Chapter 15: Evolution Notes Darwin’s Theory of Evolution: page 368
Evolution: change over time; the process by which modern organisms have descended from ancient organisms
Scientific Theory: a well supported testable explanation of phenomena that have occurred in the natural world
Charles Darwin: born in England 1809, on the same day as Abraham Lincoln
· traveled around the world on H.M.S. Beagle
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· Made numerous observations and collected evidence that led him to propose a hypothesis about the way life changes over time, the theory of evolution
Darwin’s Observations:
· Patterns of diversity: he observes such a large variation of species in similar environments
· EX: in England but not in Australia even though the environment was similar
· Living Organisms and Fossils: He observes that fossils of ancient organisms resembled organism that were still alive. Others looked unlike anything he has ever seen
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· Galapagos islands: many islands close together with very different climates. Darwin observes that the characteristics of many animals and plants varied noticeably among different islands
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These differences are related to the environments in which the respective tortoises live, and the types of food they eat. The domed tortoises tend to live in the moist high-lands and take their food from grasses and low-lying shrubs. The saddle-backed tortoises, on the other hand, live in arid regions and feed on plants that are mostly above their head, most notably the tree-like Opuntia (prickly pear) cactuses. The arched shell permits them to stretch their heads high, giving them a longer vertical reach.
On the origin of species: Darwin’s book published in 1859 that summarized all his findings from his trip around the world.
Artificial Selection: nature provides the variation and humans select those variations that they find useful. ** Used to improve crops and livestock
Selective breeding transformed teosinte's few fruitcases (left) into modern corn's rows of exposed kernels (right).
· Natural Selection: Over time, results in changes in the inherited characteristics of a population and increase fitness (ability of an individual to survive and reproduce in its specific environment) of a species.
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· Darwin's illustrations of beak variation in the finches of the Galápagos Islands, which hold 13 closely related species that differ most markedly in the shape of their beaks. The beak of each species is suited to its preferred food, suggesting that beak shapes evolved by natural selection.
· Adaptation: an inherited characteristic that increases and organism’s chance of survival.
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· Struggle for Existence: members of each species compete regularly to obtain food, living space and other necessities of life.
· Organisms produce more offspring than - given the limited amounts of resources - can ever survive, and organisms therefore compete for survival. The Atlantic cod for instance lays around five million eggs a year while Darwin calculated that even the low reproducing elephant produces more young than will ever live to maturity. Only the successful competitors will reproduce themselves. Darwin referred to this competition as the "struggle for existence".
· Descent with modification: natural selection produces organisms that have different structures, establish different niches or occupy different habitats. This causes today’s species to look different from their ancestors.
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· Common Descent: all living organisms are related to one another
Evidence of Evolution:
· Fossil record
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· Geographic Distribution of Living Species
· Homologous Body Structures: structures that have different mature forms but develop from same embryonic tissues (Wings and legs all descended from the fish fin)
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· Vestigial Organs: remnants of organs that have no function now
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· Similarities in Embryology
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Evolution of Populations:
A population is a group of individuals of the same species that interbreed and share a common group of genes
Gene pool- all the genes, including all the different alleles, that are present in a population
Relative frequency of an allele- the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Expressed as a percentage
Evolution- any change in the relative frequency of alleles in a population
Single-gene traits – any traits controlled by one gene. (EX: widow’s peak in humans)
· natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution
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Polygenic Traits- any traits controlled by 2 or more genes (EX: height in humans)
· Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection or disruptive selection.
· Directional selection: when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. (EX: seed size and bird beak size)
· Stabilizing selection: when individuals near the center of the curve have higher fitness than individuals at wither end of the curve. (EX: weight of human infants at birth)
· Disruptive selection: when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle (EX: seed size and bird beak size)
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Genetic Drift: in a small population, individuals that carry a particular allele may leave more descendents than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population
Founder effect: genetic drift due to the migration of a small subgroup of a population. (EX: Fruit Flies migrating from mainland to different Hawaiian Islands)
Hardy-Weinberg Equilibrium: allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. Also called genetic equilibrium
· Five condition are required to maintain genetic equilibrium from generation to generation
· Random mating
· Large population
· No movement into or out of the population
· No mutation
· No natural selection
Speciation: formation of new species
· Reproductive isolation: when the members of 2 population cannot interbreed and produce fertile offspring
· Behavioral isolation: when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.
· Geographic isolation: when 2 populations are separated by geographic barriers such as rivers, mountains, or bodies of water.
· Temporal isolation- when 2 or more species reproduce at different times