EMBRYOLOGICAL EVIDENCES
Earnst Haeckel (1866) studied embryos of various groups of animals and was struck by the resemblances of early embryos of all chordates. It is only at the later stage that they start looking different and show characteristics of the group. He postulated the famous phrase; Ontogeny recapitulates phylogeny, which means that the evolutionary history of an animal is repeated in the embryological development of the animal concerned. Thus, all animals start their life cycle as a unicellular zygote, and then become multicellular morula, hollow blastula and finally the trioploblastic animal. Early embryos of all vertebrates possess typical chordate characters and resemble each other.
PALAEONTOLOGICAL EVIDENCES
Paleontology is the science that deals with the study of fossils of animals and plants in order to draw inferences in support of evolution. Fossil can be anything that can give an indication of the existence of prehistoric organisms. Majority of them are bones buried deep in the soil, which in the course of time turns into rock. Very old bones get petrified and no organic matter is left in them.
Often, impressions, footprints or molds and casts give a fairly clear idea of the animals to which they belonged. Most of the bird fossils, including that of Archaeopteryx, are impressions on the rocks as their bones are too fragile to be fossilized. Fossil footprints of dinosaurs found in America, Australia and also in India, give an idea of not only their size but also the way they walked. Rarely though, we are sometimes lucky to find complete animal preserved including its skin and hairs intact. Discovery of a frozen woolly mammoth in Siberia was such a lucky event but complete insect fossils preserved in amber are not a rarity for entomologists.
TAXONOMIC EVIDENCES
Taxonomy is the science of classifying organisms. The whole exercise of classification takes into account not only the morphological similarities and differences but also evolutionary relationships among different groups. The classification therefore reflects evolution.
The Swedish naturalist, Carl von Linne (1758) proposed the natural system of classification in his book, Systema Naturae and advocated that animals placed in the same group evolved from the common ancestor. For example, snakes and lizards belonging to order Ophidia have evolved from a common ancestral group; and monkeys, apes and man, which have been placed in order Primates, also have common ancestry.
CONNECTING LINKS AS EVIDENCES
While classifying animals we encounter certain animals, mostly living fossils, which fall between two groups as they exhibit intermediate characters. Such connecting links prove that major animal groups have not evolved suddenly and independently but have modified gradually through intermediate stages.
Viruses are capable of living in both nonliving (crystallized) and living phases when they use host cell’s machinery to multiply, suggesting that biochemical molecules must have combined to produce the most primitive life in the early atmosphere of earth. Connecting links are found between all major groups. For example, Proterospongia falls between Protozoa and Porifera, since many collared and amoeboid cells live in a common matrix as in sponges. Neopilina, which was caught from 3500 meter depth off the Pacific coast of South America has a single dome-shaped molluscan shell and a foot but is segmented and possesses nephridia as in Annelida.
Peripatus connects Annelida with Arthropoda by having characters of both the groups. It has annelidan pseudosegmented body, nephridia and simple eyes but also has arthropod clawed segmented legs, antenna and tracheal respiration. The egg-laying mammals, Monotremes, are so primitive that they still carry reptilian characters. The famous fossil of Archaeopteryx has long been considered a connecting link (fossil connecting links are sometimes called missing links) between reptiles and birds.
ZOOGEOGRAPHICAL EVIDENCES
It is generally believed that animals live and propagate in areas of suitable climate and abundance of food. But this contention does not get support from the actual distribution of animals in different continents and islands. Why animals are different in continents where climate is similar, such as in South America, Africa and Australia. Elephants, lions, giraffes, zebras, rhinoceros, apes, hippopotamus etc. that are so common in Africa are absent in South America and Australia.
Why bird fauna of South America is so different and new world monkeys are different from those found elsewhere. But South American tapirs and alligators also occur as far away as in Malaysia and eastern China respectively. On the other hand we come across endemism in monotremes and marsupials of Australia, which are not found anywhere else in spite of the similarity in climate in many places. Sclater (1857) was the first one to address these questions and based on his studies he divided the continental masses into six realms.
Later, A.R. Wallace (1876) carried out detailed studies on the subject and is aptly called father of zoogeography. The peculiarity of distribution of animals can be explained by the fact that animals have a tendency to disperse in all directions in areas of suitable environmental conditions but are restricted by the barriers and hostile environment. Camels and tapirs occur in Asia and South America, two widely separated continents. Similarly, alligators are found in America and China. Apparently these animals have distributed to these areas through land bridges whenever they appeared during the process of continental drift.
Zoogeographical studies reveal how evolution can proceed in different ways in different environmental conditions. Uneven distribution of animals over different continents clearly demonstrates that evolution is the direct result of adaptations of animals to mosaic environment.
PHYSIOLOGICAL AND BIOCHEMICAL EVIDENCES
The composition of protoplasm and nuclear material is similar in all animals. The biochemical reactions and the hormones and enzymes involved in them are also similar. For instance, all animals have glycolysis, Kreb’s cycle, electron transport chain, urea cycle etc. that shows their relationships. Physiology of digestion, respiration, excretion, heart beat and endocrine system is similar with minor differences. This shows that all animals have evolved from the common primitive ancestral animals and developed complex physiological processes as they progressed through evolution.
George Nottal developed Precipitin Test to find out physiological relationship between different groups of animals. In this test blood of man (or any other animal) is injected into a rabbit to produce antibodies against it. Then serum of rabbit is taken and mixed with the blood of other animals to find out relationship. Coagulation of blood after mixing indicates close relationship, as in the case of chimpanzee. There will be no precipitation with the blood of a cat or dog. The precipitin test indicates physiological relationship between animals that has been produced by evolution.
CYTOLOGICAL EVIDENCES
Fundamental structure of cell remains the same in all animals whether lower or higher, suggesting that all animals have evolved from the primitive unicellular animals. Cell organelles, namely, mitochondria, golgi body, lysosomes, ribosomes, nucleus, nucleolus and chromatin are strikingly similar in all animals.
Also the process of mitosis and meiosis are identical, suggesting that all animals have a common origin and therefore inherited the same processes. RNA and ribosomes take part in protein synthesis in all animals and composition of DNA from adenine, guanine, cytosine and thymine in double helix is also the same. Had there been no evolution, animals should have developed different types of cellular compositions independently. Similarity in chromosomal bands in chimpanzee and man shows their close evolutionary relationship.
On the other hand plant cells are slightly different from animal cells by having cell wall made of cellulose and having chlorophyll but within plant kingdom, there is cytological similarity.
GENETIC EVIDENCES
Principles of heredity were discovered my Mendel by experimenting on plants but the same principles apply to animals as well. The mechanisms of mutation, chromosomal aberration, aneuploidy, polyploidy and hybridization are similar in all organisms. Composition and expression of genes is also similar, showing relationship among all animals.
New species are produced by gradual accumulation of genetic changes over long periods and finally producing reproductive isolation between two populations. Mutations can produce sudden changes and evolution of new types. In micro-organisms, such as bacteria and viruses, evolution can actually be seen happening as they mutate quickly and evolve new strains.
Closely related species are known to be genetically compatible and can produce hybrids, e.g. male donkey and female horse can produce mule, which is sterile. But in some species of insects fertile hybrids are known, which can give rise to new species almost instantly. Evolution can actually be demonstrated through cross-breeding experiments in animals.
EXPERIMENTAL EVIDENCES
Best argument in favor of evolution would be to experimentally demonstrate evolution happening. Since evolution takes a long time, sometimes millions of years, it is not possible to show species evolving, particularly in the case of higher animals such as vertebrates. But micro-organisms that have a short life cycle and therefore can complete hundreds of generations in short time, can be used in experiments to demonstrate evolution.
Lederberg’s Replica plating experiment
Lederberg (1952) designed experiment in which he grew bacterium, Escherichia coli, under optimum conditions on a broth and could isolate any streptomycin resistant strain from the culture by growing the colony on sterile nutrient agar plate for easy identification. The colony growing on the agar plate could be transferred to another streptomycin containing plate by imprinting it on velvet and by pressing another plate against it.
The experiment demonstrated that mutations appeared spontaneously but were not selected in a streptomycin free environment. But when the culture was exposed to streptomycin, natural selection operated and the culture transformed gradually into a resistant one. Bacteria and viruses evolve so quickly that the process of evolution can actually be demonstrated.
Experiment by Luria and Dulbruk
Salvador Luria and Max Delbruck (1943) designed a Fluctuation Test Experiment in which they grew populations of Escherichia coli in flasks containing viruses and then checked the growth of colonies on nutrient agar plates. The experiment demonstrated that the bacteria can change from virus sensitive to virus resistant forms when exposed to virus containing cultures.
Melanic moth
The industrial melanic moth (Biston betularia) occurred in light grey form in England before the industrial revolution, because the light colour of its wings provided it with selective advantage to camouflage against the lichen-covered tree trunks. After the industries came up in 1848 and smoke coming out of them killed lichens, baring the dark tree trunks against which light coloured moths could no longer camouflage, the mutant dark coloured forms appeared and replaced the light coloured ones within a span of 50 years. This shows how species can change along with the changing environment.