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Transcript
Unit 1: Evolution chapter 22: descent with modification chapter 25.2 & 3 & 4 : history of life on earth chapter 23: evolution of populations chapter 24 and 25.6 : origin of species chapter 26.1 and 26.3: phylogeny Chapter 22: Descent with Modification Observations of the natural world reveal three things: • how well organisms are suited (adapted) to their particular environments • the many shared characteristics of life (unity) • the vast diversity of life What scientific explanation accounts for these observations? Answer: evolution • broad definition: descent with modification, change over time, origin of new species and groups of species • narrow definition: change in genetic composition of a population between generations (What’s the biological definition of “population”?) Biologists study the unity and diversity of life. Evolutionary adaptation Organisms adapt to their environment over many generations through natural selection. In a population of organisms, there will be natural differences from individual to individual (because of inherited variation in the population). That means that some individuals will be better suited to the local environment. Those individuals will have the best reproductive rates and pass on the beneficial traits to their offspring. THEME: Biological evolution is the fundamental organizing principle of biology. The theory of evolution, as developed by Darwin and others, accounts for the unity and diversity of organisms in the biosphere. Unity – All organisms share similarities because they are all descendants of a common ancestor. Diversity – Organisms have different adaptations because they are necessary for optimal survival and reproduction in different environments. The traits have been inherited from previous generations, and natural selection has acted on populations to produce groups of organisms best suited for their environment. Descent with modification (unity) (diversity) History of evolutionary thought: What was the progression of ideas and contemporary context that led to the formulation of Darwinian evolution to explain biological unity, diversity, and adaptation? Aristotle (384-322 B.C.) • scala naturae • Species are fixed. Linnaeus (1707-1778) • physician, botanist • binomial classification (nested system) • rejected evolutionary kinship to explain categorization of living things Cuvier (1769-1832) • developed paleontology • Fossils in older, lower strata are dissimilar from those found in newer, upper strata. • Used catastrophism (not evolution) to explain changes in fossil record Hutton (1726-1797) • Geologic features result from gradual mechanisms. • Same processes are still operating today. Lyell (1797-1875) • Incorporated Hutton’s explanation of gradual geological change into principle of uniformitarianism (mechanisms of change are constant over time) • Earth had to be much older than a few thousand years in order for uniformitarianism to explain current geological features of the planet. Hutton and Lyell had great impact on Darwin, but Darwin was not the first to use gradual mechanisms as a way to explain biological change over time. Lamarck (1744-1829) • Compared living species with fossil record and discovered several lines of descent • Explained his observations with two principles: – use and disuse – inheritance of acquired characteristics • (Mistakenly) thought organisms were “driven” to evolve • Vilified at the time by those who rejected evolution (Cuvier, et al) • He was on the right track even though his proposed mechanism of evolution was ultimately wrong. Charles Darwin (1809-1882) • Begins medical school training but quits to become clergyman • Traveled on HMS Beagle • Read Lyell’s Principles of Geology • Collected specimens (fossils, living species, etc) during stops along South American coastline and Galapagos islands that were particularly influential in formulating his theory of evolutionary change • Darwin observed multiple examples of adaptations. – finches, tortoises, iguanas, etc. • Darwin explained the occurrence of adaptations by invoking natural selection (his mechanism of evolution). • Darwin was initially reluctant to publish his findings and ideas about natural selection. • Evolution by natural selection was independently proposed by a second scientist: Alfred Russel Wallace. Wallace “scooped” Darwin. Darwin’s The Origin of Species is published in 1859. He proposes natural selection as the mechanism of gradual biological change to explain life’s unity and diversity. His explanations and the “avalanche” of evidence to support them convinced most scientists within a decade that life’s unity and diversity is the product of evolution (descent with modification). The Origin of Species proposed two main ideas: (1) descent with modification (2) by means of natural selection Darwin’s book provided a plausible explanation for the 3 general observations about the living world: •Unity of life results from common descent (ancestry). •Diversity of life results from diverse modifications arising via natural selection. •Descendants of ancestral organisms accumulate adaptations that fit specific ways of life (producing good match between organism and environment). The success of Darwin’s work attributed to: •Use of scientific method •Proposal of logical mechanism •Mountain of supporting evidence Darwin illustrated his concepts using branching tree diagrams. Darwin’s reasoning and explanation used artificial selection as a framework for understanding natural selection. Darwin made two key observations and formulated two key inferences in explaining that natural selection operated like artificial selection. • observation #1 – Members of a population vary in their inherited traits. • observation #2 – All species produce more offspring than their environment can support. – Malthus (1798), An Essay on the Principle of Population Darwin made two key observations and formulated two key inferences in explaining that natural selection operated like artificial selection. What’s an inference? •Inference #1 – Individuals whose inherited traits give them a higher probability of surviving/reproducing leave more offspring. •Inference #2 – Favorable traits accumulate over generations. Natural selection leads to a better fit between organisms and their environment and, given enough time, can produce new species and groups of species. • Natural selection is a process: beneficial adaptations are passed on at higher rate – Natural selection requires heritable variation to be present in a population. Why? – Organisms produce more offspring than the environment can possibly support. Individuals with certain (advantageous) traits are likely to have more offspring and pass along those traits to their offspring. (And the process continues in an iterative manner.) • Over time, natural selection increases the match between organisms and their environment. • Can ultimately give rise to entirely new species • IMPORTANT: Natural selection acts on populations. Individual organisms do not evolve! Natural selection leads to a better fit between organisms and their environment and, given enough time, can produce new species and groups of species. Multiple lines of evidence support evolution by natural selection. (Chapter 22.3 and 25.2, 3, 4) 1. Direct observations of evolutionary change Example: MRSA methicillin-resistant Staphylococcus aureus Example: Biston betularia (peppered moths) exhibiting industrial melanism in pre- and post-Industrial Revolution England Example: Glucose-averse insect populations 2. Comparative anatomy homology = similarity resulting from common ancestry – Homologous versus analogous structures – Vestigial structures Vertebrate forelimbs are homologous structures. All vertebrate embryos display homologous structures at some point during development. 3. Biochemical evidence (molecular homologies) – All living organisms use same basic molecules and mechanisms: • Universal genetic code • Use of DNA and RNA, similar genetic sequences • ATP as energy “currency” in metabolism • 20 amino acids • Common metabolic pathways (glycolysis, etc) 4. Fossil record – Fossils are remains or evidence of some organism that lived long ago. – Often found in sedimentary rock layers called strata. – Shows great changes over time Fossil record documents how new groups of organisms arose from previous ones. Fossilization has a limited scope. Why? Gaps in the fossil record continue to be filled. How do you determine age of a fossil? •order tells us relative ages •radiometric dating tells us absolute ages What we can learn from fossil evidence: evolution of sea mammals astragalus, an ankle bone: = deer, pig, camels, cattle = whales, dolphins, porpoises Clock analogy – if Earth’s history occurred in 12 hours… Era Period (mya) important events in the history of life 450 - 470 535 - 525 Era Period (mya) 6 225 important events in the history of life Fossil record also shows five major mass extinction events in the last 500 million years. Permian extinction Cretaceous extinction Cretaceous mass extinction event, 65.5 mya All five mass extinctions have been followed by adaptive radiations. Era Period (mya) adaptive radiations following mass extinctions important events in the history of life 5. Biogeography – – geographic distribution of species influenced by continental drift: movement of continents on tectonic plates • forms mountains, islands, earthquakes; alters habitats, climate Plates move few cm per year. Cumulative effects of shifting land masses over millions of years can be dramatic. Examples of biological effects of tectonic plate movement: Island species vs mainland species Australian marsupials Same fossil species found in Brazil and Ghana Final sentence of The Origin of Species: “There is grandeur in this view of life…. [in which] endless forms most beautiful and most wonderful have been, and are being, evolved.” -Charles Darwin One of the founders of modern evolutionary theory: “Nothing in biology makes sense except in the light of evolution.” -Theodosius Dobzhansky