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EVOLUTION AND NATURAL SELECTION FEBRUARY 18, 2013 CAPE BIOLOGY MRS. S. HAUGHTON DEFINITION • Evolution means change over a period of time. • “…evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next." • - Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974 • Individuals don’t evolve, populations do. • There is no implied “improvement” in evolution. • Things don’t change because organisms want or need them to. • 1. 2. 3. 4. The study of evolution provides a focus for investigations into The nature of life The origins of life The diversity of living things Understanding the similarities in structure and function they share. THEORIES • 1. 2. 3. 4. 5. Theories to explain where life on Earth came from are varied and uncertain: Special creation Spontaneous generation Steady-state Cosmozoan Biochemical evolution SPECIAL CREATION • Life was created by a supernatural being at a particular time. • Supported by the world’s religions. • Calculated that creation took place in 4004 B.C. • A case where faith accepts something for which there is no empirical evidence. SPONTANEOUS GENERATION • Life arose from non-living matter on numerous occasions. • Misconceptions: mud/frogs; moldy grain/mice; raw meat/flies • van Helmont-dirty shirt / rats (bad science) • Francesco Redi- meat jars and flies (good) • Pasteur –soup/germs STEADY-STATE • Life has no origin and Earth was always capable to support life and has changed very little. • Theory does not support the evidence of fossils, however. COSMOZOAN • Life arrived on this planet from elsewhere (outer space). • Supported by person’s stories of extraterrestrial and cave drawings of UFOs. • Claims made about primitive organisms being found in meteorites. • More evidence required. BIOCHEMICAL EVOLUTION • Life arose according the chemical and physical laws. • Earth very different from it is now (only simple inorganic substances and high temperatures). • Probability of B.E. proven by Stanley Miller’s experiments. EARLIEST LIFEFORMS • Evidence suggests they were heterotrophic. • Evolved to become photosynthetic, but unstable and sis not produce energy. • Later evolved to produce oxygen. • New oxygen-using lifeforms emerged. THEORY OF EVOLUTION • Attempts have always been made to try and explain the diversity among organisms • The theory states that evolution is the development of differentiated organisms from pre-existing, less differentiated organisms over a period of time. • Darwin-On the Origin of Soecies • Lamarckian – use/disuse theory (giraffe neck and legs, toe webbing, tails). EVIDENCE FOR EVOLUTION Ever wondered about the monkey man thing? • Palaeontology • Geographical distribution • Classification • Comparative anatomy • Adaptive radiation • Comparative embryology • Comparative biochemistry Palaeontology • The study of fossils. • Supports the theory of progressive increase in complexity • Suggests that geographical regions and climatic conditions changed througout Earth history • Fossil types: hard skeletons, petrifaction, moulds, impressions, imprints, coprolites Geographical distribution • Species originate in a particular area • They disperse out from that area • Dispersal occurred where land masses were close together • Absence of more advanced organisms in a region indicates prior separation of that region from the area of origin of those organisms. • Similar animals in different locations were the product of different lines of descent Classification • Structural similarities between organisms suggests the existence of an evolutionary process. • Result of progressive adaptation. • Suggest biological organization. Comparative anatomy • One of the strongest forms of evidence. • Organisms with similar anatomical features are assumed to be relatively closely related. • Some organisms have anatomical structures that are very similar in form, but very different in function. These are called homologous structures. • Since these structures are so similar, they indicate an evolutionary relationship and a common ancestor of the species that possess them. Adaptive radiation • The evolution of an ancestral species, which was adapted to a particular way of life, into many diverse species, each adapted to a different habitat. • Adaptive radiation has occurred in the evolution of many groups of organisms. • E.g. Darwin's finches, 14 species of small land birds of the Galápagos Islands. • All the finches derive from a single species of grounddwelling, seed-eating finch. • Because the environmental niches, or habitats, were unoccupied on the isolated islands, the ancestral stock was able to differentiate into diverse species Comparative embryology • The embryonic stage for many animals is very similar. • For example, fish, bird, rabbit, and human embryos are similar in appearance in the early stages. • They all have gill slits, a two-chambered heart, and a tail with muscles to move it. • Later on, as the embryos grow and develop, they become less and less similar. Comparative biochemistry • The occurrence of similar biological chemicals in different organisms suggests the existence of biochemical homology (similarity) among organisms. NATURAL SELECTION • The mechanism by which new species arise from already existing ones. • Natural selection was proposed by Darwin and Wallace. • Takes into account 3 observations and 2 deductions made from the observations. OBSERVATIONS 1 AND 2 1. Individuals in a population produce more offspring than necessary. 2. The numbers of organisms in populations remain constant. DEDUCTION 1 • Therefore, many individuals fail to survive or reproduce. • This is a “struggle for existence” within a population. • May be interspecific or intraspecific competition. OBSERVATION 3 • Variation exists within all popluations. DEDUCTION 2 • In the “struggle for existence” the individuals showing variations best adapted to their environments have a “reproductive advantage” and produce more offspring. • This is survival of the fittest and leads to natural selection. VARIATION AND NATURAL SELECTION DEFINITION of VARIATION • Variation refers to the differences among individuals of the same species. • Whereas mitosis leads to identical offspring as the parent cells, meiosis is said to lead to variation. • Except for identical twins, no two organisms produced by sexual reproduction are alike. • Therefore, variation or variety exists between all persons, races and species. • There are two types of variation: • Continuous variation and • Discontinuous variation. CONTINUOUS VARIATION • In continuous variation, the characteristic, trait or quality being studied shows a gradual difference from individual to individual. • There is therefore a range of difference. • Examples are height and weight. DISCONTINUOUS VARIATION • In discontinuous variation, the characteristic being studied shows clear-cut differences or boundaries. • There is therefore no range. • Examples are blood group or ability to roll the tongue. QUIZ • Are the following examples of continuous or discontinuous variation? • • • • • • • • • • • Possession of a tail __________ Length of middle finger ____________ Length of hair ___________ Hair colour ____________ Number of legs _____________ Length of hair _____________ Eye colour ______________ Height _____________ Weight _____________ Ability to smell a certain substance ___________ Gender _____________ GENETIC AND ENVIRONMENTAL VARIATION CAUSES OF VARIATION • The two main causes of variation are: • GENETIC FACTORS • ENVIRONMENTAL FACTORS GENETIC FACTORS • Due to the genes that are passed on from generation to generation, there are always certain gene combinations in different species and even families. • Also, the segregation and many different ways in which chromosomes group up together (random assortment) during meiosis to make eggs and sperm will lead to many differences in variation even within species or our families; e.g. hair colour, eye colour, blood groups, height etc. • Genetic variation arises by: 1. Meiosis: during segregation of chromosomes in making eggs and sperm. 2. Fertilization: when eggs and sperm, both carrying unique genes from mom and dad come together to make off-spring. 3. Mutation: when a gene changes without warning. ENVIRONMENTAL FACTORS • Characteristic caused by an organism’s environment are sometimes called acquired characteristics, because the variation is due to the limitations presented by their surroundings. • E.g. a plant grown in a small pot will limit its growth to fit in the pot. • E.g. if water is deep underground, the normally short roots of plants will grow very long to reach it. • Variation leads to natural selection and evolution over millions of years. NATURAL AND ARTIFICIAL SELECTION DEFINITIONS • Selection: the process in which certain populations are favoured for survival over others because of useful traits they may possess. • Selection pressure: a factor acting on a population which favours certain varieties for survival e.g. climate change • Natural selection: occurs when the selection pressure comes from the natural surroundings or environment of the populations. • Artificial selection: occurs when man determines which population possess the favourable trait and so encourages the survival and continuation of that population. NATURAL SELECTION IN NATURE DARWINIAN THEORY • In 1859, a scientist called Charles Darwin proposed the following theories to support evolution. • His theory was called the Theory of Natural Selection which occurs every day in nature and is an ongoing process. 1. Variation helps some organisms adapt better than others 2. Most organisms over-produce their offspring to ensure some survive. 3. Organisms in populations will always be struggling and competing with other members. 4. The fittest (best adapted) will survive 5. This ensures that only advantageous traits are passed on to future offspring. 6. So, over a long period of time, the population will be left with the stronger individuals, which are best adapted. EVIDENCE FOR NATURAL SELECTION • Changes over long periods can be observed in the fossils of organisms • The case of the peppered moth (polymorphism) • Antibiotic resistance in bacteria POLYMORPHISM • Polymorphism occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph. • In order to be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population (one with random mating). • Polymorphism is common in nature; it is related to biodiversity, genetic variation and adaptation; it usually functions to retain variety of form in a population living in a varied environment.Polymorphism results from evolutionary processes, as does any aspect of a species. • It is heritable, and is modified by natural selection. PEPPERED MOTH • Natural selection does not always cause change especially if the environment remains the same and the organism is already well adapted. • In this case, this type of natural selection is called stabilising selection. ADVANTAGES AND DISADVANTAGES OF NATURAL SELECTION ADVANTAGES • The fittest, best adapted organisms remain in the population. CARRYING OUT ARTIFICIAL SELECTION 1. Inbreeding: where Man mates related organisms to maintain desired traits, e.g. mating “sibling” dogs to keep size and fur colour for dog shows. 2. Outbreeding: where Man mates unrelated organisms from same or different species to attain desired traits, e.g. a zebra with a horse to get a unique striping pattern ADVANTAGES AND DISADVANTAGES OF ARTIFICIAL SELECTION • Overall advantages are: Man gets the desired traits that he wants • Advantage of inbreeding: the desired trait is maintained throughout the generations with only slight or insignificant differences. • Advantage of outbreeding: outbred organisms are stronger than inbred • Overall disadvantage: Loss of diversity; all individuals are similar • Disadvantage of inbreeding: unwanted genes are also passed along through the generations e.g. diseases • Disadvantage of outbreeding: the desired trait is more difficult to attain and maintain NATURAL SELECTION VS. ARTIFICIAL SELECTION • NATURAL • ARTIFICIAL • Selection pressure • Selection pressure from environment from MAN • Fittest individuals • Individuals with favoured desired traits favoured • Change is gradual as • Change is quick to attain desired environment organism changes • Resultant organisms • Resultant organisms very different usually different in small ways • Offspring remain • Future offspring may healthy develop complications