(B) Darwin's Theory of Natural Selection Charles Darwin was born on 12th February 1809 in England, Darwin began his career as a ‘Naturalist’ of the ship 1.M.S. Beagle, The voyage of the Beagle started on 27th December 1831 to make a long cruise around the world. He visited many islands like Galapagos islands of Pacific ocean, coasts of South America, New Zealand, Australia etc, and finally returned to England on 2nd October 1836. Darwin took notes on the flora, fauna and the geology of the places he visited, Fig. 2.3 : Darwin (1809-1882). Fig. 2.4 : Wallace (1823-1913). Darwin was also influenced by following : (i) Sir Charles Lyell : Sir Charles Lyell was one of the celebrated geologists of that period. Darwin was greatly influenced by his book "The Principles of Geology". In this book, it was pointed out that the natural forces of the past were the same as those existing at present. Darwin could observe changes in the earth's crust as a result of these forces and also could observe the effects of these changes 00 the plants and animals. In the Galapagos islands, Darwin observed s birds called finches and giant tortoises. These were different from finches and tortoises which were present on the South American main land.There are about 14 varieties of finches on these islands. These scror from ONC another mainly in body size and in shape and size of ae due to different feeding habits. " These variations in finches and tortoises between the members of aifferent islands influenced his thinking on evolution. (ij) Robert Malthus : In October 1838, Darwin came across Essay on Population’ written, by Robert Malthus. It was published in year 1798. In this essay, it was stated that the population increases by ometric progression whereas the food production increases by arithmetic progression. All this results in competition and struggle for existence. . (iii) Artificial selection : It is an ancient practice performed by man to alter characters of certain domestic animals and cultivated crops. The breeders select and perpetuate those variant types that suit their interests or seemed to be useful to them. bi Brassica oleracea (A common wild Mustard) Selection for FaCCHONI selection for stem Selection for Selection for __ Selection for terminal buds leaves stems & flowers _ flower clusters othy Cabbage Paeeee Kohirabi Kale Broccoli Cauliflower sprouts Fig. 2.5 : Some crop plants produced by selective breeding. Man has produced several different varieties of pigeons, dogs, fowl etc. He also has produced several different varieties of ornamental Plants and cultivated plants. For example, man has produced cabbage and cauliflower from a common wild mustard plant Brassica oleracea. th As artificial selection can produce new varieties, then why not ere should be a similar process performed by nature ? And hence,d the word Natural Selection. Darwin believed that No Darwin use but it proceeds too slowly to gj Selection definitely operates, , | bse, argued that if so much change can be achieved by artificial se] * He . eCti + a short period of time, then Natural Selection should be ean in considerable modifications of species over millions and totes of generations. Sof (iv) Alfred Wallace : Alfred Wallace (1823-1913) was a na from Dutch East Indies. In June 1858, Darwin came across an a published by Wallace. Its title was on the Tendencies of Varieties Depart Indefinitely from the Original Type. fo Wallace was also inspired by Malthus's essay on population, Th, Wallace independently conceived the idea of natural selection, “s, On: Ist July 1858, Wallace's essay and a portion of Darwin! manuscript were published simultaneously. Then, Darwin published his book called "Origin of Species" in November 1859; in which he explained the "Theory of Natural Selection". Darwin's Theory of Natural Selection is as follows : (@) Prodigality of Production : Reproductive potential of all the living organisms is far more than their actual need of the offsprings, tural turalise Fig. 2.6: The route of the Beagle. (Note : Galapgos islands in extreme left part) ea But fomeow the population gets restricted, otherwise th ve been able to hold all these organisms. For example : (a) An oyster lays 60 million eggs per year, The progeny of on? pair, if all of them survived, i i tt i , in fi i ells, et times the size of the earth, id Sen oe et e worldEven the slow breeding elephant Produces six young one: betwee” the age of thirty to ninety years. If the Progeny of a single fais sived, within 500 years, there would be 15° million elephants, i ded each one of them lived for 90 years, , (i) Struggle for Existence + Even though, the organisms have this fsach capacity ; of reproduction, the number of organisms is fairly constant regarding each species. Thus, it is clear that there must be the death of several organisms. this is because, though the population tends to increase, the space, food and water are limited. This leads to the struggle for existence. Fig. 2.7 : Darwin's Finches - 1 to 6 Pairs are tree dwellers. 7 and 8 are bush dwellers. 9 to 14 are ground dwellers. Each pair has a male and female. The beaks indicate food preferences which range from insects to fruits, Nuts, seeds. The nuts and seed eaters have stout beaks while the insect eaters have some what elongated and pointed beaks.90° m Darwin was on these islands G&S Culpeper from 16-9-1835 to 20-10-1835 @® Werman pvingdon 3 sincloo GD Gs" . Jarvis Indefatigabe ; ca cig @G Barrington Chatham Charles @® Fig. 2.8 : The Galapagos Islands. Albemarle « Hood “90° The struggle for existence is of three types. (a) Intraspecific : It is the struggle between the members of the same species. The needs of the members of a species are common. Every mem- ber of a species tries to fulfil these needs and therefore there is an intense struggle. Thus, the intraspecific struggle is much intense. For example, a struggle between two frogs of the same pond. s (b) Interspecific : It is the struggle between the members of the different species. It may be a severe struggle or not. For example, @ struggle between a fish and a frog of the same pond. (©) Struggle with environmental factors : All species have © struggle with environmental factors like drought, high temperature, floods, famine, humidity etc, a Gif) Variations and Heredity : The transmission of characters from th cha to the offsprings is called heredity. The dissimilarities between racters of parents and offsprings is called variation. In a broader sense, i ol : se, the resemblance of an individual to its P™ and relatives is called heredity, eaFig. 2.9 : H.M.S. Beagle (H.M.S. stands for His Majesty's Ship) The differences of an individual from its parents or relatives are called variations. An organism may have a variation, which will help it to survive in the struggle for existence. This variation is called an advanta- geous variation. Some organisms have this type of variation right from the birth. All the variations are not useful. Some are neutral, while others may be harmful. The useful variations give rise to new characters and heredi passes them to the next generation. (iv) Survival of the Fittest : In the struggle for existence, those organisms survive, which haye more advantageous variations. These must also show adaptational qualities with the environment. Thus, in the struggle for existence, nature selects the fittest organism to survive. Hence, this is also called a natural selection. (vy) Changing Environment : Environment is never constant. Sooner or later, it undergoes changes. Therefore, the organisms have to show adjustments continuously. But several factors attribute in changing the environment and therefore limitations are bound to be for adaptations. Thus, under the pressure of natural selection, the animals have continuously adjusted themselves to the changing environment by structural or physiological modifications. Not only the environment changes, but animals also migrate to new environments, and in order to survive, they have to acquire some Modifications to suit with new environment, otherwise they will die.(vi) Origin of Species : Continuation of variations and theig gradual accumulation produces new and different types. According to Darwin's theory of evolution, giraffe has been evolved due to following factors. (A) Due to variations, ancestral giraffes were having variable length of necks. (B) The offsprings with longer necks were successfiy in getting foliage of higher levels and therefore, they only survive during scarcity of food. (C) The long neck was favoured by natural selection, so also there might have been some adaptations also. This ultimately gave rise to modern giraffe. Proved Facts 1. Rapid increase in the number of offsprings. 2. The total number is approximately stationary. Consequences Struggle for existence. Natural selection or survival of the fittest. 3. Struggle for existence. 4. Variations and heredity. 5. Survival of the fittest. 6. Changing environment. Structural and physiological modifications leading to origin of new species. Objections to Darwins Theory : Darwin could not explain how the variations arise. Darwin could not explain regarding the formation or presence of vestigeal organs.