Evidences of Evolution
By Dr. Girish Chandra
Fossils are the only direct evidences to suggest that evolution has actually taken place and that the species are not fixed but ever changing entities. Unfortunately fossil history of almost every species is incomplete; thereby the evidence of fossils cannot be entirely relied upon in evolutionary studies. Therefore, other evidences, albeit circumstantial, have to be taken into consideration while studying evolution. Some of such evidences are outlined below.
MORPHOLOGICAL AND ANATOMICAL EVIDENCES
While comparing different organs of animal body of various groups, some interesting facts emerge that can be used to demonstrate that gradual change has actually taken place in species.
Homologous organs: When organs of similar origin and ancestry are found to perform different functions they are called homologous organs. For example, limbs of all tetrapods have evolved from the same basic structure found in amphibians but they have modified to perform varied functions in different groups, such as fore limb of man, horse, frog, bat’s and bird’s wing, flipper of dolphin and fossorial leg of mole, all have the same basic arrangement of bones. They all have humerus, radius and ulna, carpals, metacarpals and phalanges in the fore leg and femur, tibia and fibula, tarsals, metatarsals and phalanges in the hind leg. Only their shape and size has modified to suit the specific need of the animal. Serial homology in the limbs of different groups of arthropods from the basic crustacean plan is another striking example of evolutionary modifications.
Analogous organs: Analogous organs are those which perform the same function but have not evolved from the identical ancestral form but have independent origin. For example, wings of insects, bats and birds are used for flight but have evolved independently and thus structurally different. Insect wing is made of chitinous venation and membrane, wing of bat is made of skin called patagium stretched between fingers while bird wing is made of feathers but they are all used for flying. Bones that support the wing of bat and bird are homologous but the patagium and feathers are not.
Adaptations: Environment and ecological conditions change continuously, more so over a long period of time. This puts pressure on the animals and plants to either change accordingly or perish. We, therefore, find animals belonging to the same group, living in different environmental conditions and undergoing different modifications not only in their morphology but also in habits. This may lead to convergent, divergent or parallel evolution in species.
Convergent evolution: Whales and dolphins having lived in aquatic situations for such a long time have changed into fish-like form because the environmental conditions demanded it but they still continue to carry mammalian features such as suckling the young and breathing air. This is called convergent evolution in which unrelated organisms start resembling each other due to environmental influence. Pectoral fins of fish and flipper-like fore limbs in dolphin, sea turtle and penguins demonstrate similarity due to convergence.
Parallel evolution: Sometimes related animals evolve side by side and resemble each other due to evolving in the similar environmental conditions, e.g. ungulates: deer, antelopes, goats, sheep, cow, bison etc. offer identical adaptations for grazing and fast running. All carnivores belonging to cat and dog family show parallel evolution in modifications that help in hunting down the prey. In Australia, marsupials exhibit parallel evolution to the placentals of the world because they occupy identical ecological niches.
Adaptive radiation: Populations of widely distributed species often encounter various types of environmental conditions to which they must adapt gradually. Such populations diverge from each other and in the long run become different species or genera. This phenomenon is called adaptive radiation and was noticed by Charles Darwin in different species of finches found in Galapagos Islands. All these finches had evolved from a single population that was accidentally blown off the coast of South America and upon finding all ecological niches vacant, adapted to feeding on different kinds of food.
Coevolution: An interesting case of evolution can be seen in coevolution, in which two or more species depend so heavily on each other for their survival and propagation that they constantly evolve together. Predator-prey, host-parasite and symbiotic species offer such reciprocal adaptations. While the prey constantly looks for new methods of escaping from predation, the predator manages to acquire equally efficient techniques to catch the prey and the coevolution goes on. The most remarkable coevolution is often seen in flowering plants and their insect pollinators. The nectaries of flowers are often so concealed as to be found only by a particular species of insect pollinator.
Mimicry: Mimicry is a phenomenon in which two different animals resemble each other for gaining protection from the predators. In Batesian mimicry the model is protected and the mimic gets protection by resembling morphologically with the model. So the predators are confused and let the mimic go too. For example, the poisonous Danaus butterfly is mimicked by Hypolimnas female and is avoided by the birds. In Mullerian mimicry both the model as well as the mimic are protected but still resemble each other so that damage done to the population by the learning predators is shared by the two species. Several tailed amphibians having poisonous glands resemble each other and so do the wasps and unpalatable moths.
Vestigial organs: They are those organs which were functional in the ancestors but have lost their significance due to changing circumstances. When organs lose their utility they start to atrophy and eventually become rudimentary. In man 180 such organs are present, e.g. vermiform appendix, ear muscles, plica semilunaris in the eye which is a rudiment of nictitating membrane, rudimentary tail vertebrae called coccyx, small canines, third molar, body hairs etc. These organs were in use in prehistoric man who lived in caves and fed on raw diet but lost their utility when he started to cook food and live in protected situations. Similarly, ostriches and other flightless birds have functionless wings, which lost their value when these birds started depending on their running speed to escape from predators.
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.
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.
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.
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.
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.
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.
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.
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.