Lecture #19. Evolution. Overview: Darwin Introduces a Revolutionary Theory. Chapter 22~ Descent with Modification: A Darwinian View of Life

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1 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,

2 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 Linnaeus (classification) Hutton (gradual geologic change) Lamarck (species can change) Malthus (population limits) Cuvier (fossils, extinction) Lyell (modern geology) Lyell: uniformitarianism Darwin: evolution Mendel: inheritance Wallace: evolution Darwin (evolution, nutural selection) Mendel (inheritance) Wallace (evolution, natural selection) Revolution U.S. Civil War American Revolution French Hutton proposes his theory of gradualism Malthus publishes Essay on the Principle of Population Lamarck publishes his theory of evolution Lyell publishes Principles of Geology Darwin travels around the world on HMS Beagle Darwin begins his notebooks on the origin of species Darwin writes his essay on the origin of species Wallace sends his theory to Darwin The Origin of Species is published Mendel publishes inheritance papers. Descent with Modification, I 5 observations: 1- Exponential fertility 2- Stable population size 3- Limited resources 4- Individuals vary Galapagos Islands 5- Heritable variation Darwin s Finches 2

3 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) Biogeography Geographical distribution of species Examples: Islands vs. Mainland Australia Continents The Fossil Record Succession of forms over time Transitional links Vertebrate descent 3

4 Comparative Anatomy Homologous structures (homology) Descent from a common ancestor Vestigial organs Ex: whale/snake hindlimbs; wings on flightless birds 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

5 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

6 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 Molecular Biology Similarities in DNA, proteins, genes, and gene products Common genetic code Species Human Rhesus monkey Mouse Percent of Amino Acids That Are Identical to the Amino Acids in a Human Hemoglobin Polypeptide 100% 95% 87% Chicken 69% Frog 54% Lamprey 14% Final words... Absence of evidence is not evidence of absence. 6