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Lecture #19 Date ________ • Chapter 22~ Descent with Modification: A Darwinian View of Life Overview: Darwin Introduces a Revolutionary Theory • A new era of biology began on November 24, 1859, the day Charles Darwin published On the Origin of Species by Means of Natural Selection – Focused biologists’ attention on the great diversity of organisms – He presented evidence that the many species of organisms presently inhabiting the Earth are descendants of ancestral species – He proposed a mechanism for the evolutionary process, natural selection as the cause of adaptive evolution Evolution • Evolution: the change over time of the genetic composition of populations • Natural selection: populations of organisms can change over the generations if individuals having certain heritable traits leave more offspring than others (differential reproductive success) • Evolutionary adaptations: a prevalence of inherited characteristics that enhance organisms’ survival and reproduction November 24, 1859 1 Influences on Darwin • Carolus Linnaeus: founder of taxonomy, classifying life’s diversity “for the greater glory of God” • Georges Cuvier: opposed the idea of gradual evolutionary change, instead advocated catastrophism, speculating that each boundary between fossil strata represented a catastrophe • Geologists Hutton and Lyell: perceived that changes in Earth’s surface can result from slow continuous actions still operating today • Lamarck: hypothesized that species evolve through use and disuse and the inheritance of acquired trait (But the mechanisms he proposed are unsupported by evidence) Evolutionary history • • • • • Linnaeus: taxonomy Hutton: gradualism Lamarck: evolution Malthus: populations Cuvier: paleontology • • • • Lyell: uniformitarianism Darwin: evolution Mendel: inheritance Wallace: evolution Linnaeus (classification) Hutton (gradual geologic change) Lamarck (species can change) Malthus (population limits) Cuvier (fossils, extinction) Lyell (modern geology) Darwin (evolution, nutural selection) Mendel (inheritance) Wallace (evolution, natural selection) American Revolution French Revolution U.S. Civil War 1800 1850 1900 1750 1795 Hutton proposes his theory of gradualism. 1798 Malthus publishes “Essay on the Principle of Population.” 1809 Lamarck publishes his theory of evolution. 1830 Lyell publishes Principles of Geology. 1831–1836 Darwin travels around the world on HMS Beagle. 1837 Darwin begins his notebooks on the origin of species. 1844 Darwin writes his essay on the origin of species. 1858 Wallace sends his theory to Darwin. 1859 The Origin of Species is published. 1865 Mendel publishes inheritance papers. Descent with Modification, I • 5 observations: • • • • • 1- Exponential fertility 2- Stable population size 3- Limited resources 4- Individuals vary 5- Heritable variation Galapagos Islands Darwin’s Finches 2 Descent with Modification, II • The phrase descent with modification – Summarized Darwin’s perception of the unity of life – States that all organisms are related through descent from an ancestor that lived in the remote past • 3 Inferences: • • • 1- Struggle for existence 2- Non-random survival 3- Natural selection (differential success in reproduction) Evolution evidence: Biogeography • Geographical distribution of species • Examples: Islands vs. Mainland Australia Continents Evolution evidence: The Fossil Record • Succession of forms over time • Transitional links • Vertebrate descent 3 Evolution evidence: Comparative Anatomy • Homologous structures (homology) • Descent from a common ancestor • Vestigial organs Ex: whale/snake hindlimbs; wings on flightless birds Evolution evidence: Comparative Embryology • Pharyngeal pouches, ‘tails’ as embryos • Ontogeny Recapitulates Phylogeny – Ernst Haeckel proposed that the embryonal development of an individual organism (its ontogeny) followed the same path as the evolutionary history of its species (its phylogeny) – Refuted, but new things can be learned from his insights Natural Selection and Adaptation • Evolutionary biologist Ernst Mayr has dissected the logic of Darwin’s theory into three inferences based on five observations (a) A flower mantid in Malaysia (b) A stick mantid in Africa 4 • Observation #1: For any species, population sizes would increase exponentially (if all individuals that are born reproduced successfully) • Observation #2: Nonetheless, populations tend to be stable in size (except for seasonal fluctuations) • Observation #3: Resources are limited – Inference #1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving • Observation #4: Members of a population vary extensively in their characteristics…no two individuals are exactly alike 5 • Observation #5: Much of this variation is heritable – Inference #2: Survival depends in part on inherited traits…individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals – Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations Evolution evidence: Molecular Biology • Similarities in DNA, proteins, genes, and gene products • Common genetic code Species Percent of Amino Acids That Are Identical to the Amino Acids in a Human Hemoglobin Polypeptide 100% Human Rhesus monkey 95% Mouse 87% Chicken 69% Frog Lamprey 54% 14% Final words…... • “Absence of evidence is not evidence of absence.” 6