Populations evolve not individuals Adaptations Inherited traits that enhance survival and thus reproduction in a particular environment Charles Darwin Theory of Evolution Based on many observations. Evolution – change over time. Path to Darwin’s Theory.
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Populations evolve not individuals Adaptations Inherited traits that enhance survival and thus reproduction in a particular environment Charles Darwin Theory of Evolution Based on many observations Evolution – change over time
Path to Darwin’s Theory • Similar idea: simpler forms of life preceded more complex forms from ancient Greeks (2500 years ago) • Aristotle believed that species are fixed • Judeo-Christian thought (book of genesis) • Divine creator; earth to be 6,000 years old • Early fossil studies; extinct spp.; earth may be older • Lamarck: inheritance of acquired characteristics • Lyell: Geologist; gradual change by nat. forces • Darwin’s voyage 500 BC 322 BC ~2000 yrs old 1700s 1809 1830 1831-1836
Charles Darwin: British naturalist born 1809 • expanded on some existing ideas • Geologists, naturalists, and scientists • Did not address origin of life, but rather focused on explaining the vast diversity of life • provided supportive evidence; 5 year voyage around the world 1831-1836 • Galapagos Islands • Wrote essay on evolution based on his observations/experiences 1844 • Published “On the Origin of Species” 1859; under competition by Wallace • Descent with modification: ancestral species could diversify into many descendent species by accumulation of adaptations to environment • Hypothesized natural selection as driving force
What is natural selection? • Over production of offspring • Limited natural resources • Heritable variations • Differential or unequal reproductive success Offspring w/in a varied population, whose characteristics best adapt them to the environment are most likely to survive and reproduce • more fit individuals leave more offspring than less fit individuals
even more diversity over thousands of generations • With natural selection over vast time allows for changes to accumulate
Evidence for evolution • Fossils: preservation of dead organic matter examples: 35 mya 1.5 mya 375 mya 190 mya 40 mya 5,500 ya
Layers of sediments Deposits pile up over millions of years forming strata Young on top; older on bottom Read the layers compare preserved fossils track changes over time Fossil record
Is Earth 6,000 years old or 4.6 billion? • Radiometric Dating • Dating geologic structures by rate of radioactive decay • Atomic elements decay at a fixed rate • Half-life = time it takes for half of an element to decay • C14 or radiocarbon dating (plants and animals) • unstable carbon isotope, 5730 years to decay half of a sample • C14 : C12 ratio is half in fossil than atm = 5730 • If C14 : C12 ratio is ¼ of atm = 11,460 • K40 has half life of 1.3 billion years • Famous tests: • Iceman (Italy, 1991); dated to 5500 years old • Shroud of Turin; evidence shows to be about 700 years old rather than 2000 years old Dr. Willard Libby – Atomic physicist – Nobel Prize for C14 dating work. Before that, was a key researcher in the development of the atomic bomb.
Homology Similarities in form and structure from common ancestory E.g. mammal forelimbs with different functions Similar embryological stages Common structures during similar early development (e.g., gill slits) Comparative anatomy & embryology
Hereditary background and proteins encoded in DNA Compare gene sequences Similar sequences – more recent ancestor More dissimilar – more distant ancestor Molecular Biology
Why is evolution the best explanation to the vast natural diversity? • Mountains of evidence of various types • e.g. fossils, radiometric dating, comparative anatomy & embryology, molecular biology • each agrees with the other • provides tremendous support of evolution theory… • disagreement would be falsifiable evidence • Theory still challenged • Theory guess or based on any belief • Theory = falsifiable idea supported by extensive evidence • Theory of Gravitation, Theory of Relativity…principles based on facts (e.g. earth is round)
Although natural selection acts on individuals, which affects survival & chances to reproduce w/in an env., a population changes over time Population genetics Darwin’s and Mendel’s ideas together Populations change genetically over time Gene pool All alleles in a population Microevolution Change in allele frequencies over time e.g. pesticide resistant allele will increase while its alternate decreases frequency Populations evolve
Agents of potential changes in allele frequency • Natural Selection – pesticide example • Non-random mating • Plants closer to each other may get fertilized • People sometimes choose similar mates (short couples…) • Mutation - creates new alleles • Gene flow – gain or loss of alleles in a popln. • Immigration or emmigration • Genetic drift – change in gene pool due to chance • Founder effect – colonization of small group • Bottleneck effect – reduction of population
Drastic reduction of popln. size Earthquakes, floods, fires, etc Surviving popln has underrepresented alleles Genetic drift – bottleneck effect e.g. elephant seals were hunted down to 20; restored now to 30k; found only 1 allele in ea. of 24 genes; no variation
Selection pressures • A particular phenotype selected for or against depending on the environment • Guppy example: • 2 forces of natural selection working against each other: mate preference and predator vulnerability • Balance where females are attracted to males with brighter colored tails; risk of attracting predators • How could you test these selection pressures? ♀ ♂
Observe many generations… • In predator-free environment? • More brightly colored males with large tails evolved • Re-introduced predators • Less flashy males became more “fit”
Affects the distribution of phenotypes Normal distribution of varied fur frequencies Stabilizing selection Reduces extremes; favors intermediates Most common Natural selection affects populations
Directional selection Acting against one extreme or environment favors one extreme e.g. darker landscape or, insects exposed to pesticides
Disruptive selection • Environment is varied to favor both extremes • e.g. patchy landscape with light soil and dark rocks