Chapter 22 Descent with Modification: A Darwinian View of Life Question? How did the diversity of life originate? Through the process of Evolution. Big Idea 1 Evolution The processes that have transformed life on earth from its beginnings to today's diversity. Evolution is the most pervasive principle in biology (Big Idea 1). Theory vs theory Theory Hypothesis supported repeatedly by data Makes testable predictions theory Layman’s and TV use of the word Confused with hypothesis in Science Examples of Theory Cell Theory Big Bang Theory Atomic Theory Theory of Gravity Theory of Evolution Teaching Philosophy on Evolution as a Theory Evolution Has itself "evolved" or changed over time. Illustrates “Science as a Process”. Students should be able to give the main points of several views. Pre-Darwinian Views 1. Greeks 2. Fixed Species 3. Catastophism 4. Hutton and Lyell 5. Lamarck Greek Philosophers 1. Plato - Organisms are already perfectly adapted to their environments. 2. Aristotle - Organisms arranged on a “scale of life” from simple to complex. Result No evolution. Life is already perfect and doesn’t need to change. All the rungs on life's "ladder" are already occupied. Fixed Species Concept The creator had designed each and every species for a particular purpose. Result No evolution. Created the viewpoint that all species could be identified and named (Taxonomy). A major factor in the Linnaeus classification system. Catastrophism Georges Cuvier (1769-1832). Attempted to relate fossils to current life. Theory Fossils were the remains of species lost due to catastrophe. No new species originated; species could only be lost over time. Result - No evolution. James Hutton 1795 - Gradualism Profound change is the cumulative product of slow, but continuous processes. Result Changes on the earth were gradual, not catastrophic. Charles Lyell 1797 - 1875. Incorporated Hutton’s gradualism into a theory called Uniformitarianism. Uniformitarianism Geological processes have operated at the same rate over the Earth’s history. Result The Earth must be VERY old. (much older than 6000 years of the fixed species concept). Idea that slow and subtle processes can cause substantial change. Jean Baptiste Lamarck Published theory in 1809. Theory - Life changed from simple to complex over time. Lamark Fossils were the remains of past life forms. Evolution did occur. Mechanisms 1. Use and Disuse Body parts used to survive become larger and stronger. Body parts not used to survive deteriorate. Mechanisms 2. Acquired Characteristics Modifications acquired by use/disuse were passed on to offspring. Problem No knowledge of genetics. Acquired traits are not transmitted offspring. To Lamarck’s Credit He did suggest correctly the role of fossils in evolution. He did suggest that adaptation to the environment is a primary cause of evolution. Thomas Malthus Essay on human population growth in 1798. Disease, famine, homelessness, and war are inescapable because human populations grow faster than food supplies. Darwin read Malthus. Wallace read Malthus too. Charles Darwin Father of the modern theory of evolution. Theory Descent with Modification. Darwin's Background Trained as a Naturalist (after trying religion and medicine). Voyage of the Beagle Result Darwin's training and travel opportunities allowed him to formulate and support his ideas on Natural Selection. Galapagos Finches Alfred Wallace - 1858 Paper on Natural Selection identical to Darwin's ideas. Result - July 1, 1858 Dual presentation of the Wallace-Darwin ideas to the Linnaean Society of London. Side note – The president of the Linnaean Society later remarked that “nothing remarkable happened in 1958” Darwin - 1859 Publication of "The Origin of Species”. Sold out and has been reprinted many times. Comment Darwin best remembered for the theory because of his overwhelming evidence and because he published. He was also better “known” than Wallace. Darwinian View History of life is like a tree with branches over time from a common source. Current diversity of life is caused by the forks from common ancestors. Example “The Origin of Species” Documented the occurrence of evolution. Suggested that the mechanism for evolution was Natural Selection. Observations: Observation 1 – Members of a population may vary greatly in their traits. . Observation 2 Traits are inherited from parents to offspring. Observation 3 All species are capable of producing more offspring than their environment can support. Observation 4 Owing to lack of food or other resources, many of these offspring do not survive. Inference 1 Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals. Inference 2 This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations. Nature Determines which characteristics are favorable. Determines who survives. Result - “Natural Selection” (or selection by Nature) Natural Selection in action Artificial Selection When man determines the characteristics that survive and reproduce. Result - the various breeds of animals and plants we’ve developed. Ex - Mustard Plant Original Cultivars Artificial Selection Evolution Success Measured By Survival Reproduction Whoever lives and has kids is the “winner” in evolution. Requirements In order for Natural Selection to work, you must have: Variations within a population. Long periods of time (according to Darwin). Subtleties of Natural Selection 1. Populations are the units of Evolution. 2. Only inherited characteristics can evolve. Comment Acquired characteristics may allow a species to evolve "outside" of Natural Selection. Ex: culture, learning Evidences for Evolution Direct observation of evolutionary changes. Fossils Homology Convergent Evolution Biogeography Molecular Direct Observations Color patterns in guppies Drug resistant HIV Beak size in Birds PitX1 gene Others Color Pattern in Guppies Field Experiment (see text for full details) Changed the selection pressure on male guppies Result – color pattern change in 15 generations (22 months). HIV Drug Resistance Drug resistance strains selected for by treatments Result – resistant strains became 100% dominant in 4-5 weeks. Beak Size Study – measured the beak size of all birds in a population over several years. Result – drought and food competition changed beak size. Field Fossils Relics or impressions of organisms from the past. Problem: Show changes over time from simple to complex. Many fossils don't have descendants. Evolution Viewpoint Life has changed over time. Many species failed to survive and became extinct. Comments 1. Fossilization is a rare event. 2. Only hard parts fossilize well. 3. Problem in finding fossils. 4. Interpretation. 5. Missing Links. Assignments Read Chapter 22 or Chapter 15 in Hillis Lab – Population Genetics – due Friday of next week Discussion board - today Exam 2 – Tuesday of next week Chapter 22 – Wed. Homology Homologous Structures Common "building plan” with divergent functions. Mammal forelimbs Problems Vestigial Organs - Rudimentary structures of marginal, if any, use. Whale Legs Human Example Evolution Viewpoint Remodeling of ancestral structures as their functions or adaptations changed. Homology in Embryos Problem - closely related organisms go through similar stages in their embryonic development. Ex: Gill pouches in vertebrates Evolution Viewpoint Ontogeny is a replay of Phylogeny. (Development reflects descent) Development reflects inheritance of common control genes such as Hox genes. Convergent Evolution Unrelated organisms show similar adaptations. Cause – lived in a similar environment with similar selection pressures. Biogeography The geographical distribution of species. Problem: Species mixtures on islands Marsupials in Australia Evolution Viewpoint Biogeographical patterns reflect descent from the ancestors that colonized that area. Molecular Biology Study of Evolution at the DNA or protein levels. Problem - related species have similar DNA sequences. Universality of DNA as the genetic code supports a common ancestor. Evolution Viewpoint Related species share a common ancestrial DNA. The closer the relationship, the more similar the DNA sequences should be. Summary Darwin's ideas now a "Theory”. Predictions of a Theory are tested by experiments and observations. Be familiar with the pre-Darwin views of evolution. Summary Know Darwin’s “observations” and “inferences”. Be able to discuss the various evidences of Darwinian evolution.
