian Gulf, the entire penihsula of India, the portion of China south of Nan-ling Range (Malaysia). The islands of Sumatra, Borneo, Java, Celebes and the Philippines are also included within this region.
This region is subdivided into the following sub- regions.
(1) Indian,
(2) Ceylonese,
(3) Indochinese and
(4) Indomalayan.
This region is temperate with moderate climatic conditions. Luxuriant growth of forest with rich biotic fauna characterises this region.
The characteristic animals are: Fishes: silurids, notopterids, anabantids, cyprinoids, etc. Primitive fishes are absent. Amphibians: caecilians, rachophorias, tree frogs and toads, few urodeles. Reptiles: vipers, sea-snakes, pythons, Crocodylus, Gavialis, geckos, iguanas, etc. Birds: king-crows, sunbirds, passers, woodpeckers, cuckoos, kingfishers, pigeons, doves, fowls, peacock, etc. Mammals: hedgehogs, shrews, “flying foxes”, old world monkeys, cats, dogs, bears, elephants, rhinoceros, rodents, orangutan, gibbons, tarsiers, etc.
The Indian subregion includes whole of Indian peninsula extending from the Himalyan slopes to Cape Comorin. The Ceylonese subregion includes the island of Srilanka, Indochinese subregion comprises of China, south of Palaearctic boundary, Burma, Thailand, the island of Andamans, Formosa, Haiaan, etc.
The Indo- malayan subregion embraces the Malayan Peninsula and the islands of Malayan Archipelago (Borneo, Java, Sumatra, Nicobar, etc.).
10. Australian realm:
This region includes Australia, New Guinea, Tasmania, New Zealand and the oceanic islands of the Pacific. The Australian region is characterised by both tropical and temperate climates. The inner part of this region has arid climates having deserts. The animals and plants are peculiar because of their isolation from other continents of the world. Amongst the mammals, all the famlies of monotremes and marsupials are present.
They are the predominant mammalian fauna. It is subdivided into the following subregions: (1) Austro- Malayan, (2) Australian, (3) Polynesian and (4) New Zealand. The following animals, Dingo, Parrots, Neoceratodus, etc., are the characteristic fauna.
New Zealand contains Sphenodon and Kiwi. The Austro- Malayan subregion comprises of the islands of Malayan Archipelago (Aru, Mysol, Waigion and Mollucas, New Guinea and Solomon islands); Luxuriant forest, excessive moisture and stable high temperature are the characteristic features of this subregion.
The peculiar animals of this subregion are: Dendrolagus, Petaurus, paradise birds, cassowaries, tree frogs, narrow mouthed toads, Dasyureres (native cat), Gouaidae (crowned pigeons), honey eaters, etc. The Australian subregion includes Australian mainland and Tasmania. Wombats, Duckbilld platypus, Marsupial moles are some of the peculiar animals.
The Polynesian subregion includes Polynesian and Sandrich islands (Fiji, Caroline, Society, New Caledonia, New Hebrides, etc.) Tooth-billed pigeons are the peculiar faunal forms. New Zealand subregion includes New Zealand, Auckland Island, Campbell Island, Norfolk Island, Macquarie Islands, etc. Sphenodon, Liopelma, typical and free-tailed bats are some of the peculiar animal forms.
Owing to the resemblances of their respective fauna, the Nearctic and the Pale- arctic regions are grouped together as Holarctic realm. Lydekker (1896) has proposed the division of the entire terrestrial surface into three major divisions.
According to him Nearctic, Palearctic, Ethiopian and Oriental regions are included under one region—the Arctogaeic realm or the North land. Neotropical is known as Neogaeic and the Australian region is called as Notogaeic realm as shown in Fig. 1.2.
Six bio-geographical or zoogeographical regions are separated from one another by nearly impassable barriers. The Nearctic and Palearctic regions are separated by the Atlantic and Pacific oceans. The Ethiopian region is separated from the Palearctic region by the vast stretch of Sahara desert. The Oriental region is separated from the Palearctic by the Himalayas and Nan-Lings mountain chains.
South America and North America are connected now by the Isthmus of Panama, but during geological eras this connection was submerged. As a consequence South America-was completely isolated from the land masses. Sometimes the physical conditions prevailing within one region may be much alike to those of another. South America and Africa have similar physiography and climate.
These two regions have few organisms in common, which represent the living relics of once world-wide groups. The present-day distribution of dipnoans reveals the same truth. The African genus (Protopterus) and South American genus (Lepidosiren) belong to the same family while the Australian genus (Neoceratodus) is placed in another family.
Considering the distribution of the surviving genera of the dipnoans, it is clear that these three continents of the globe have special relation although these are separated by oceanic barriers. This is testified by the fact that the fossil dipnoans had once enjoyed world-wide distribution.
The geographical distribution of animals on the surface of the earth is easily understandable if one assumes that each group of animals has been originated in one of the central regions of the globe, then they spread to inhabit different zones separated by ecological barriers.
The divergence from the central region results into adaptation of the animals to various conditions. This phenomenon signifies the main theme of evolution.
Evidence # 2. Morphological:
Morphology is a special branch of biology which deals with the form and structure of living organisms. Morphological study of the vertebrate bodies reveals the truth that the different structures are built on a basic plan. Minor variations encountered in some forms are the effect of adaptation to diverse mode of living.
If vertebrates are examined in detail, a striking similarity existing in structures from fish to man is observed. Scientists coined the term “homology” to express the similarity in structure.
Morphological evidence can be discussed under the following heads:
Comparative study of different structures in vertebrates:
A comparative study of the organ systems in different vertebrates shows that they are built on the same structural plan and at the same time shows gradual blossoming up of complexities. There are many examples from the comparative anatomy of vertebrates which provide source of evidences for organic evolution.
The vertebral column in all vertebrates originates from four mesodermal masses in each somite. The vertebral column is composed of vertebrate, each having its centrum, neural arch, neural canal, neural spine, transverse processes and attaching processes.
The heart in vertebrate series relates the same story (Fig. 1.3). The two- chambered heart of fishes become three- chambered in amphibia in response to the change of habitat.
The main purpose is to prevent the admixture of oxygenated and deoxygenated blood. In reptiles the heart is also three-chambered, but the ventricle is incompletely divided into two by an incomplete median vertical partition. In crocodiles the heart is almost four-chambered. But in birds and mammals the heart is completely four-chambered and there is a complete separation of oxygenated and deoxygenated blood.
The aortic arches in different vertebrates also relate the same story. In all the vertebrates, the adult arches develop from six embryonic arches. The variation and modification in different vertebrates (Fig. 1.4) are due to specialisation of respiratory system and heart. The brain in different vertebrates also furnishes the same truth.
Starting with fish to mammal, the brain has five subdivisions and shows progressive evolutionary changes, especially in the development of the cerebral hemispheres and cerebellum (Fig. 1.5). Other homologous structures also relate the same story. Let us take for example of the forelimbs in vertebrates .
The fundamental part of the fore- limbs is its bony structure. In all the vertebrates excepting fishes the upper portion of the forelimb is called the humerus. The humerus is attached to the shoulder girdle. Below the humerus lie the radius and ulna.
The radius and ulna are followed by carpal bones, metacarpals and phalanges. If comparison is made with the corresponding structure of vertebrates a striking similarity exists throughout the series.
Main bony elements are comparable section for section. Modifications are due to adaptation to diverse mode of living (Fig. 1.6). In birds, the forelimbs are modified into wings for flying. In horses, the limbs show cursorial adaptation. In whales, the forelimbs are modified into paddle-like flippers for swimming in water.
In human being, the forelimbs are pentadactylous in nature and are adapted for grasping objects and for manipulation. But examination of the skeletal architecture of the aforesaid types of limbs indicate that they are fundamentally composed of same bony elements and are also built on the similar basic plan. The hind limbs also reveal the same truth and exhibit slight deviation in comparison to the forelimbs.
The survey of any particular system discussed above will show that a particular system is based upon a prototype which becomes modified in class to class.
Vestigial organs:
Vestigial organs are the useless rudimentary remains of the organs that are believed to be fully formed and functional. These structures are useless to their possessors but resemble very closely and correspond to the useful organs of other forms. A large number of examples of the vestigial organs is found in animal kingdom. In Greenland whale, the hind- limbs had no use and have become abortive.
They are represented today by bony remnants which do not even protrude over the body (Fig. 1.7). In seal, the hind limbs become incorporated with the caudal appendage to form an efficient and powerful propelling organ.
The python has the vestiges of the hind limbs among the ventral scales. Practically in all vertebrates the remains of a membrane (nictitating membrane) is found in the inner corner of the eye. This membrane is complete and is perfectly functional in birds.
The greatly reduced wings in some running birds, like Kiwi and Ostrich are also the examples of vestigial structures. Human body has a number of vestigial organs (Fig. 1.8). The muscles concerning the movement of the pinna in man arc small and useless. Vermiform appendix in the caecum in man is a typical example of vestigial organ.
The appendix is well developed in other primates, but in man it remains as a degenerative legacy from ancestors. Weidersheim has recorded as many as 100 vestigial organs in man. Wisdom teeth or third molars, vestigial caudal vertebrae (3-5 in number) of man are some of the typical examples.
Limitation of space will not permit lo give an extensive survey of the vestigial organs in organisms. They are really un-movable and there is hardly an organism whose body does not contain the tokens of the past. In human beings these vestigial organs are the roots of infection, because these are abortive and less vascular structures.
It is extremely difficult to explain these useless structures, but we may assume that they represent the remains of the fully formed functional organs of some remote ancestral forms.
Intermediate forms:
A survey of the animal kingdom reveals the existence of certain intermediate forms which bridge the gap in the sequence of evolution. Monotremcs, as we see today, possess admixture of mammalian as well as reptilian features. They are actually 50% mammalian and 50% reptilian in their anatomical organisation.
Monotremes form an intermediate bridge linking the reptiles to the mammals. Intermediate forms linking closely related groups are quite obvious to be present because of the fact that the existing forms evolved from pre-existing forms lower in organisation. But the intermediate forms are very rare amongst living forms. This may be due to the fact that such forms are usually weak and often fail to survive.
Palaeontology reveals a large number of fossil intermediate forms, viz. Archaeopteryx, linking the reptiles .with birds; Theriodonts linking the reptiles with mammals. Amongst living representatives, the example of Peripatus can also be cited. Anatomically it virtually forms an intermediate stage between annelids and arthropods.
The cumulative sources of morphological evidence cannot be taken as the final proof of evolution but the existence of fundamental similarities of structures naturally gives the indication that evolution has occurred. Homologous organs can be interpreted as the evidence of structural relationship existing throughout the animal kingdom.
Evidence # 3. Embryological:
Embryology deals with the early development of an organism and is also concerned with the changes that take place in an organism from fertilization to birth or hatching. The importance of the embryo- logical evidence rests upon the fact that the embryogeny of higher forms seem to recapitulate the racial history of its ancestors or Ontogeny recapitulates phylogeny.
This concept of recapitulation has given a lot of support to the evolutionary theory and gives indirect evidence of evolution. If evolution is taken to be a fact, embryo- logical evidence appears to be very simple and straightforward.
Recapitulation theory:
It is a celebrated fact that there exists a close similarity in the life history of different organisms, especially in different vertebrates. The development of an individual exhibits general conformities to the early creatures of evolution enroot to finality.
This idea of recapitulation was first struck in the minds of the Embryologists and created a havoc in the minds of the Scientists and an epoch in the evolutionary history. This idea of recapitulation was criticised from many corners but it still occupies a prominent status in the discussion of evolution as a process.
Meaning of recapitulation:
Embryos undergo development and their ancestors have undergone evolution. Recapitulation in biology may be regarded as a relationship existing between embryology and evolution on the assumption’ that the developmental stages of an individual (ontogeny) repeats the post racial history (phylogeny) in an abbreviated and/or accelerated way.
In brief, ontogeny is a short resume of phylogeny or ontogeny repeats phylogeny.
Historical review:
Men from early days tried to link ontogeny with phylogeny. The very term recapitulation finds its expression after much, travel and difficulties. It is an wonderful blending of a number of ideas sponsored by the historic figures of Biology. Greek embryologists have established the fact that the embryos of higher animals resemble that of lower forms.
Aristotle profounded the “Scale of beings” and on the basis of viviparity he put mammals at the summit of the animal society. Mecket stressed the “parallelism” between the embryos of higher forms and the adult stages of lower forms. Von Baer objected to Mecket’s idea of parallelism and puts more weight to his views.
He has expressed the results of his embryological studies in four points:
(1) During ontogeny the general characters appear prior to specialised character.
(2) From the more generalised characters, the less generalised and lastly the specialised characters are developed.
(3) With the development, an animal gradually departs more and more from the forms of other animals.
(4) The young stages in the development of an animal are not exactly similar to the adult stages of other animals lower on the scale but resemble their young stages.
Muller, based on the concept of recapitulation, advocated that evolution might occur in two ways:
(i) By divergence from the ancestral path during development and
(ii) By the addition of new stages at the end of development.
But the real credit for enunciation of the concept of recapitulation goes to the German scientist, Ernst Hacckel who set forth the “Biogenetic law” or the “Recapitulation theory”. Haeckel placed the re- capitulation concept on a solid basis.
Haeckel’s theory of recapitulation:
The spark of Haeckel’s genius had lighted the <4rkness of embryology. The theory of recapitulation indicates that the entire phylogenetic processes occurred for billions of years are telescoped together in the short process of differentiation found in an individual sequence of development. To sum up the recapitulation theory of Haeckel, it can be stated that ontogeny is condensed as a recapitulation of the phylogeny.
His theory has two aspects:
(i) Ontogenesis is the recapitulation of phylogenesis and
(ii) Phylogenesis is the mechanical cause of ontogenesis.
To an embryologist the striking similarity between developing embryos of different vertebrates is a well-known fact (Fig. 1.9.). Haeckel supposed that each animal during its development from the egg to adult, passes through a series of stages reflecting the evolutionary history of the species to which it belongs.
Fig. 1.10 relates the relationship between the ontogenetic and phylogenetic relationship of frog. Presence of gill-slits in mammalian embryo may be a case of ‘hang-over’ as Wadding- ton calls it, from the time when the ancestors of the mammals were fishes. The embryonic history gives sure information not only about the facts of evolution but also the general course it took in a species.
Evidences in support of recapitulation theory:
Haeckel and many embryologists put forward many evidences in support of recapitulation theory.
These are:
(1) Existence of close similarity between embryos of different vertebrates in early stages of development.
(2) The heart in vertebrate series is built on common basic plan. It consists of two portions—receiving parts and forwarding parts. The receiving parts comprise of auricle and sinus venosus while the forwarding parts consist of ventricle and conus arteriosus.
(3) In embryonic stages of all vertebrates, the arterial arches are alike. In all vertebrate embryos, there are six pairs of arterial arches.
(4) Presence of gill-slits in all vertebrate embryos.
(5) In vertebrates, nervous system originates by infolding of dorsal ectoderm.
(6) Nearly all the crustaceans pass through nauplius stage.
(7) Tadpole larva of frog resembles fish.
(8) Mollusca and annclida pass through Trochophore stage.
(9) Covering of hair in human babies (Lanugo).
(10) Antlers of some living deer resemble those of fossil forms.
(11) Often tail is present in human being.
(12) In the ontogenetic development of kidney in higher vertebrates there is a succession of Pro-, Meso- and Metanephros types.
(13) Almost all the coelenterates have ‘planula’ larva.
(14) The vertebrae develop from the same source and in a similar wav in different vertebrates.
Haeckel was not so clear, as to the anatomical construction of the later ancestral forms. An important part of his theory is directed to the distinction between ancestral and adaptive characters in development. So, in order to explain the difficulties and to strengthen the idea of recapitulation Haeckel used some terminologies.
These are: Palingenesis. When characters of the ancestors are conserved by heredity and are faithfully recapitulated at the time of one’s development. Presence of gill-slits in vertebrate embryos is a case of palingenesis. Caenogenesis. The characters which are new adaptations. The young stages of a developing animal present certain structures which no adult ancestor could possibly have possessed.
Their appearance, is due to the result of secondary adaptation. The foetal membranes can be cited as instances of caenogenesis. Trachygenesis. The characters which are accelerated and become crowded back in the embryonic life. Lifiogenesis. When some stages are omitted in the development. Bradygenesis. When some stages are lengthened during development.
Supporters of Haeckel’s theory of recapitulation:
To confirm Haeckel’s view on recapitulation a large number of workers from different corners tried to convince the world by forwarding views. Weismann said that ontogeny arises from the phylogeny by condensation of its stages.
Mac- bride advocated that the larval phase of development of an individual represents a former condition of the adult of the stalk to which it belongs. Graham Kerr advocated that the vertebrate larvae differ very little from that of the common ancestral type. Agassiz observed that the developmental stages of present echinoid show parallelism with the fossil genera.
Merits of recapitulation theory:
After the establishment of the recapitulation theory by Haeckel the idea of recapitulation was accepted readily by most of the workers. But recently the idea of recapitulation is modified and is interpreted correctly. However, Haeckel’s theory of recapitulation must be discussed while explaining evolution as a process. This theory is of great historical importance and lays the foundation of descriptive embryology.
The theory gives the strongest support to the embryological evidence of evolution. Recapitulation theory helps to determine correct systematic position when other evidences are unknown, e.g., Saculina, Ascidia and many parasitic copepods.
Demerits of recapitulation theory:
The theory of recapitulation became a stumbling block for the advancement of embryology. Every worker, whatever, they found about the facts of embryology tried to focus that on the light of recapitulation. But the discovery of organiser action by Spemann and exogastrulation of Holtfreter gave a dread blow to the conservativeness of recapitulation and paved the way of experimental embryology.
Evidences against recapitulation theory:
(1) Thymus gland in vertebrate series:
According to recapitulation theory similar structures must develop in the same way. But in Salmo the thymus gland develops from ectoderm and endoderm. In Talpa it develops from ectoderm and in man it develops from endoderm. So the diversities in the development of thymus gland in vertebrates goes against recapitulation concept.
(2) Tooth and tongue relationship:
Excepting man, in almost all vertebrates teeth develop before tongue, but in man tongue develops first then come the teeth. This is a case of deviation of recapitulation.
(3) Germ- layers and their potencies:
Haeckel regarded that the ectoderm, mesoderm and endoderm are the three fundamental germ layers from which similar differentiation occurs in the same way in all vertebrates. But modern experimental embryologists have changed the concept of rigidity of germ layers,
(a) X-ray irradiation experiments on chick development shows degeneration or stoppage of differentiation of the embryo due to interference.
(b) Transplantation experiments in embryos can defy normal path of differentiation. Transplantation experiment with eye on tail are some of the instances. Weiss has produced a tail in the neck region of the embryo by grafting.
(c) If sugar is lacking in vitro culture of chick embryo, it develops in an abortive way.
(d) The famous exo-gastrulation experiment of Holtfreter is a typical instance of deviation, where the direction of morphogenetic movement is reversed.
(e) Eyes can be made to develop to the median position by applying MgCl2 in the embryonic condition of fish.
(4) In most vertebrates, neural tube is formed as a groove whose dorsal edges fuse to form a tube. But in Petromyzon, Lepidosiren, teleost, etc., it arises as a solid rod which becomes hollowed out subsequently.
(5) Emergence of pro-, mcso- and metanephros in succession in the embryo of higher vertebrates is not a case of recapitulation. Wad- dington, by experimental means, has shown that the pronephros gives the necessary stimulus for the development of mesonephros, similarly mesonephros in turn provides the necessary stimulus for the development of metanephros.
(6) Cetacca originated from ancestors who had well- developed hindlimbs but no trace of hind- limbs is present in their embryo.
Recapitulation theory criticised. The concept of recapitulation of Haeckel is severally attacked and the modern Biologists have modified the original concept. W. His was the first man who challenged the recapitulation theory.
At early stages, developing animals possess the characteristics of the class, order, genus, species and sex to which they belong as well as the individual characteristics. F. R. Lillie also pointed out that not only the final result but all the stages of ontogeny are modified in evolution.
Roux altogether rejected the theory and deduced mechanical process by which different structures arise from particular area of germ. Garstrang gave a new interpretation and regarded that the phyletic line of succession of zygotes run more or less parallel with the adult sequences and ontogeny does not recapitulate phylogeny.
Ontogeny, as he said, is to be the modifications of its predecessor and to a limited sense ontogeny of an animal may best be regarded as an epitome of its phylogeny. The presence of tadpole of frog is not a modification of an adult fish-ancestor but a modification of the larva which that ancestral fish undoubtedly possessed.
Present status of recapitulation theory:
At the present time recapitulation theory is no longer regarded to be correct. In different embryos, there is no doubt, striking similarities exist but this is not the case of repetition of phylogenetic stages as visualised by Haeckel.
Most of the parallelism existing in ontogenetic development may best be regarded as the reflection of characters not of the adults but of the embryos. Recapitulation theory of Haeckel may be said a theoretical explanation having no practical support.
Presence of vestigial structures is not the fulfilment of phylogeny. Experimental embryologists have altogether denied the notion, “ontogeny repeats phylogeny”. By suitable experimental studies, J. Needham in his book “Biochemistry and Morphogenesis” has regarded recapitulation as a phenomenon and not as a law.
The recapitulation theory in its original sense is untenable and has many things to do with the embryos but has little to derive out of it.
Relationship between phylogeny and ontogeny has been explained by many Russian scientists who regarded that the relationship between the two processes does not follow any rule. They have coined several terms to explain the relationship existing between them.
Addition:
When development follows the racial pattern up to the last racial ontogenetic stage. To this, new stages are added to give rise to new adult.
Anchallaxis:
Ontogenetic development deviates from the racial pattern so that recapitulation is absent.
Deviation:
Ontogeny of the organism follows the ancestral pathway of development up to a certain stage and then deviates along a new line.
Abbreviation:
When the last stages of the racial ontogeny are omitted.
Acceleration:
During ontogenetic development the intermediate stages of the racial ontogeny are dropped, so that the later characteristics appear in the ontogeny earlier than they did in the intermediate racial ancestors.
The science of embryology has a long history, many of its pages are torn and others are lost. Haeckel, with his uncanny efforts, tried to insert some pages in that old history. But the magnificent advancement of experimental embryology has broken the backbone of the recapitulation concept of Haeckel. However, it must be admitted that Haeckel had indirectly laid the foundation of the development of embryology.
Nowadays his original idea is not taken as such but has been modified. As the embryonic development is an epigenetic phenomenon, superficial similarities between the embryos are likely to be present.
Haeckel’s recapitulation theory can be interpreted by saying that the individual developmental stages (ontogeny) may at best repeat the ontogenetic stages of the racial forms (phylogeny) and not the phylogenetic stages as such.
Evidence # 4. Geological:
The science of geology furnishes many clues in understanding the process of evolution. The most important branch of geology which provides the greatest support is Palaeontology (Gr. palaios=ancient) which deals with the fossils. This particular branch of science links Geology with Zoology.
Estimation of time:
Estimation of the geological time scale is necessary to determine the age of various fossils. There are several methods to estimate the geological time.
Sedimentation rate technique:
The sedimentary rocks are formed by settling down of the eroded materials and other sediments. The time and rate of formation of sedimentary rock are fairly known at present. Geologists are of the opinion that the sedimentation rates are more or less constant and by examining the sedimentary rock deposits containing fossils the age of the strata can be estimated.
But this method is not taken to be accurate, because the sedimentation rates are not always uniform.
Radiation of heat from earth:
Physicists estimated the age of the earth from the radiation of heat from the earth. It is assumed that the earth was once a molten mass and has cooled down subsequently to its present state.
“Radioactive clocks”—to date the age of fossils:
Scientists determine the age of the fossil deposits from the fact that the radioactive elements undergo decay at regular rates and thus form the ‘radioactive clocks’. There are several ‘radioactive clocks’. They are: uranium-lead ‘clock’, carbon ‘clock’ potassium and argon ‘clocks’, etc.
Uranium-lead method:
The most accurate method of estimation the age of the geological strata comes from the principle of disintegration of radioactive element, uranium (uranium-238) into stable lead (lead- 206), at a constant rate. It is calculated that through such natural disintegration one per cent, of uranium disintegrates in 66,000,000 years.
By comparing the weights of uranium and lead in a particular strata, the actual age of the strata can be determined more accurately.
Radiocarbon method:
The uranium lead method is proved to be very accurate, but uranium is very rare. So technique of dating by using other radioactive elements other than uranium becomes more convincing. The most commonly employed method is the radiocarbon (carbon-14) method. Carbon is utilised by all organisms.
Of the total quantity of carbon used by the organisms, a constant and insignificant quantity is radioactive. It is known that a particular quantity of radioactive carbon loses 50% of its weight in 5,760 years.
Unfortunately materials of more than 40,000 years old cannot be determined by radiocarbon technique. But this method is highly accurate to determine the age of the recent fossils. In the fossil forms the quantity of the radioactive carbon may be estimated and by comparing it with the quantity present in fresh forms—the age of the particular test piece can be determined.
Very recently dating of fossils has been based upon isotopes of potassium and argon.
Fossils form the basis of understanding the process of evolution:
The ever-changing variabilities of life are incomplete if not for the contribution made by the palaeontological aspects of geology. The buried animals and plants in the earth’s strata give us strongest support to the principles of organic evolution. The sole basis of support is the fossils—the nature’s heiroglyphics.
The vanished species imprinted in the pages of the earth furnish the documentary evidences of organic evolution and provide the alphabets with which the language of the biological history is written. Nature has preserved these relics to carry the tale of evolution. They are actually the milestones in the path of evolutionary progression.
This history of the study of fossils dates back to a very ancient time. Fossil was discovered first by Empedocles who collected a fossil of Hippo in Sicily and regarded that as the remnant of God. Aristotle thought the fossil forms as the evidences of an attempt of the inorganic materials to organise the shape and form of organic life. Henrion (1718) regarded fossils as casts and molds left over in the creation of plants and animals.
Leonardo da Vinci (1452-1519) first recognised the fossils as the evidences of animal life of ancient ages. Georges Cuvier (1769-1832) made thorough study of the fossils. He published an account of fossil elephants in the year 1800 and contributed much to the new approach towards the study of fossils and their significance in understanding evolutionary process.
Fossils are the remains of organic life present long ago preserved by natural process in the strata of the earth. Fossils are of diverse forms. Nature has performed many physicochemical experiments to preserve them to carry the thread of evolution. There are certain preconditions for fossilisation. Fossilisation is a very slow and gradual process.
Fossilisation is only possible if the living organisms are immediately buried under earth to escape the contact of atmospheric air to prevent oxidation and must also overcome other destructions. Commonly it is regarded that fossil means petrifaction, meaning thereby turning to stone. Several types of fossils are recorded which can be grouped under four heads:
Petrifaction or pseudomorph:
In this form, original structural pattern is more or less preserved and it has to undergo mineralisation. In this category of fossil usually the hard skeletal parts of the animal body are preserved. Petrification is a gradual process where replacement of molecule for molecule occurs.
The petrified fossils retain not only the external features, but the original histological picture also. Fig. 1.11 shows the petrified fossils of Trilobites.
Preservation intact:
Certain animals are preserved by nature with original substance more or less intact. Many pre-historic animals are preserved in nature’s cold storage, specially in the snowy bed of Siberia. The notable example is the Mammoth preserved in the frozen bed of Siberia.
Similar remains have also been discovered in Alaska. Certain animals, particularly the insects are preserved in crystal clear amber—a fossil resin of pines (Fig. 1.12). At the time of exudation, the resin is soft so as to engulf small insects. The resin becomes subsequently hard and changes over to amber. The amber preserves the delicate part of the insect without causing slightest injury.
Moulds ok casts. Natural casts or moulds are formed when the surrounding region which encircles the organism becomes hard and is converted to stone. The organism, thus enclosed is disintegrated and subsequently removed by natural process. The surrounding stone leaves a cavity which gives the exact contour of the body of the vanished organisms. In this category of fossils no internal structure is preserved.
Foot-prints or imprints:
The impression of the foot-prints or other parts of many organisms are kept preserved on the stone.
Fossilisation occurs when a sector of the earth containing the buried animals are converted into stone by natural processes. In such formation the oldest layer must be the deepest and the newly formed strata will be topmost.
This original orientation of the earth’s strata may be greatly changed by natural catastrophe but the fossils embedded in the strata help to determine the exact age of the strata. Because along with the fossils, certain radioactive substances like uranium which undergoes change into stable lead are also present.
The actual remains (such as mammoths in ice and insects in amber), minute replacements (petrifaction), coarse replacements (molds and casts) and impressions or prints gave direct evidences of organic evolution. Besides, there are many fossils which furnish indirect evidences of organic evolution.
They are:
(A) Coprolites:
Solidified excreta or the casts of the same.
(B) Artifacts:
Prehistoric fashioned flints or ant-hills.
(C) Burrows, tracks, trails:
Burrows, tracks, trails, etc., of living animals.
(D) Geologic:
Geologic formations from organic sources like graphite, flint, limestone, coal, petroleum, etc.
Uses of fossils:
Fossils, the “medallions of creation” help to analyse
(1) Racial history of plants and animals.
(2) Past climatic conditions of the earth and
(3) To measure the geologic time.
Geological time scale:
Based on the study of rocks with its contained fossils geologists have divided the earth’s past history into a number of eras.
Like human history, the earth’s history is also divided into a number of major eras. The eras are divided into periods. The periods are subdivided into epochs (Fig. 1.13).
Geological chronology:
Down to the unimaginable corridors of the geological history, the thread of life has passed from generations to generations ever-varying but unbroken.
Precambrian—the darkest period of geology:
Archeozoic and Proterozoic are the first known eras of the earth’s history. Palaeontology cannot lay its hand on these eras. It was the darkest period of geology when nature was nursing the new-born life.
Postcambrian—the brighter period of geology:
Real geological era starts from Cambrian, when true fossils were discovered. The first creature whose body was perfectly preserved was that of Trilobites. In the Cambrian period almost all major invertebrate groups were evolved. In the Ordovician, progressive evolution of the Cephalopods was observed and the first fossils of fishes were also recorded.
During Devonian period animals first came to land from aquatic home. The entire sea-water became overpopulated and there was struggle for existence. Scorpions were the first to leave the aquatic home and came to land. This was a transformation from water to land and scorpions were the first to come. During this period there was further specialisation of fishes.
Ichthyostega holds the key of amphibian ancestry. The bodily emergence was all right but there was a hindrance in the reproductive system.
They had to go back to the primal aquatic home for the purpose of reproduction. During Carboniferous a peculiar amphibia, Eryops evolved where an initial adjustment of reproductive phenomenon occurred. These amphibians became very sensitive and due to lack of physiological adjustments, they had to bid good-bye to this world.
The last amphibia which could stand was the Stymouria. Seymouria carried the burden of evolution and gave the reptilian line of evolution— the Ldmnoscetis which in turn diverged into two lines—Ophiacodont and Sphenacodont.
Mesozoic—the unique and interesting era:
The entire era is regarded as the Age of Reptiles. There were fluctuating geological conditions. Triassic was the age of turtles, snakes and Saltophosuchus. This period also observed the first sign of birds and mammals. From Saltophosuchus “explosive evolution” occurred and it gave rise to the two branches of Dinosaurs—the Saurischians and Orinthcschians.
But during this era geological changes made environmental conditions unfavourable and inhospitable for the huge dinosaurs to continue. They had to leave the world keeping behind a sad story. The key of dominance of life fell on some humble creatures.
Small in size but unique in their endowments, they constituted a group of animals—the mammals. Hugeness of size has nothing to do in evolution but it was the brain faculty which was of paramount importance.
Cenozoic—the age of mammals:
The Cenozoic era was the age of mammals and birds, on which geology has a lot to say. The fossil remains of the horse, camel and elephant furnish vivid instances of unmistakable transformations of life through geological history.
Palaeontological discoveries provide the strongest evidence of evolution. Fossils in the earth’s crust in almost all cases bridge the gaps left in the march of life through the geological time. Palaeontology provides many ‘missing links’. The most famous transitional form linking the reptiles and the birds is the fossilised bird, Archaeopteiyx.
It had feathered wings like bird, but possessed reptilian teeth and lizard-like tail. Palaeontology also records the complete history of the origin and evolution of many modern mammalian forms. A comprehensive documentary record of the said animals has been vividly discussed. It must be recorded that the fossils buried in the earth’s strata give an admirable support to the principles of organic evolution.
It helps us to understand the basic trends of evolutionary processes. It makes us convinced that the living organisms change in course of time from simple to complex. This progressive trend is amply recorded in the successive geological strata.
Evidence # 5. Taxonomical:
Taxonomy, the science of classification of animals, gives convincing evidence in favour of organic evolution. Modern classificatory scheme of animals is based on structural similarities amongst organisms and such structural resemblances also indicate their relationships. In the realm of taxonomy a phylum is regarded as the largest group of animals having a common ground plan of organisation.
Possession of common characteristics by the members of phylum makes it reasonable to conclude that all of them have evolved from a common ancestral stock. By this way it is possible to arrange the phyla according to their complexities beginning with the Phylum Protozoa and culminating in the Phylum Ghordata.
In the line of evolution there are many unbridged gaps. To connect such gaps we may have to go back to the past history of the animal forms and much of the information is kept hidden amongst extinct forms.
The existing animals represent only the terminal twigs of the evolutionary tree. Direct connection between terminal twigs may sometimes be absent, but the phylogenetic relationship can be established if we follow the branches and stem of the phylogenetic tree.
The interrelationship amongst different phyla gives a clue to the progressive changes in evolution. The taxonomic evidence of evolution stands on morphological and physiological similarities amongst organisms.
Evidence # 6. Comparative Physiology and Biochemistry:
In course of evolution, the change of structure is always accompanied by physiological and biochemical changes. Evolution is basically a summation of different biochemical phenomena. All living organisms are basically built on one substance —protoplasm which varies slightly from species to species.
The chemical composition and functions of protoplasm can be described in a similar way with few exceptions throughout the organic realm. This impressive fact strongly suggests community of origin.
The fundamental properties of living .things remain rather constant, while variation has produced immensely varied forms in the living world. The chemistry of chromosomes—the physical basis of heredity also relates the same story.
The chromosomes, in all living cells consist of basic proteins in combination with nucleic acids. Histone and protamine are the simplest types of proteins present in chromosomes, but globulin and some other proteins are also identified.
The nucleic acids are rather similar. The nucleic acids differ mainly in the sequence of base pairs which unite the nucleotide chains together. The specificity of genes upon which the traits depend is largely due to the sequential arrangement of base pairs joining the nucleotides. The constitution of chromosomes, like that of protoplasm, also indicates the general uniformity of the fundamental units of life.
A survey of the evolution of living organisms reveals that the primitive members of the animal phyla inhabited the palaeozoic seas while their terrestrial relatives became adapted to the terrestrial condition. Physiologically the terrestrial vertebrates have retained many characteristic traces of their descend from the aquatic ancestors. In both aquatic and terrestrial animals, the ionic composition is more or less same.
It is recorded that parasitism is an ancient phenomenon and goes far back into the geological time. The existence of parasites has always been a part of the general pattern of vertebrate life. The parasites, indirectly or directly, furnish evidences of evolution. Certain parasites are recorded to attack only organisms that bear a close resemblance to each other. This indicates uniformity of body chemistry.
Malarial parasites occurring in reptiles, birds, insectivores, some primates, suggest their origin in Mesozoic era before the origin of reptiles. Pinworms occur in amphibia, reptiles, some rodents, primates and indicate their early origin at the time when vertebrates arose.
Similarly parasites like Sclerostomes, Tapeworms that occur in isolated animals, such as South African Strutheo and South American Rhea suggest common origin and their subsequent diversification.
Enzymes and Hormones:
The chemical composition and physiological functions of the enzymes and hormones are closely identical. A protein-splitting enzyme, trypsin, is present from the protozoans to mammals. The starch-splitting enzyme, amylase, is found from sponges to man. The hormones in different mammals are fundamentally alike. Insulin extracted from other mammals is used in diabetic human beings.
The thyroid hormone is similar in vertebrates. Beef thyroid is used biochemically for the treatment of human thyroid deficiencies. Thyroid hormone regulates the metamorphosis of frogs. If thyroid glands are removed surgically the metamorphosis is stopped. But administration of human thyroid tissue extract into such frogs will rectify the deficiency and accelerates metamorphosis.
Similarity in chemistry and physiological role of enzymes and hormones, indicate the fact that some identical genes controlling the activities of the specific glands are present amongst the vertebrates.
Phosphagens:
The chemistry of muscle contraction relates that ATP (adenosine triphosphate) breaks down during contraction to release energy. Phosphagen, an energy-rich compound, breaks down and liberates energy for the resynthesis of ATP. Phosphagen is a specific compound in the muscles of vertebrates. Phosphagen is present in echinoderms and hemichordates—a fact suggesting the phylogenetic relationship between them.
Serological studies:
The comparative study of the body fluid, especially the blood in different forms according to their chemical nature suggests resemblance between different organisms. If foreign protein (antigen) is introduced into the blood of an organism, counteracting substances (antibody) are produced.
This antigen- antibody reaction is very significant in our body-system. Nowadays immunisation against particular diseases may be artificially produced by causing antibody production. Such antigen-antibody reactions are highly specific. These antigen-antibody reactions are used as tools to determine the genetic relationship.
It is seen that antibodies against the blood of an animal react vigorously with the blood of closely related forms and less vigorously with the blood of distant related forms. Serological tests have been invaluable in taxonomy, because they help to establish natural relationships amongst animals.
Serological tests depend on the property possessed by the living body to protect itself against any foreign intruders. An entry of bacteria and viruses into the body system results into the production of defensive substances called the antibodies.
A substance of protein nature which induces the formation of antibodies is designated an antigen. For an example, if the serum of horse’s blood is inoculated into a rabbit, the latter will form antibodies against the serum of horse.
The antibodies are protein substances and an antibody-containing serum is called the antiserum. If the blood serum from a rabbit containing antiserum is removed and mixed it with the serum of horse, the antibodies in serum of rabbit will react with that of horse resulting in the formation of a white precipitate.
These kinds of antibodies are called the precipitins and these serological tests are known as the precipitin test. The precipitin tests have recently been extensively employed to study the evolution of animals.
Similarity in serum proteins:
Domestic rabbits have been extensively used as experimental antibody producers. Inoculation of human serum into the blood of rabbits results in the formation of antibodies against the human serum. The antiserum of rabbit will contain antibodies against human serum. If a little quantity of antiserum is mixed with human serum in a test tube, a white precipitate will form and will settle at the bottom of the test tube.
Mixing of antiserum containing antibodies against human serum with the serum of chimpanzee, baboon and dog in three separate test tubes shows:
(i) The test tube with chimpanzee serum shows similar amount of precipitate like that of human serum. This fact signifies that the serum of chimpanzee and the serum of human are exactly alike in chemical substance.
(ii) The test tube containing baboon serum results formation of a smaller amount of precipitate. These results indicate that the baboon serum contains a few proteins seems
identical to those in the serum of human, while the major proportion of proteins in the serum of baboon are different to those found in human serum.
(iii) The test tube containing dog serum docs not form any precipitate. This is because of the fact that the proteins of dog serum are completely different from those of human serum, because the antibodies formed against the human serum fail to react.
Sometimes a small reaction might occur if the antiserum is very potent. This is because of the fact that all mammalian forms have some chemical similarity in their serum proteins.
The above-mentioned precipitin tests show that the serum of chimpanzee and human is indistinguishable. This similarity is due to common chemical organisation of blood proteins which is due to inheritance from a common ancestry of chimpanzee and man.
The baboon serum is less like human serum than the serum of chimpanzee. This can be explained that in distant past, possibly in the oligocene period, the baboon, old world monkeys and man shared a common ancestry.
They inherited similar type of serum structure from that common ancestor. Subsequent evolutionary divergence from the common ancestral stock has led to the modification of the pattern. Man and chimpanzee shared a common ancestry more recently than the baboon and man. That is why the serological tests show close similarity in serum proteins in man and chimpanzee than the baboon and man.
Similarity in serum structure explains evolutionary process and the serological tests afford a device to detect the degree of phylogenetic relationship between animals. Serological tests advocate that the degree of chemical similarity is always proportional to the degree of phylogenetic relationship. Closely related animals have similar serum proteins while distantly related forms possess serum proteins which are less alike.
Evidence # 7. Cytogenetical:
The similarities and dissimilarities amongst organisms depend upon the similarities and differences in their genetic constitution. Various types of relationships occur at gene level. Cytogenetical studies on Drosophila exhibit close genotypic similarities amongst different species.
The chromosomal configuration of various forms of Drosophila shows close similarity. Cytological and genetical differences and resemblances are used as tools to determine their ancestry. Cytogenetic resemblances amongst – several species are indicative of the fact that they share a common ancestry.
In recent years genetics have contributed much in understanding how the heritable changes, the basis of evolution of a particular type of organism, have operated. Heredity is a conservative process and in rare cases allelomorphic genes may mutate which causes the alteration of the structures dependent upon them.
Such changes or variations are also heritable. In the laboratory and in nature, origin and continuation of such changes can be explained. Like all other evidences, cytogenetic studies provide good evidence of evolutionary process.
Evidence # 8. Domestication:
Different types of animals can be produced artificially under domestication. The great range of size and form of the different breeds of horses is a most striking feature. The small long-haired pony appears as pigmy beside the heavily built large draft horse.
But it is a fact that all these types developed from two or three wild species of horses. By artificial selection in breeding man has produced large variety of forms in cattle, dogs, pigeons, etc. The domestic pigeon furnishes classical example.
All the diverse types of pigeon encountered now are derived from a single wild rock pigeon, Columba livia. The example of Porto Santo rabbit can be furnished to show that living organism may change or evolve into other forms under natural condition. Gonzales Zarco, a Portuguese navigator imported a female rabbit with her litters in the Porto Santo island. The mother rabbit was of European domesticated variety.
The climate of the said island was very favourable for the rabbit and due to the absence of carnivores the rabbit family grew excellently and multiplied at a prolific rate. After a few hundred years when the representative of the island rabbits are compared with the original European stock it was noted with surprise that the two types are quite different which lead some biologists to describe them as distinct species.
The foregoing discussion makes it crystal clear that the living organisms can be changed within a very brief span of time under Artificial Selection. If this be the fact, it will not be Unscientific to hold that natural forces are instrumental in producing various new types in millions and millions of years.
Evidence # 9. Specific Adaptations:
Evolutionary changes are possible because living organisms can adapt successfully to existing environmental conditions. Geological history reveals two categories of evolutionary changes in living organisms. Some organisms are extremely plastic and can successfully adapt themselves to escape any eventuality that they confront, while others cannot respond suitably and as a result they become extinct.
The evolutionary history of elephants furnishes both the facts. About 350 species of fossil elephants are recorded in the geological history, but most of them disappeared from the globe excepting the two surviving genera living in two widely separated countries of India and Africa. The cause of overall extinction is due to the failure of response to the changing environmental conditions.
Few ancestral forms were plastic enough to evolve into the existing types. Of the two surviving types, the African elephant, Loxodonta is fast disappearing because of the ivory-hunters. The Asian elephant, Elephas, has comparatively better chance of survival, because of two possible reasons—the tusks do not yield high-grade ivory and they can be domesticated.
Animals do change in a most striking way to meet specific needs. As a result structural and functional adaptations occur in living organisms. All the adaptive changes furnish the most convincing evidence that living things change in course of time by making new adaptations.
Multitudes of striking adaptations are observed in animals. The protective adaptations of mountain Ptarmigan can be cited as a single instance. They are protected in a wonderful way by changing the colour of the plumage season-wise.
In winter the body is covered over by white feathers to match with the snowy covered surrounding while in summer the colour of the feathers is more or less brown to correspond the background of rocks, earth, leaves, etc. The instance of an Indian butterfly, Kallima, may be presented here as a remarkable instance of protective mimicry.
The adaptations in animals are the outcome of interaction of the organisms with the environmental dynamics. This phenomenon is usually a very slow and gradual process and it requires sufficient time. But there are examples where the adaptive changes can be recorded within a very short time.
The evidences discussed so far in understanding the process of evolution are cumulative. The claim of evolution as a theory rests upon the cumulative effects of all the evidences. No one of the evidences is able to explain the evolution alone but in conjunction with others it gives the final and positive support.
Morphological evidence can only explain that living organisms have changed in course of time and are phylogenetically related with one another.
The resemblances in ontogenic development establishes the hereditary relationship. The geographical evidence reveals that the variation in geographically isolated forms are the adaptive feature to different environmental condition.
Paleontological evidences as fossils preserved in the earth’s crust show gradual evolution of the organic forms. One amongst them cannot prove with finality but all of them jointly point to the same conclusion.
Evolutionary changes are very slow and far- reaching and such changes cannot be observed in the brief life span of a human being. But the evidences force us to believe in unprejudiced manner that evolution has actually occurred.