CHAPTER – 7 : EVOLUTION
Theory of Special Creation:
According to this theory, life originated on this earth from super natural powers like god. He created all plants and animals, which appeared on earth in the form they exist today.
Theory of Spontaneous generation or Abiogenesis:
According to this theory life originated on earth from non-living objects spontaneously by a process called Abiogenesis (origin of life from non-living matter). It was believed that fishes and frogs originated from mud, maggots arouse from decaying meat and insects from plant juices and microorganisms from air & water. But later Louis Pasteur disproved this theory and stated that life originate from pre-existing life.
Conditions of Primitive earth/ Origin of life:
It is believed that earth has originated about 4,600 million years ago. It is formed by the condensation and cooling from a cloud of gases and dust. At first the earth was very hot and had various gases and vapour of several elements. With the passage of time, the earth gradually cooled down and gases condensed. Thus a solid crust of earth was formed. There were torrential rains for thousands of years resulting in the formation of large water bodies like oceans.
The earth’s atmosphere at the time was a reducing atmosphere and not an oxidizing one as today. There were large quantities of hydrogen, nitrogen, water vapour, carbon monoxide, methane and ammonia in the primitive atmosphere. However, free oxygen was not present, so the atmosphere is known as reducing atmosphere and this led to the continuous series of chemical reactions among the gases to form amino acids. Hence life originates from reducing atmosphere.
The present atmosphere is oxidizing one and no life is originating today because oxygen will not allow any continuous series of chemical reaction and if any product is formed among the gases that will be oxidized.
A.I. Oparin and J.B.S. Haldane believed that methane, ammonia and water vapours contain the kinds of atoms needed to form various substances such as alcohol and amino acids. Accumulation of such organic compounds within the oceans, lakes, ponds, pools, etc. over million of years must have produced a kind of ‘hot soup’. In this ‘hot soup’ or ‘Darwin’s warm little pond’ smaller organic compounds must have combined together to form larger organic compounds and various macromolecules like polypeptides, proteins, nucleic acids, carbohydrates etc. These compounds then interacted to produce the first living cell. So according to them, the first living cell arose from simple inorganic and organic non-living elements – a process called Abiogenesis.
The energy for such chemical reactions must have come from the heat of the atmosphere and from the electrical energy of lightening.
The most important compound that initially formed is a nucleoprotein (nucleic acid and protein) since it is the chemical characteristic of genes. They might have aggregated in various combinations and must have formed the colloidal masses at the base of oceans. They formed the small globules. They are then covered by fatty acids to form their surface membranes. This membrane also became selectively permeable so a specific organization inside was maintained. Experimental evidences have also shown that such types of cells formed are called as coacervates (pre-cell) and then they gradually transformed into a living cell.
Then enzymes and other important compounds inside were formed. In the present day cells, all these macromolecules are formed by the actions of enzymes. But the enzymes are protein in nature. So initially all macromolecules were formed by non-enzymatic actions.
Urey and Miller experiment:
Stanley Miller and Harold C. Urey in 1953 tested the Oparin-Haldane theory. They made an apparatus to circulate methane, ammonia, water vapour and hydrogen gases. All these gases were put in a flask fitted with electrodes. In another flask, water was being boiled continuously. The electrical charges, to provide energy similar to lightening etc. were passed for one week or more. After that they collected and analysed the contents of the apparatus. He was able to get a number of amino acids, some of which are known to be present in the proteins e.g., glycine, alanine, aspartic acid and glutamic acid. Miller also got several of the simple acids that are known to occur in the living organisms such formic acid, acetic acid, propionic acid, lactic acid and succinic acid.
Hence they proved the Oparin and Haldane theory and now it is clear that reducing atmosphere was essential for such abiotic synthesis.
It is defined as the process of gradual and orderly changes in organisms from one form to another over a period of millions of years. It is a slow and continuous process.
Homologous organs (Divergent Evolution):
Organs having similar embryonic origin and basic plan, but differing in their functions are known as homologous organs. E.g., The arm of man, the leg of a horse, the wing of a bat, the wing of a bird and the flippers of a seal have the same basic plan of development but they are used for different works. All of them possess humerus in upper arm, radius and ulna in the forearm, carpals in the wrist, metacarpals in the palm and phalanges in digits. They also show similarities in the arrangement of the muscles and nerves and also show same pattern of embryonic development.
Homology in plants:
In plants, the homologous organs are a thorn of Bougainvillea and a tendril in Cucurbita both arising in the axillary position, but perform different functions. Thorn for protection and tendril for support.
Analogous organs (Convergent Evolution):
Organs having similar functions but different in their basic plan of development are known as analogous organs. For example, the wing of insects and that of birds or bats are analogous structure. Their basic plan of development is different but has a similar function of flying. In insects wing is an extension of the integument whereas a birds wing is formed of bones covered with flesh, skin and feathers.
- In Opuntia/Cactus, a stem is modified to look like a leaf and may perform the function of a leaf (photosynthesis).
- In potato and sweet potato, potato is a stem tuber and sweet potato is a root tuber. Storage of food is the same function.
Geological time scale:
It shows the ages of the various eras and periods together with the major groups of plants and animals that are believed to have existed during that period. It helps in the study of palaeontology.
It has been divided into 6 eras which are further divided into periods or epochs. Each being characterised by some specific living forms and climatic changes geological time scale is the calender of earth past history indicating the evolution of life through time recorded in sequence of rocks.
The essence of Darwinian theory about evolution is natural selection. Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution.
Lamarck’s concept of evolution/Inheritance of acquired characters:
This theory states that characters are acquired by animals in two ways,
- The effects of environment
- Use and disuse of body parts.
For example, the long neck of giraffe is explained by Lamarck on the same principle. Giraffe, which lived in the dry and arid deserts of Africa, tried to reach the foliage high up on the trees to eat them as there was no vegetation on the ground. In the process its neck and forelegs got stretched a bit and this was inherited to the next generation. Then in the next generation same efforts were continued. Gradually through many successive generations, we got giraffe having such a long neck and forelegs.
Lamarck’s idea of the use and disuse of body parts and the inheritance of acquired characters was not accepted by the scientists. It was disproved by August Wiesmann. He showed that even after cutting the tail of rats for several generations, no rat was born without a tail.
Charles Darwin and Alfred Russel Wallace independently gave the theory of evolution. This theory is known as ‘Darwin’s theory of natural selection’ and is published in a book, “Origin of Species by Natural Selection”. The main features of this theory are as follows,
Reproduction: All organisms reproduce and multiply enormously. Eg. A pair of mice produces dozens of young ones, insects lay thousands of eggs and plants also produce thousands of seeds.
Variations: No two individuals are alike. They differ from each other in size, shape, behaviour, etc. even the offspring of the same parent are never exactly alike except identical twins.
Struggle for existence: All the offspring are not able to reach adulthood. When offspring
become adulthood, then they start to reproduce. This reproductive capacity varies from animal to animal; some reproduce more and some minimum. This differential capacity of reproduction is known as differential reproduction.
Since the number of individuals is far more than actually can survive, so they compete among themselves for food, shelter and space.
Survival of fittest/ Natural selection: Only those individuals which have favourable variations survive and reproduce while others not suited by the environment perish away. Thus nature exercises its selection and only those individuals that are ‘fit’ to survive and reproduce successfully.
Origin of Species/Speciation: This continuous process of variation and natural selection will ultimately result in elimination of certain individuals; while others will gradually establish. In this process new characters, which are good, will set in. Thus new species may be produced in due course of time.
Mechanism of Evolution:
Hugo deVries believed that it is mutation which causes evolution and not the minor variations (heritable) as Darwin said. Mutations are random and directionless while Darwinian variations are small and directional. Evolution for Darwin was gradual while deVries believed mutation caused speciation and hence called it Saltation (single step large mutation).
- According to this law, if all the factors / conditions remain constant, the frequency of particular genes and their alleles will remain constant in a population of sexually reproducing organisms from generation to generation.
- The difference between the observed frequencies of alleles and those predicted by Hardy-Weinberg Principle indicates the degree of evolutionary change. Evolution occurs when the genetic equilibrium is disturbed.
Factors affecting Hardy-Weinberg Equilibrium:
- Gene migration / Gene flow
- Genetic drift
- Genetic recombination
- Natural Selection
Examples of Natural Selection:
A case of natural selection was seen in Great Britain in a peppered moth (Biston betularia). This moth had two forms: grey colour and black colour (Carbonaria). In the early part of the nineteenth century only the grey coloured forms of moths were present; the dark forms were rare. The grey coloured moths were seen on the tree trunks covered with lichens and so they were able to escape from their enemies. Later on, due to the development of industries the lichens were killed and the tree trunks looked dark due to the deposition of industrial soot. Birds, now were able to spot these moths and feed upon them. So the grey coloured moths were eaten by the birds and the dark coloured moths escaped from the birds. Then now the coal is replaced by the industries and oil and electricity is used. This has reduced the soot production and ultimately less deposition of soot on the tree trunks. These tree trunks have, now, again become grey in colour. Consequently, grey coloured moths have again increased in number. This example clearly brings out the action of natural selection.
2.Resistanceof mosquitoes to pesticides.
When DDT was introduced to control mosquitoes it was tremendously successful. Most of the mosquitoes were sensitive to DDT and were therefore killed. In that population of mosquitoes, few mosquitoes became resistant to DDT and survived. They multiplied and now almost total population of mosquitoes became resistant to DDT.
Hence the principle of natural selection shows that the chemical insecticides can remain effective only for a limited period.
The Process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called Adaptive radiation. Ex. Darwinian Finches, Australian Marsupials.
A Brief Account of Evolution:
- About 2000 million years ago the first cellular forms of life appeared on earth. Some of these cells had the ability to release O2.
- Slowly single-celled organisms became multi-cellular forms and by the time 500 mya, invertebrates were formed and active.
- Jawless fish evolved around 350 mya.
- Organisms started to invade from water to land. Fish with stout and strong fins could move on land and go back to water. These fishes evolved into the first amphibians
- Later, these amphibians evolved into reptiles. They lay shelled eggs. Then reptiles of different shapes and sizes dominated on earth (dinosaurs).
- Some of the reptiles evolved into birds and later some of them to mammals. Mammals were viviparous and more intelligent in sensing and avoiding danger at least.
Origin and Evolution of Man:
- About 15 mya, primates called Dryopithecus and Ramapithecus were existing. They were hairy and walked like gorillas and chimpanzees. Ramapithecus was more man like and Dryopithecus was more ape-like.
- Two mya, Australopithecines probably lived in East aftrican grasslands. Evidence shows that they hunted with stone weapons but essentially ate fruits.
- Fossils of first human like being the hominid were found and their brain capacity were between 650-800cc, they were called as Homo habilis. They did not eat meat.
- Fossils discovered in Java 1891 revealed the next stage, i.e., Homo erectus about 1.5 mya and had a large brain around 900 cc and they ate meat.
- Neanderthal man with a brain size of 1400 cc. They used hides to protect their body and buried their dead.
- Homo sapiens arose in Africa and moved across continents and developed into distinct races.
- During ice age between 75,000-10,000 years ago modern Homo sapiens arose.