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EVOLUTIONARY THEORIES Over the years, there have been many theories on evolution. (1) Branching Theory * most scientists agree with the theory that living things have changed, from simple organisms to complex organisms, over a long period of time * species that are most closely related share a more recent common ancestor while distantly related species have a common ancestor further in the past Tree Diagram (2) Spontaneous Generation – * the concept that living things come from nonliving things * ex – flies came from the rotting bodies of animals * this theory made sense to people because they could not see the tiny eggs laid by organisms, therefore, they didn’t know organisms came from the eggs (3) Theory of Use and Disuse – * early 1800s * proposed by __________________________________________________ * organisms develop new structures because they need them * these structures become ___________________ _______________________, which is a trait produced during an individual/s lifetime and these useful traits are then passed onto the organism’s offspring * example – the Theory of Use and Disuse was disproved by ___________________________, who performed experiments with mice (4) Theory of Natural Selection – * proposed by ___________________________________________ in the 1850s * stated that evolution occurs because of natural selection * nature acts as a selecting agent of an organism’s traits * traits that help an organism survive in a changing environment are passed on o the next generation Darwin’s Theory of Natural Selection (A) Overproduction: A population generally produces more offspring than can survive in the environment (B) Competition: Because of overproduction, there is competition, or a struggle for survival, between organisms for space, food, water, light, minerals, and other limited resources (C) Variations: Members of a population show variations (differences) in traits that make certain individuals better adapted to survive. Differences in structure, size, and color are examples of variation. (D) Natural Selection: Since some variations are more helpful than others, there is a natural selection against organisms that cannot adapt. Organisms that cannot adapt die. (E) Survival of the Fittest: Survival of the fittest applies to those individuals that have variations that enable them to live and reproduce. Ex – in a woodland area, brown fur would be advantageous over white fur (could blend in) (F) Inheritance of Variations: Organisms with helpful variations are more likely to survive and to reproduce, passing these helpful variations to the next generation (offspring). (G) Evolution of New Species: Over long periods of time, variations accumulate in a population. Eventually there are so many variations that the population changes. This is called _____________________________________. Darwin’s theory was not accepted right away – he could not explain one aspect of his theory – What were the major differences between Darwin’s and Lamarck’s two theories? (5) Mutation Theory – * 1901 - Hugo De Vries * experimented with the plant, evening primrose * determined that variations came from _____________________(changes in genetic material) which occurred randomly and those ___________________ that were favorable were inherited by offspring (6) Modern Theory – * combines Darwin’s ideas of variation and natural selection with recent studies of mutations, DNA, genes, chromosomes, and sexual reproduction and genetic recombination Modern Theory of Natural Selection (A) The genes of inherited variations that give an organism a better chance for survival tend to be passed on from parent to offspring. These new inheritable characteristics can result from new combinations of existing genes or from mutations of genes in reproductive cells. (B) Favorable genes tend to increase in numbers within a population because some characteristics give individuals an advantage over others in surviving and reproducing, and the advantaged offspring, in turn, are more likely than others to survive and reproduce. (C) Genes for traits with low survival value decrease in numbers from generation to generation. (D) If the environment changes, genes that previously were neutral or had low survival value may become favorable and increase in numbers. The variation of organisms within a species increases the possibility that at least some members of the species will survive under changed environmental conditions. How does the Modern theory of evolution use Darwin’s ideas?