Theories of Evolution

THEORIES OF EVOLUTION  Evolution a process that causes changes in a population's genetic material over time. It can result in altered genes, new traits, and new species. Evidence of Evolution The Fossil Record Geographic Distribution of Living Things Homologous Bod...

THEORIES OF EVOLUTION  Evolution a process that causes changes in a population's genetic material over time. It can result in altered genes, new traits, and new species. Evidence of Evolution The Fossil Record Geographic Distribution of Living Things Homologous Body Structures Similarities in Early Evidence for Evolution The Fossil Record- Layer show change Geographic Distribution of Living Things Homologous Body Structures Similarities in Early Development  The Fossil Record Geographic Distribution of Living Things-similar environments have similar types of organisms Homologous Body Structures Similarities in Early Development Evidence of Evolution Homologo us Structures Homologous Structures - structures that have different mature forms in different organisms or functions, but develop from the same embryonic tissue Analogous structure - structures that same function but different origin Evidence for Evolution Vestigial organs-organs that serve no useful function in an organism i.e.) appendix, miniature legs, arms Similarities in Early Development  Jean-BAPTISTE de Lamarck (1744 - 1829) A great French naturalist. Lamarck sought a naturalistic explanation for the diversity of modern organisms and the animals seen in the fossil record. Proposed --‘THEORY OF INHERITANCE OF ACQUIRED CHARACTERS ’ in 1809. Postulates of lamarckism A) NEW NEEDS- Changes in environment factors like light, temperature , medium, food , air , etc or migration led to the origin of new needs in living organisms. To fulfill these new needs , living organisms have to exert special efforts like changes in habits or behaviour. B)USE AND DISUSE OF ORGANS- The new habits involve the greater use of certain organs to meet new needs , and the disuse or lesser use of certain other organs which are of no use in new conditions.  C)INHERITANCE OF ACQUIRED CHARACTERS He believed that the favorable acquired characters are inheritable and are transmitted to the offspring so that these are born fit to face the changed environmental conditions and the chances of their survival are increased. D)SPECIATION- Lamarck believed that in every generation , new characters are acquired and transmitted to next generation so that new characters accumulate generation after generation. After many generations, a new species is formed.  EVIDENCE: a) GIRAFFE - Ancestors of giraffes are supposed to be deer-like quadrupedal grazing upon the grasses in Africa. Due to disappearance of ground vegetation and availability of trees , long-necked and long fore-limbed giraffe s developed from short-necked and small fore-limbed deer-like ancestors.  b) SNAKES - Ancestors of limbless snakes were lizard-like reptiles with fully developed pentadactyl limbs. Due to continuous disuse of limbs and stretching of their body to suit their creeping mode of locomotion , limbless snakes evolved. c) HORSE – Ancestors of modern horse ( Equus cabalus) used to live in the areas with soft ground and were short- legged with more no. of functional digits. These gradually took to live in areas with dry ground. This change in habit was accompanied by increase in length of legs and decrease in functional digits for fast running over hard ground.  CRITICISM OF LAMARCKISM A) August Weismann , a German biologist proposed the ‘Theory of continuity of germplasm’ in 1892. This theory states that a multicellular organism is formed of 2 types of cells- GERM CELLS(have genes for inheritable characters to the offsprings) and SOMATIC CELLS (which have genes of particular organ during ones life time only). Environment affects only somatic cells. As link between the generations is through germ cells and somatic cells are not transmitted to next generation so the acquired characters must be lost with the death of an organism so these should have no role in evolution.  B) Pavlov , a Russian physiologist , trained mice to come for food on hearing a bell. He reported that this training is not inherited and was necessary in each generation. C) Kellogg and Bell experiment – they fed larva of silk moths on reduced quantity of mulberry leaves. They found decrease in size of larva in the next two generations even though these fed normally. D) Castle and Philips experiment- they transplanted ovary of black guinea pig into body of white guinea pig before sexual puberty. This female when mated with white male guinea pig , produced all black young ones. This observation shows that environment does not influence germ plasm. SIGNIFICANCE OF LAMARCKISM a)It was first comprehensive theory of biological evolution. b)It nicely explains the existence of vestigial organs in animals due to their continuous disuse. c)It explains the development of strong jaw muscles and claws in the carnivores due to their continued extra use. d)It stimulated other biologists to look for the mechanism of organic mechanism.  DARWINISM (THEORY OF NATURAL SELECTION Proposed by Charles Darwin (1809-1882 ) A.D.) , an English naturalist. He went on a voyage on H.M.S Beagle and explored S. America , the Galapagos islands and other islands. He was highly influenced by essay entitled ‘On the Tendency of varieties on Depart Indefinitely from the original type’’ by Alfred Russell Wallace and another essay ‘Principle of geology’’ written by Charles Lyell. POSTULATES OF DARWINISM A) Geometric increase B)Limited food and space C)Struggle for existence D)Variations E)Natural selection or survival of the fittest F)Inheritance of useful variations G)Speciation What happened to the giraffe’s? Survival of the fittest or natural selection Natural selection said the giraffes with short necks had less food to eat Why the food resources changed to leaves only on the upper branches What happened short necks could not reach upper branches and did not survive (couldn’t pass on genes. Long neck giraffes survived & reproduced because they were able to reach the food EVIDENCE IN FAVOUR OF DARWINISM A)Close parallelism between natural selection and artificial selection. B)The remarkable cases of resemblance e.g. Mimicry and protective coloration. C) Replacement of earlier giant dinosaurs by small sized carnivorous reptiles due to scarcity of food , space and global cooling. D)pedigree of horse and other animals also support Darwinism. E)Correlation between position of nectaries in flower and length of proboscis of pollinating insect which van be developed only gradually. EVIDENCES AGAINST DARWINISM A)Inheritance of vestigial organs. B)Inheritance of over-specialized organs. C)He didn’t explain the cause of variations and the mode of transmission of variations. D)It doesn’t include the transitional stages which have no fossil record. E)Darwin did not differentiate between somatic and germinal variations. F)It doesn’t explain the evolution of terrestrial animals from aquatic animals.