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Evolution for Beginners Evolution HistoryCarolous Linnaeus • 1700’s • Swedish Botanist • Developed a system of classification • Theorized that organism changed over time due to hybridization just in plants. Georges Louis Leclerc De Buffon • • • • French Naturalist 1700’s Proposed species shared ancestors Suggested the Earth was much older than 6000 years. Erasmus Darwin • Charles Darwin’s own Grandfather • English Doctor • Proposed all living things were descended from a common ancestor. • More complex forms of life arose from less complex ones Jean-Baptiste Lamarck • 1809 • Proposed all livings things evolved towards perfection • Believed species did not become extinct only that they evolved into other forms. • Believed that traits that were needed were passed on to offspring. (inheritance of acquired characteristics). Geologic Change • French Zoologist- Georges Cuvier • Observed deeper layers of rock held quite different fossils than the upper layers • Fossils- are traces of living animals from the past • Developed the theory of Catastrophism – states that natural disasters caused some specied to become extinct and then other animals moved in from elswhere. Geologic Change Continued • Scottish geologist James Hutton proposed gradualism • Gradualism- is slow changes that happened over time in the past • It is often used as term to explain the gradual change in species though evolution. Geologic Change Continued • Charles Lyell an English Geologist • Proposed the theory of uniformitarianismthis combined Hutton's theory of gradual change over time with his idea that things are occurring at constant rate and are ongoing. • This had a major impact on a young Carles Darwin. What is evolution? A basic definition of evolution… “…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 So what does the definition mean? • Evolution is a change in the number of times specific genes that code for specific characteristics occur within an interbreeding population • Individuals don’t evolve, populations do • There is no implied “improvement” in evolution Genetic Variation and Evolution • Evolution: changes through time 1. Species accumulate difference 2. Descendants differ from their ancestors 3. New species arise from existing ones Charles Darwin Charles Darwin was born on February 12, 1809 in Shrewsbury, England. From 1831 to 1836 Darwin served as naturalist aboard the H.M.S. Beagle on a British science expedition around the world. He observed much variation in related or similar species of plants and animals that were geographically isolated from each other. These observations were the basis for his ideas. Darwin presumed that populations of individuals changed over time, and, in 1844, he developed the concept of the driving force for evolution. It wasn’t until many years later that he published his idea. “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.” —Charles Darwin from "The Origin of Species“, 1859 Darwin’s Observations • He was struck by the many different variations of individual species he observed. • He observed tortoise shell differences – Lots of low food no hump in shell. – Food at a higher level tortoise has hump • Finch beak differences. – Soft food thin beak – Hard nuts larger stronger beak Darwin’s Observations • Fossil evidence • He found fossils of animals that look similar to animals alive today, but are different species. • He saw marine fossils high on mountains and concluded that Lyell’s theory that geological change happens slowly over time. Darwin’s Thinking • After the voyage he spent 20 years thinking about what he saw. • Local farmers helped him with his ideas because he saw them use artificial selection to specify what traits they wanted in captive animals. • Realized the traits had to be heritable or passed from one generation to the next. Evidence • He theorized the animals on the island had migrated from the mainland because they very closely resembled those animals. • This is what we now call biogeography. • Embryology- Organism look very similar in as they are developing, but develop differently as adults. Evidence Anatomy • Homologous structures- structures that are similar in structure but appear different in different organisms and have different functions. • Example forelimbs of humans bats and moles. Evidence Anatomy • Analogous Structures- Structures that have similar function but are not similar in origin. - Example wings of bats and flies. • Vestigial Structures- organs that no longer have any useful function, but probably did in an ancestor. – Example Human appendix Paleontology • Study of fossil remains of ancient animals. • This was a very new science in Darwin’s time. • Fossil evidence shows how animals change over time. Genetic Evidence • Today we can use genetic sequences and analyze similarities between species. • There are Pseudo-genes that are like vestigial organs that no longer are used, but may have been used in the past . Natural selection: mechanism of evolutionary change Natural selection: proposed by Darwin as the mechanism of evolution • species have a variation of inherited characteristics • they produce many offspring- overproduction • the population includes more individuals with specific characteristics that help them surviveadaptation • the population evolves and is better adapted to its present environment- Descent with modification Natural Selection Darwin knew nothing of genes, but what he did have were two observations and a little inference that provided the motive force for evolution. •Darwin: Evolution is descent with modification Natural Selection Observation 1: Organisms generally have more offspring than can survive to adulthood. Observation 2: Offspring are not identical. There is variation in their appearance, size, and other characteristics. Natural Selection Inference: Those organisms that are better adapted to their environment have a greater likelihood of surviving to adulthood and passing these characteristics on to their offspring. Survival of the “fittest.” Survival of the “fittest.” Darwin’s theory for how long necks evolved in giraffes The evolution of populations Gene pool- the combined alleles of all the members of a population. 1. Sources of variation 1. Sources of variation Change can be rapid • Ecologists Peter and Rosemary Grant observed finches in the Galapagos. • A drought in 1977 reduced soft seeds finches preferred, but large tough seeds that only the large beaked finches could eat were still available • The next year there was a drastic increase in large beaded finches. Selection • Artificial selection: a breeder selects for desired characteristics Selection • Natural selection: environmental conditions determine which individuals in a population produce the most offspring • 3 conditions for natural selection to occur – Variation must exist among individuals in a population – Variation among individuals must result in differences in the number of offspring surviving – Variation must be genetically inherited 3. Maintenance of Variation • Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population – Negative frequency-dependent selection: rare phenotypes are favored by selection – Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population • Strength of selection changes through time Maintenance of Variation • Fitness of a phenotype does not depend on its frequency • Environmental changes lead to oscillation in selection Fitness and Its Measurement • Fitness: A phenotype with greater fitness usually increases in frequency – Most fit is given a value of 1 • Fitness is a combination of: – Survival: how long does an organism live – Mating success: how often it mates – Number of offspring per mating that survive Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes Stabilizing selection • The intermediate phenotype is favored and becomes more common in the population Disruptive Selection • Both extreme phenotypes are favored in a population. Gene Flow • The movement of alleles from one population to another. Genetic Drift • A small population can produce changes in the frequency of alleles by chance. (Example picking flowers) • Bottle neck affect- occurs after a population has been greatly reduced the few members left do not have the original diversity of the larger population. • Founder affect- Occurs when a small number of individuals colonize a new area. • Evolution by drift is aimless, not adaptive. (may be good or bad) Sexual Selection • This is referred to as non-random mating • This occurs when certain traits increase mating success. – Intrasexual- males compete to mate. Example bighorn sheep. – Intersexual- males display traits that attract females. Example birds of paradise Genetic equilibrium is a basic principle of population genetics. Hardy–Weinberg principle states that the genotype frequencies in a population remain constant or are in equilibrium from generation to generation unless specific disturbing influences are introduced. Those disturbing influences include 1. Non-random mating- no sexual selection 2. New mutations- no new alleles introduced 3. large population-no random genetic drift 4. Gene flow- no immigration or emigration 5. All traits must equally aid survival- no natural selection Five agents of evolutionary change Speciation Through Isolation • Speciation- the rise of two or more species from one existing species • Reproductive isolation- occurs when members of two or more population can no longer mate successfully. Last step in becoming new species. • Behavioral isolation- differences in courtship or mating behaviors • Geographic isolation- physical barriers • Temporal isolation- reproduction at different times. Example - cicadas Patterns of Evolution • Natural selection is not random! • Convergent evolution- when two unrelated species develop similar characteristics. • Divergent evolution- When closely related species become increasingly different. • Coevolution- Two or more species evolve in response to one another. – Examples Pg 328 Extinction • Extinction- The elimination of a species from the Earth. – Background extinctions- continuous low rate of extinction that are a part of the life cycle – Mass Extinctions- caused by catastrophic event Patterns of speciation • Punctuated equilibrium- bursts of activity followed by long periods of stability • Adaptive radiation- the change of one species into many different descendents