Organic Evolution PDF

## Organic Evolution ### Definition Evolution is the process by which different kinds of organisms develop from complex form to simple form, due to change of adaptations. The concept of evolution can be explained by theories that are of two types (categories):- - Theories of origin of life - Theor...

## Organic Evolution ### Definition Evolution is the process by which different kinds of organisms develop from complex form to simple form, due to change of adaptations. The concept of evolution can be explained by theories that are of two types (categories):- - Theories of origin of life - Theories of organic evolution ### 1. Theories of Origin of Life Many scientists have tried to describe the origin of life on Earth. The theories that describe the origin of life on Earth are:- - **Special creation theory** - **Spontaneous generation theory** - **Stead state theory** - **Cosmozoan theory** - **Biological evolution (naturalistic) theory** #### (a) Special Creation Theory This theory states that "Everything on Earth, including organisms was created by God or super natural power". It also explains that once organisms are created, they never change (they remain unchanged). **Strength of the theory** - It opened up the minds of scientists to argue (discuss) on the origin of life. **Weakness of the theory** - It cannot be subjected into experiments (it is not scientific). - Organisms can never remain unchanged; they change from time to time. #### (b) Spontaneous Generation Theory In this theory, it is believed that "Life originated from non living matter." It was also believed that living things are originated from non living matter (materials). For example, they believed that;- - Worms and tadpoles (young frogs) originated from the meat. - Fish originated from mud and sand. - Lice originated from dirty etc. **Strength of the theory** - It can be experimented (tested). **Weakness of the theory** - Life can not arise (originate) from non living things. #### (c) Steady State Theory This theory states that "Life had no origin, but the Earth has been able to support it". It also claims that fossils do not indicate the origin or distinction of species. **Strength of the theory** - It is true that life is supported by the Earth. **Weakness of the theory** - It is not true that life had no origin, life must have an origin. #### (d) Cosmozoan Theory This theory states that "Life was brought on this Earth from elsewhere in the universe." This theory doesn't show the specific part of the universe where life was brought but it shows that life is supported on the Earth. **Strength of the theory** - It is true that life is supported on Earth. **Weakness of the theory** - It does not tell the origin of life on Earth. - It does not show the mechanism of how life was brought on the Earth. #### (e) Biological Evolution Theory In this theory, it is believed that "one billion years ago temperature and moisture became suitable for life." The atmosphere at that time consisted of gases like ethane, ammonia, hydrogen, and water vapor. When these gases were exposed in ultraviolet light or electrical charges, ammonia acids were formed. Since ammonia acids are the raw materials of organic molecules, such as proteins and nucleic acids, then it is believed that life originated from the above mentioned gases. **Strength of the theory** - It is scientific and therefore can be experimented. **Weakness of the theory** - The theory does not show how the resulting molecules are mobilized into an organism. ### 2. Theory of Organic Evolution Organic evolution is the process by which organisms undergo gradual (slow) changes in a course (period) of time, so as to adapt the environment. Organic evolution is the process which involves gradual and progressive changes of living organisms over a million of years. The theories which explain the concept of organic evolution are:- - **Lamarck's theory** - **Darwin's theory** #### (a) Lamarck Theory Jean Baptist Lamarck (1774-1829) was the first French scientist to propose the theory of organic evolution in 1809. He formulated his theory and he called it Lamarckism based on two theories which are:- - The use and disuse of organs (parts) - Inheritance of acquired characters In explaining his theory, he suggested that:- - A change in the environment creates new needs in the organisms living in that environment. - The changes are caused by increase in use and disuse of body parts. - He believed that during the lifetime of an organism, those parts which are used tend to develop and become strong, while those parts which are not used weaken and disappear. - He further more explained that the acquired characters as a result of constant use of organ (part) became part of heredity and hence passed from one generation to another. He used the following points (examples) to explain his theory:- ##### (a) Development of Long Necked Giraffes According to Lamarck, the ancestral giraffes had short necks. Then it happened that there was a lack of food on land (environmental needs), hence they had to look for food on leaves of the trees. Due to constant strengthening of the neck muscles, while eating, the giraffes developed a long and strong necks and front legs (use and disuse theory). This acquired character (long necks) became part of heredity and was inherited generation to generation (inheritance of acquired characters). ##### (b) Development of Webbed Feet in Ducks Jean Baptist Lamarck assumed that today's water birds were originally terrestrial (land) birds. Lack of food on land led the ducks to move into water for searching food, due to the constant spreading of their toes whilst swimming they developed webs in their digits. This acquired character (webbed feet) was inherited generation to generation. **Strength of Lamarck's theory** - His work opened up the way for scientists to debate on the course of organic evolution. - He emphasized the role of environment in the course of evolution. - It is true that when the body part is used it becomes strong. **Weakness of Lamarck's theory** - Acquired characters cannot be inherited generation to generation, since they are not controlled by genes. - The disuse of the body part cannot cause it to disappear. - He didn't distinguish inheritable from non- inheritable characters. #### (b) Darwin's Theory Charles Erasmus Darwin was the second scientist to explain the theory of organic evolution. By using his ship HMS Beagle, he travelled for five years from South America to various parts of the world. In his journey, he noted that:- - Organisms of the island are related (but not the same) to organisms of the mainland. - Organisms of the given area appeared related to the fossils of that area. - Organisms of the given area with a given climatic condition appeared related to the organisms in another area with comparable climatic condition. This news made Darwin to put forward his theory of organic evolution in form of a booklet titled Origin of species by means of natural creation. This is called the theory of Survival of the fittest. Darwin's theory is based on three parts:- - Survival of the fittest. - Struggle for existence - Natural selection. Darwin believed that organisms which are well adapted to the environment survive (survival of the fittest), where as those which are less adapted are selected out by nature (natural selection). Darwin's theory of natural selection is based on the number of observations and deductions (conclusions):- **1^st Observation** Organisms in a population tend to over-reproduce than the available resources. In other words, organisms have a greater reproductive ability. **2^nd Observation** The number of individuals in a population remains fairly constant. From these two observations, he made a deduction:- **Deduction 1** Many individuals fail to survive or reproduce due to struggle for existence, that is a competition for the limited resources - food, shelter, water and mates. **Deduction 2** In the struggle for existence, those individuals that are well adapted to the environment survive and reproduce themselves, while those individuals that are less adapted are selected out by nature. ##### Development of Long Necked Giraffes According to Darwin According to Darwin, both types of giraffes (long and short necked giraffes) were present. Due to lack of food on land, they both had to look for food on leaves of trees. The long necked giraffes were able to win this struggle for existence, since they were fit. The short necked giraffes failed in the struggle and therefore they died in the course of natural selection. The characteristic long neck was then inherited from one generation to another. **Strength of Darwin's theory** - It explains evolution more scientifically. - It is true that variations occur among individuals. - It shows the role of environment in evolution. - It stopped the debate on the theory of special creation. **Weakness of Darwin's theory** - He failed to distinguish continuous variation from discontinuous variation. - He accepted inheritance of acquired characters by Lamarck. - He failed to explain variations genetically. - The theory failed to distinguish inheritable from non inheritable features. - It doesn't show how variations are inherited from one generation to another. ### Mechanism of Organic Evolution This means what factors cause organic evolution and how they cause it. The factors which cause organic evolution (agents of organic evolution) are:- - **Mutations** - **Gene drift** - **Gene flow** - **Isolations** - **Selections** #### (a) Mutations Mutation is a change in a gene on chromosome which may result into change in DNA. This change results into difference in characteristics (variations) and hence organic evolution. #### (b) Gene Drift This is the tendency of gene pairs which are heterozygous to become homozygous for one or more alleles. This may cause changes in organisms and finally organic evolution. #### (c) Gene Flow This is the passage (flow) of characters from one generation to another within a population. Gene flow may alter, add or remove certain characters in a population and hence gradual changes in organisms (organic evolution). #### (d) Isolation Isolation is categorized into two categories which are:- - **Geographical isolation** - **Reproductive isolation** ##### (i) Geographical Isolation This involves barriers such as seas, oceans, rivers, mountains, valleys, etc. Since organisms occupy different habitats, then they never meet to carry out mating process. ##### (ii) Reproductive Isolation This involves all factors which enable related population to keep their identity and remain distinct from each other, hence free inbreeding is prevented due to different habitats. #### (e) Selection As in isolation, selection is also categorized into two categories:- - **Natural Selection** - **Artificial Selection** ##### (i) Natural Selection This is a process by which the environment itself selects the organisms that are well adapted to live in it. New species develop from the pre-existing species by natural selection. The process by which new species arise (develop) naturally from the pre-existing species is called Speciation. Natural selection is accompanied (caused) by factors like predation, diseases, droughts, hunger, etc. ##### (ii) Artificial Selection This is a careful and planned breeding done by man. Artificial selection produces new species of plants and animals with disease resistance, high yield of milk, meat, or harvests, etc. Artificial selection is categorized into two groups:- - **Inbreeding** - **Out breeding** ##### (i) Inbreeding This is the mating of organisms which are closely related, such as brother and sister or father and daughter in animals. ##### (ii) Out Breeding This is the mating of organisms which are not closely related. So artificial selection causes variations and, hence, organic evolution. ### Evidences of Organic Evolution Various information and theories provide a considerable evidence to support organic evolution. Among of those evidences are:- #### (a) Evidence from Fossil Records (Paleontology) Fossils are preserved remains of animals and plants (organisms), e.g. bones, teeth, stems, leaves, etc. Fossils are dug up in the sedimentary rocks, those fossils found deeper resemble primitive forms of life, where as those fossils found in the upper resemble modern forms of life. This assures that organic evolution takes place among individuals in specie. #### (b) Evidence from Comparative Embryology Embryos of all vertebrates look resembled at early stages of development. This ensures that all vertebrates originated from the same ancestors (old form of organisms). #### (c) Evidence from Comparative Physiology This involves the chemical composition and functions of the organism's bodies which are similar for the related organisms e.g. respiration in all animals, photosynthesis in all plants, digestion in all animals, etc. These similarities in body functions (processes) indicate that animals originated from the common ancestors. #### (d) Evidence from Geographical Distribution (Biogeography) Organisms are found (distributed) in different geographical locations. This may be due to adaptive features developed by those organisms or certain barriers. Hence here adaptations is said to be the agent of evolution. #### (e) Evidence from Comparative Anatomy/morphology This involves body structures of various organisms. If the structures of the some organisms are compared, they show some similarities which indicate a common origin. These structures are grouped into three groups, namely:- - **Basic Structures** - **Homologous Structures** - **Vestigial Structures** ##### (i) Basic Structures These are structures which are common (shared) in a group of organisms, e.g. mouth parts in insects, beaks in birds, fins in fishes, etc. ##### (ii) Homologous Structures These are body structures of different organisms that are similar but perform different functions due to adaptations, e.g. fore limbs of vertebrates (in man for catching, in birds for flying, in whales for swimming, etc). Those body structures which have different origin but they perform the same functions (e.g. lungs and gills for gaseous exchange) are called analogous structures. ##### (iii) Vestigial Structures These are structures which have lost their function in a course of evolution. However, in other organisms, they may have useful roles, e.g. Appendix in man has no function, but in rabbits it is useful for digestion of cellulose materials. Other evidences of organic evolution can be derived from:- - **Taxonomy** (the study of classification of organisms based on their relationships) - **Cytology** (the study of the cell structure of organisms) - **Bio-chemistry** (the study of compounds found on food samples, e.g. glucose).