BIO 152 Chpt 23 Evolution and Variation PDF

# Chapter 23: Evolution and Variation ## The origin of life, theory of evolution, Natural selection and Artificial selection, speciation and diversity Dr. Greg E. ODO ## 23.1 Introduction Many intellectual threads led to the modern theory of origin of life and organic evolution, a theory that re...

# Chapter 23: Evolution and Variation ## The origin of life, theory of evolution, Natural selection and Artificial selection, speciation and diversity Dr. Greg E. ODO ## 23.1 Introduction Many intellectual threads led to the modern theory of origin of life and organic evolution, a theory that requires recognition that Earth is ancient, that there is a common inheritance within a biological group, and natural events can be explained by discoverable natural laws. But it took a long time before these threads were woven into an evolutionary tapestry (Odo, 2010). Plato's idealistic concept, that all natural phenomena are imperfect representations of the true essence of an ideal unseen world was for centuries the prevailing philosophy in Western Europe. Following Platonic ideas, Aristotle suggested that not only were species immutable but that there was a hierarchical order of species from most imperfect to most perfect, a concept refined over the centuries as the 'Great Chain of Being'. In hindsight, this philosophy profoundly inhibited the development of evolutionary ideas because it maintained that the world of essences is perfect and all change is illusory. This unchanging order remained unquestioned until inexplicable gaps in the chain of nature prompted philosophers such as Gottfried Leibniz to propose that the universe was not perfect, only that it might go through successive intermediate stages on the way to perfection. ## 23.2 Evolution Change in a population's genes from one generation to the next, change over time. **Natural Selection:** Nature selects who stays!! This process drives evolution. **Adaptation:** A trait that helps an organism to survive & reproduce (born with it). This process drives evolution. **Variations:** Differences between individuals of the same species. Examples: color, shape, size, behaviour, chemistry. Variations come from mutations, meiosis, and crossover (challenge topic). **Microevolution:** Change in allele frequency over time in a population via mutations, selection, gene flow, drift. **Macroevolution:** "new species" / separate gene pools. Individual organisms live, reproduce and die. Individuals, however, do not evolve; populations do. Evolution is the change in gene frequency that occurs in a population over time. **Population:** all the individual members of a particular species living in a given area (Odo, 2010). ## 23.3 Pre-Darwinian Ideas of origin of life and Evolution The idea of evolution dates back to the earliest period of human civilization. These oldest theories discussed mostly the Origin of Life, but they were mere speculations rather than facts. These may be summarized under three heads: * Theory of eternity of the present conditions * Theory of special creation * Theory of catastrophism The believers of the first theory argued that there was neither beginning nor end to the universe. The life forms which existed many millions of years ago have remained unchanged till the present day and would continue to be the same throughout eternity. Also, the theory of special creation was preached for many centuries by the Christian Church. The basis of their belief was that the account of the creation of the world was by God and everything in this world including animals, plants and man, Genesis 1, 1-31. The theory of catastrophism was introduced by Cuvier, a paleontologist, who carried out research on fossil fauna for a long time in Paris. He believed that at one time world-wide catastrophes brought about the death of the old fauna; that extinction caused the creation of a new fauna which, according to him, took a long time covering millions of years due to the changed conditions of the environment, The idea of organic evolution, although believed to be a modern one, can be traced many centuries back. The idea of evolution emanated from the researches of many Greek philosophers of whom Empedocles was the first. He believed that organisms did not improve through successive generations but nature tried to produce perfect organisms many times, and during this period unfit form were eliminated. The great philosopher Aristotle then came. He believed that there was an inherent tendency among the organisms in nature to attain greater and greater perfection according to the changes in the environment, and that was the reason why there was such gradation from the lowest to the highest evolved in nature. After this period there was a halt in the speculation of organic evolution until the coming of such evolutionists as Linnaeus, Buffon, Erasmus Darwin, Lamarck, Charles Darwin and others. ## 23.4 Lamarck's Theory: Inheritance of Acquired Characters His theory resolves itself into three factors: * Influence of the environment * Use and disuse of parts * Inheritance of acquired characters Lamarck held the view that environment plays the principal part in the evolution of living organisms. He noted many instances where individuals of the same species grown under different environmental conditions showed marked difference. Plants grown in the shade develop larger leaves than those grown in the open. In dry soil the root system becomes more extensive than in wet soil. In darkness leaves do not develop chlorophyll and the stem becomes weak and drawn out (etiolated). Many plants leading an amphibious life show heterophyll. From such observations Lamarck concluded that plants reacts to external conditions, and that as a result of cumulative effect produced by the changed conditions through successive generations new species make their appearance. In the case of plants, according Lamarck, changes in character (or adaptations) are brought about by the direct action of the environment, and in the case of animals they are brought about by the use and disuse of parts. The use or exercise of certain parts results in the development of those parts; while disuse or want of exercise results in the degeneration of the parts. He further believed that new characters, however minute, acquired in each generation under changing conditions of the environment, are preserved and transmitted to the offspring (inheritance of acquired characters). The classic example cited in this connexion is that of the giraffe. Lamarck's view was that horse-like ancestors of these animals living in the arid region in the interior of Africa had to feed on the leaves of trees. They had necessarily to stretch their limbs to reach up to the leaves. This use of exercise resulted in the lengthening of the neck and the front legs, and thus a new type of animal made its appearance from a horse-like ancestor. His theory is open to certain objections, environment are very slight and superficial. Another objection is that the inheritance of acquired characters has not been proved yet. In fact, if seeds collected from plants growing elsewhere for many years under a new environment and acquiring new characters be brought back and grown in their original habitat, the plants are seen to revert to their original forms. ## 23.5 Darwin's theory: Natural Selection The next theory of evolution was put forward in 1859 by this English biologist Charles Darwin (1809 - 82) and published in his *Origin of Species by Means of Natural Selection*. His theory called the theory of natural selection is based on a mass of accurate observations and prolonged experiments which led the whole scientific world to believe in the doctrine of evolution. His theory called the theory of natural selection is based on three important factors: * Over-production of offspring and a consequent struggle for existence * Variations and inheritance * Elimination of unfavourable variations (survival of the fittest). The classical illustrations are shown below: [Diagram of a tree with a "Blue feet?" caption, and a diagram with adaption, biodiversity, variation, and adaptations] ## 23.4 Darwin's Notes from above illustrations: * **Observation 1:** Overproduction - Organisms make more offspring than can survive. * **Observation 2:** Resources are Limited. * **Conclusion 1:** Competition for survival and reproduction arise among species. * **Observation 3:** Individuals differ from one another within a species' population. * **Conclusion 2:** Fitness - the well-adapted individuals from one generation will usually leave the most offspring. ## 23.5 Struggle for Existence If all the seeds of any particular plant were to germinate and all seedlings to grow up into full-sized plants a very wide area would soon be covered by them in course of a few years. If other plants and also animals) were to increase at this rate, a keen competition or, in other words, a struggle for existence, would evidently be set up among them because availability of food, water and space would fall far short of the demand. This struggle would soon result in the destruction of large numbers of individuals. ## 23.6 Variations and their inheritance: It is known that no two individuals, even coming out of the same parent stalk, are exactly alike. There are always some variations however minute they may be, from one individual to another. Some variations are suited to the conditions of the environment, while others are not. According to Darwin these minute variations are preserved and transmitted to the offspring, although to cause for these variations was assigned by him. ## 23.7 Survival of the Fittest In the struggle for existence the individuals showing variations in the right directions survive, and these variations are transmitted to the offspring; others with unfavourable variations perish. This is what is called by him *survival of the fittest*. The survivors gradually and steadily change from one generation to another, and ultimately give rise to new forms. These new forms are better adapted to the surrounding conditions. Darwin's observations on the variations of domestic animals and cultivated plants served him as a clue to the elucidation of his theory of natural selection. Sometimes such extensive changes are found in course of several generations that it becomes difficult to believe that the first form has given rise to the last. Further, for the purpose of having a desired type, breeders and florists take not of certain variations among individuals select them for future generations, rejecting and destroying the rest. They grow the selected types, generation after generation, until the desired result is obtained. New types are seen to appear by this process, called artificial selection. Many cultivated flowers and vegetables often show a number of varieties, and in course of time these variations become well marked. ## 23.8 Natural Selection: Now Darwin's explanation of natural selection is thus: animals and plants are multiplying at an enormous rate, biodiversity. As we know no two individuals are exactly alike, the new forms naturally show certain variations. Some variations are favourable or advantageous so far as their adaptation to the conditions of the environment is concerned, and others are not so. Owing to an excessive number crowding together a keen struggle for existence ensures. And in this struggle those that have favourable variations and are, therefore, better fitted naturally survive, and the rest perish. Through this survival of the fittest the species change steadily owing to preservation and transmission of minute variations, and gradually give rise to newer forms. Darwin called this process 'natural selection' from analogy to artificial selection. It is the environment that selects and preserves the better types and destroys the unsuitable forms. ## 23.9 Pangenesis: This is Darwin's theory by which he assumes that every cell of the plant body or the animal body produces imaginary particles or units called pangenes, which carry in their body not only the normal parental characters but also those acquired during the life-time of an individual plant or animal. Pangenes are formed in all parts of the body, and finally they collect together to form the germ cells. Through these cells all the characters, normal and acquired, are ultimately transmitted from one generation to another. By his pangenesis theory Darwin tried to explain how the characters are carried forward from the parents to the offspring, assuming that somatic cells at a certain stage produce germ cells which in turn produce somatic cells in the text generation. The above doctrine of Darwin has however, been discarded by the modern scientists since, as now definitely known. It is only the germ cells that are the true bearers of hereditary characters, remaining practically unchanged and unaffected by the environment and thus passing down intact from generation to generation. Although Darwin receives the fullest credit for bringing about the final acceptance of the doctrine of evolution, his theory is open to certain doubts. It is true that natural selection is operative in the preservation of certain forms and destruction of others. Yet some doubts have been expressed regarding the process being the cause of evolution of new species. Some of the reasons for these doubts are as follows: * Are slight variations of any decided advantage in the struggle for existence? It is only the perfected organs that are helpful to organisms and not the organs during the process of perfection. * It is doubtful if slight variations can help the individuals by artificial selection in breeding experiments. * There are many organs which are not of any apparent use to the organisms. * If only the fittest survive, how is it that many unfit ones still exist? * If nature 'selects suitable forms and features, why were not the rest swept out of existence? ## 23.10 Weismann's theory: Continuity of Germplasm. An ingenious theory explaining the cause of variation and evolution was put forward in 1895 by the German scientist Weismann (1834-1914), a disc put forward. He divided the protoplasm of the animal or plant body into somatoplasm which gives rise only to somatic or body cells and germplasm which producelasm reproductive cells, and they flow as two separate streams through the body of the plant or the animal. Somatoplasm is responsible for differentiation of tissues, development and growth of the individual plant or animal body, and is exhausted and lost at the end of the life-cycle, i.e. it is discontinuous, whereas the germplasm is ever-young and immortal, and is continuous from one generation to another, and is actually the bearer of hereditary characters. Somatoplasm may be influenced by the environment and new characters acquired during the life-time of an individual, but germplasm is not so affected, and, therefore, as is almost universally believed, inheritance of acquired characters is not possibility. Weismann did not believe in Darwin's pangenesis. During reproduction the fertilized egg gets the paternal germplasm from the sperm and the egg-cells respectively. In the nuclei of both somatic and germ cells there are certain factors which determine the character of the cell. It is believed that each somatic cell has a single factor, whereas a germ cell contains all the factors that are found in the somatic cells of the adult plant or animal. The inheritance of characters by the spring depends upon these factors of the germ cells only. There is always a struggle for existence among these factors, and this results in a germinal selection. The stronger factors survive and are readily transmitted from one generation to another. Hence any mutation in the germplasm or any variation resulting from the struggle among the factors in the germ cells can only handed down from generation to generation. Weismann's theory supports Darwin's theory of natural selection, but it has been criticized by many scientists as purely Speculative. Also, it is not a fact, as assumed by Weismann, that germplasm is Permanently curtained off from the somatoplasm. With advance in knowledge it has been revealed that germplasm comes in direct contact with somatoplasm during nuclear divisions. ## 23.11 De Vries' Theory: Mutation. Another theory of evolution was advance in 1901 by the Dutch botanist Hugo De Vries (1848 – 1935). He held that small variations, which Darwin regarded as the most important from the standpoint of evolution, are only fluctuations around the specific type. These variations are not inheritable. De Vries held that large variations appearing suddenly and spontaneously in the offspring in our generation are the cause of evolution. These variations De Vries called 'mutations'. That large discontinuous variations are the causes of organic evolution which was first advocated by Bateson in 1984. De Vries strongly supported this view on the basis of his extensive observations. Thus, he observed an evening primrose (Oenotherolamarckiana), introduced from America, growing in a field in Holland. Among numerous plants he found two types quite distinct from the rest. These new types were not described before, and having bred true he regarded them as distinct species, O. brevistylis and O. laevifolia, as he named them Oenotheralamarckianaand the new species were removed to his garden atAmsterdam, and cultivated through many generations. It was found that among thousands of seedlings raised a few (7 new species) appeared that were different from the rest. These when raised, generation after generation, always came true to types. These new forms are known as mutants. He propounded a mutation theory as his explanation of evolution. While De Vries agreed with Darwin's view, regarding natural selection weeding out unsuitable forms, he held the view that new species are not formed, as Darwin said, by the slow process of continuous variations. Since then several instances of plant mutations (as well as animal mutations) have been found in nature. Mutation is now known to be due to changes in genes - less, degeneration, addition, recombination, etc., occurring in the gametes, zygote, or somatic cells, ultimately affecting the nature of the mature plant. ### Evidence supporting Evolutionary Theory * **Biogeographical** (distribution of living organisms around the world. Based on plate tectonies, continental drift, migration ete file. Galapagos finches and tortoises, Lemurs in Madagascar, Hawaiian honeycreepers. Chichlid fishes and Lizards in Bahamas, Salamanders in California...) * **Paleontological** (finding fossilized remains, reconstructing the organisms and making deductions about the possible relationship between the fossil and present day organisms) * **Geological** (Strata and carbon dating: Earth formed - 5x10 у.а.) * **Comparative embryology and development** (homologies, ontogeny recapitulates phylogeny, homeobox genes) * **Genetics** (DNA sequences, genome projects help reconstruct phylogenies. i.e. frozen mammoth) * **Artificial selection** - antibiotic resistance, HIV strains, milk production in cows, faster horses This is 'gene mutation' which may also be artificially brought about by treatment with X-rays. The mutation theory of De Vries has been widely accepted. ## Fossils are the remains of past life dug up from the earth's crust (See Diagram below) * Fossil formation is rare. * Most fossils form in sedimentary rock. * Fossils are the historical documents of biology. * Because the Earth's crust is layered over time, younger fossils are found closer to the top. [Image of a fish fossil] Fig. 23.3: Fossils remains e.g. bones, teeth, casts, impressions ## Summary: Evolution by means of natural selection explains the history of life on earth. This claim is supported by abundant evidence. ### Key Concepts are: * Species share common ancestors(tree of life and others below) * Descent with modification (perpetual change) * Nature chooses which species stay (extant) and which go (extinct). [Diagram of Tree of Life] Fig. 23.6: Phylogenetic tree of life (Mayo, 2001)