Darwin and the Theory of Evolution

The Breakthrough Science Society is going to observe the three years from 2007 to 2009 as “Darwin Years”. The year 2007 will mark the 125th year of his death and the year 2009 marks the 200th year of his birth as well as the 150th year of the publication of his seminal book “Origin of Species”. Darwin’s theory of evolution brought about a radical change in the outlook of the contemporary man. Its impact was felt not only in the fields of the biological sciences but also in all the fields of knowledge involving history. Even today his ideas and arguments appear as instruments of struggle in the scientific campaign against irrationalities of all sorts. So, during the next three years, we will study and cultivate the life and works of Darwin through various kinds of science programmes. On this occasion, we intend to publish a series of articles dealing with the theory of evolution as well as the evolution of plants and upto man in the form of translation from the second volume of the Bengali booklet “Vivartan Yuge Yuge” (Evolution through Ages) published by the BSS in 1999. In this issue the first two chapters are presented. – Editorial Board, Breakthrough

WHEN THE SOLAR SYSTEM was born wrote that those regions were under the about 4.6 billion years ago, the planet sea at earlier epochs, and these animals Earth was just a lifeless sphere. Today the lived in that sea. Their bodies got buried same sphere is full of life: trees, grass, in the sediments, which, in course of time insects, birds, reptiles, fish, , became compressed into rocks. When the and the greatest product of the biological sea moved away from that area, these world: thinking man. rocks were exposed. Later, geological But how could such a metamorphosis processes pushed them up to create take place? Till the 17th century, people mountains. Thus, according to him, the thought that God has created man and present unusual placement of the fossils the higher animals and birds; and lower is the result of a completely material organisms like insects, frogs and snakes process. are born spontaneously in clay and rotting Some clues were provided by the biomass. Doubts about such belief findings in geology. As regards appeared when people started finding sedimentary rocks it is generally possible skeletons, bones and other fossil remains to say which rock layer was formed earlier of animals whom nobody has ever seen. and which one later, because they occur Some people said that these were at different depths. It was found that unsuccessful creations of God. When God different types of fossils occur in did not like some of His own creations, He different layers. This implies that all the discarded them without putting life into organisms on the Earth did not appear at them. the same time. Some appeared earlier and A group of inquisitive people was not some later. Depending on the fossils’ happy with such answers, and tried to positions in the rock layers, it is possible look deeper. The famous painter and to identify the order of appearance of the engineer Leonardo da Vinci was among different extinct organisms. Moreover, them. When he saw fossils of sea-dwelling dating of rocks may also suggest the organisms in the high mountains of Italy, probable time of their existence. he did not assume that God created and A deeper probe revealed that there is a placed the fossils there. Instead, he general rule in the order of appearance. described the observations in detail and The organisms which appeared earlier

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Cover Article have simpler anatomical structure and adapted — which may happen on account physiological function. The organisms that of climatic change or migration to a appeared later have greater variety and different location. complexity in their structure and function. This does not mean that fossils of simple organisms are not found in more recently formed rock layers. Indeed, simple organisms (like amoeba) are found in all layers as they are found even now. But it is true that fossils of organisms of complex anatomy are not found in older rock layers. Before a particular time in the Earth’s history, one finds no fossil records of vertebrate animals. Mammals are not found before a specific but later time. Another point. What is said above about animals also apply on plants. It thus became clear that certain groups of species appeared on the face of the Earth at a certain specific time. Most Jean Baptiste Lamarck (1744-1829) of the species existed on the Earth for a specific period, and became extinct. Once “Lamarck was the first man whose conclusions on the subject excited an organism became extinct, it never much attention. This justly celebrated came back. The species that appeared at naturalist first published his views in different points of time, but have not 1801. . . he first did the eminent service become extinct, are found on the Earth of arousing attention to the probability today. This idea was clearly put forward of all changes in the organic, as well as by the French naturalist George Buffon in the inorganic world, being the result (1707-1788). of law, and not of miraculous The focal problem was: How did the interposition.” – Darwin different species evolve? Probing this question, people realized that the changes A change in the environment causes actually occur in the population of each changes in the needs of organisms living species – not at the level of the individual in that environment, which in turn causes organisms. Some scientists looked for the changes in their behaviour. Altered cause-and-effect relations in an attempt to behaviour leads to greater or lesser use of find the explanation. A French naturalist a given structure or organ; increasing use Jean Baptiste Lamarck (1744-1829) first would cause the structure to increase in tried to probe this question in the year size over several generations, whereas 1801. According to Lamarck, the process gradual disuse would cause it to shrink or of evolution is essentially the process of even disappear. This rule — that use or morphological change of the organs disuse causes structures to enlarge or belonging to the members of a species, shrink — Lamarck called the “First Law’’ resulting in the transformation of a in his book Philosophie Zoologique. section of one species into another. Why Lamarck's “Second Law’’ stated that all do the organs evolve? Because of the such changes were heritable. The result of influence of the environment on individual these laws was the continuous, gradual organisms. An organism may face a change of all organisms, as they became change in the environment to which it is adapted to their environments.

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This is known as Lamarck’s use-disuse but two members of the same species are theory. Though it provided a reasonable never exactly identical. This is called explanation of the evolution of many “variation’’. organisms, it also left many questions Secondly, in all species, far larger unanswered. For example, one can talk number of individuals are born than can about the use or disuse of an organ that is grow to maturity. Most die at a young age already present in a species. But how can – either out of attacks by the enemy we explain the creation of an entirely new species, or because of their inability to organ? How could lungs come into being gather enough food. Only a small fraction when marine animals first moved to the can reach maturity and can give birth to land? In philosophical terms, it explained offsprings. It would be disastrous if all only the quantitative changes, but failed rats could live — in a few years they to explain the qualitative changes. would increase in number from a hundred to a million. Even elephants — whose rate of birth is quite low — would increase in number from one pair to 1,90,000 in a matter of 750 years. This general observation applies to snakes, frogs, insects, fishes, and all other species. But we see that in nature, the number of members of every species remains within a limit. It is because for every species, the foodstock is defined, and limited in a given time and space. There is a struggle going on among the members of a species over this limited food-source. This is called the “intra-species struggle’’. Charles Darwin (1809-1882) On the other hand, for every species there are some “enemy’’ species. is In this position, Charles Darwin (1809- the enemy of deer; snake is the enemy of 1882) noticed some common features of frog. Seen from another angle, the deer the biological world. These facts are so species is also enemy of the tiger species, common that those who were engaged in because if deer are able to avoid being the observation and objective analysis of eaten by , the latter will die of the biological world cannot but notice hunger. Two plant-eating species are them. However, it was Darwin who first enemy of each other on the question of realized that they are invaluable clues in collecting food from the same understanding the process of evolution. environment. Every species has specific First, even though the members of a disease carrying germs as enemy. Every species look similar, there are always organism has to enter into struggle with some differences among them. We easily the enemy species in order to remain notice these differences between two alive. This is called the “inter-species humans, because our “human’’ eyes are struggle’’. trained that way. But even if we do not The same thing was observed by the notice such differences in other species, botanists also. Trees, shrubs and plants two tigers or two mosquitoes are never in the same land are in constant struggle exactly the same. Owing to an elementary with each other for access to nutrients, similarity, we can identify them as water, air, and sunlight. All the pollens in members of the tiger or mosquito species, the flowers cannot take part in

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Cover Article fertilization; all the seeds cannot increase. Such overall change of the germinate into saplings; and only a tiny composition of a species is generally not minority of the saplings manage to get perceptible in a few generations. But if enough water, nutrients and sunlight to this process continues for a long time, in mature into trees. In course of this inter- every species, some characteristic traits species and intra-species struggle some will be selected and will be found in larger succeed and some do not. proportion of the individuals. Some other Owing to their existence in these two physical characteristics will prove to be types of struggles, most of the members of detrimental to an organism’s adaptation any species are eliminated before reaching to the environment. Such features will be the age of reproduction. Because of the found in lesser and lesser number of variation within a species, each member is individuals, and will eventually be different from all others in some respect. eliminated from the population. Darwin Some may have features that make them called this long-drawn process as “natural better suited to succeed in this struggle. selection’’, which causes a gradual change This characteristics is called “adaptation.’’ in the overall characteristics of a species. The organism that is better adapted to a Moreover, the “environment’’ for any given environment (both physical and species is not something fixed. If biological), has a higher probability of antelopes become better adapted to their surviving to maturity and producing environment, can survive only by offsprings. On the other hand, variation improving their predatory skills. If lions within a species render some organisms improve their technique of catching prey, less suited for a given environment. They antelopes cannot survive without have a higher probability of dying young. improving their adaptation to the In the predator species, the organisms environment that includes lions. In that are faster and stronger, have a better addition, the physical environment also guarantee of food. The insects whose body changes with time due to geological shape and colour allow them to reasons. camouflage under leaves and tree-trunks, Thus we see an overall process of have a lesser probability of being seen by evolution in the biological world. All the birds and being eaten. Different species are undergoing evolution at the organisms are differently adapted to same time, in different directions. The physical environments like temperature, driving force behind this evolutionary humidity, rainfall, and soil composition. mechanism is natural selection. This is Within a species, all the organisms cannot the main content of Darwin’s theory. be equally adapted to the environment, In the above discussion, the word because of variation. Most die of attacks “species” appeared many times, and it was by enemy species like predators and tacitly assumed that the reader germs, or because they cannot get enough understands what it means. Indeed, the food. Only a tiny minority of the members meaning of the term is intuitively clear in of a species can take part in the process of most cases, as the members of a species producing the next generation. can be identified by their physical The traits in physical structure that features. Nobody ever has a problem of allowed an organism to be better adapted seeing the bovine characteristics in a cow to a given environment, will have a higher or the feline characteristics in a cat. In probability of being transmitted to the fact, Carolus Linnaeus (1707-1778) next generation. Thus, in the next started the job of classifying organisms generation, the number of individuals into separate species using such physical better adapted to the environment will characteristics.

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that produced “Litigon”. Thus the tigon was evidently not sterile. But the fact that and tiger are two separate species is clear to the naked eye! Considering such cases, the modern definition of species is “a group of organisms that reproduce among themselves in the natural environment, and is sexually isolated from other such groups.” This means that reproducibility by artificial means is not the issue. Reproducibility in natural environment is the main factor to be considered. Carolus Linnaeus (1707-1778) Darwin’s main thesis was that a

species develops variations. If organisms In the nineteenth century, many new belonging to a species somehow become regions of the globe were explored, and divided into two or more physically naturalists started visiting far-off places separated and reproductively isolated and deep forests, to collect samples of groups facing different environments, they plants, birds, insects, and other animals. tend to diverge into two species. As each It was then found that serious difficulties adapts to its environment, natural appear in identifying the species to which selection may favour the organisms with the animals belong. In many cases, the certain characteristics. If natural selection physical features of two animals were allows the group to adapt successfully to palpably different, but it was not clear its niche, it survives as a new species. If it whether it was just variation within the does not, it becomes extinct. Thus two or same species or the two belonged to two more species can be created out of a different species. In some cases, different common stock in the process of evolution. species were found to have very similar They in turn cause the creation of physical features. newer species. This way the first life-form So it became necessary to have a has, through the process of evolution clearer definition of “species”. It was found spread over millions of years, given birth that the members of different species to the millions of species that we see on normally do not mate; if they do, no the Earth today. offspring is produced; and in the rare Represented diagrammatically, it looks cases where offsprings are born, they are like a many-branched tree of Fig.1. The sterile. It is not possible to have stem started in the primordial life-form, reproduction between an elephant and a and branched over to form today’s goat. Horses and donkeys can mate and species. In some cases one can see produce . But mules are not capable unbroken lines starting from the first life of further reproduction. So efforts started upto one of the modern species. In most to classify organisms on such definition of cases however, the branches end before species. But there also problems cropped reaching modern age — when a species up. An example will illustrate this. In the became extinct. The dinosaurs are such a Kolkata , scientists have allowed blind lane in the path of evolution. If a reproduction of lions and tigers, and that species, say x, bifurcates into two species has produced the animal, “Tigon”. x and y, we see a continuous line of x. If, The tigon was then allowed to reproduce then, y becomes extinct, the continuity of with a lion, and in the next generation that branch is broken. The process of

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Cover Article evolution never again gives rise to y — This evolutionary “tree” implies that since both x and the environment changes there is a general law or direction in continuously. The existence of a species is evolution. It has not proceeded therefore particular and irreversible. haphazardly. It has proceeded from the simple to the complex, from lower to higher, towards more structural and functional specialization. Immense biodiversity has been created through addition of branches to the tree. But never has a unicellular organism evolved from a multi-cellular one. Never has a reptilian evolved from a or an invertebrate from a vertebrate. Since evolution has such an “arrow”, when a new fossil is discovered, scientists can quite accurately pinpoint a time before which that organism could not have existed. Today man has learnt this process of evolution. This has created the general concept that in this material world nothing is static, unchanging; nothing is unchangeable. Everything is changing — in a natural process of evolution — the sun, the Earth, the biological world, the Fig.1: The evolutionary tree. human race, and their society.

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