I. Darwin’s voyageA. the fossil recordB. dating fossilsC. comparative anatomy of related speciesD. distribution of plants and animalsBIOL 1020 – CHAPTER 22 LECTURE NOTESChapter 22: Descent with Modification: A Darwinian Viwe of LifeI. Darwin’s voyageA. Charles Darwin (1809-1882) received a degree in theology, but also was trained in the type of field biology common in the early 19th century (most prominent biologists of the time were field biologists who studied and classified organisms in their natural environments)B. Darwin signed on as the captain’s companion on board the H.M.S. Beagle, on which he took a five-year voyage from 1831-1836 exploring South America and surrounding islands, as well as islands in the South PacificC. His private work on the voyage was as a naturalist, collecting and cataloging thousands of speciesD. He was most impressed by the similarities between species in the Galapagos islands (in the Pacific west of Ecuador) and their similarities to species from South America; this did not fit well with the divine design model that he had been trained under and that was still prominent at the timeE. There was much discussion by immediate predecessors and contemporaries of Darwin about how the divine design model did not mesh well with observation of the extremes of variation among species, the idea of extinct species represented in the fossil record, and the functional similarities between the anatomy of extremely divergent species; the idea that evolution occurs thus was “in the air” at the time, but attempts to find a convincing mechanism fell short (such as Lamarck’s acquired characteristics model)F. Recall Darwin’s theological training – Darwin was well aware of the impact that a workable, testable theory of evolution would have, and the intense controversy and scrutiny it would draw; thus, while he worked out most of his theory of evolution shortly after his trip on the Beagle, he spent 20 years accumulating evidence and doing experiments before finally publishing the ideaG. Darwin was spurred on to publish when Alfred Russel Wallace shared his independent work where he had reached similar conclusions to Darwin; they first presented the theory of evolution by natural selection together in 1858H. Darwin published his first version of the book On the Origin of Species by Means of Natural Selection in 1859; in it, he laid out the entire argument with all of the evidence that he had been gathering ever since his voyage on the BeagleI. Darwin’s book had immediate and dramatic impact; the force of his argument and evidence convinced many scientists quickly, but of course it stirred tremendous controversy as well; Darwin made several revisions of his work in response to some of the most reasonable criticisms, as well as focusing on human evolution in The Descent of Man (1871)II. Evidence supporting the theory of evolutionA. the fossil record 1. fossils provide direct evidence for change over time- fossils range from mineralized casts or imprints (most commonly of bone, teeth, and shells, but sometimes of softertissues) to actual body parts preserved in bogs, tar, amber, or ice- fossils provide evidence of intermediates between extant and extinct forms- many relatively complete examples of transitions in body forms are known, such as the evolutionary lineage of horses and the transition of terrestrial species to modern whales- the fossil record provides tests of evolution as an explanation for the history of life on Earth – fossils can be dated, and the age of fossils invariably matches the predicted place of those body forms in the history of life on Earth2. fossils most commonly form in sedimentary rocks in aquatic environments- the fossil record is biased toward organisms with hard parts that lived in aquatic or arid environments, where decayis slow and incorporation in rocks can occur with reasonable speed- organisms that lived in places of rapid decay are thus biased against in the fossil recordB. dating fossils3. relative position in rock layers- sedimentary layers most commonly have the youngest layers nearer the surface, and are progressively older as you go deeper- large-scale geological events can be used to correlate rock strata from different sites; other dating methods are also used to correlate rock strata4. association with index fossils that have been dated by other means from other locations5. radioisotope dating- each radioisotope has characteristic, constant rates of decay- some allow for measurement of when a rock was formed or when an organism died- example: potassium-40 decaying to argon-40 when magma cools to solid rock, no argon is in the rock (escapes as the rock forms) once the rock hardens, the radioactive clock begins – potassium-40 in the rock decays to argon-40 measurement of the amounts of potassium-40 and argon-40 in the rock today are used to determine an age range for when the rock could have been formed half-life of 1.3 billion years: used for fossils tens of millions to billions of years old1 of 2BIOL 1020 – CHAPTER 22 LECTURE NOTES- another example: carbon-14 decaying to nitrogen-14 (half-life of 5730 years) – used for organic remains hundreds to tens of thousands of years old6. there are hundreds of well-studied sites with fossils that can be dated in some way; no truly incongruous fossils have been foundC. comparative anatomy of related species7. organs or structures that have similar form due to a common evolutionary origin are called homologous features- example: the similarity between the human arm, the dolphin's flipper, the bat's wing, and the bird's wing- example: plant leaves, cactus needles, flower sepals and petals8. not all organs or structures with functional similarity have a common origin- such cases are called homoplastic features, or analogous features- resemblance between homoplastic features is superficial – consider an insect's wing and a bird's wing- independent evolution of similar features in distantly related organisms is called convergent evolution9. vestigial structures – a feature that once had a role in the evolutionary history of a species but that no longer functions- natural selection will logically lead to degeneration of unused features- however, it is not easy to completely remove by natural selection – thus, vestiges are left behind- examples: human appendix, wisdom teeth; whale pelvis and hind legsD. distribution of plants and animals10.