TOK Evolutionary history is an especially challenging area of science because experiments cannot be performed to establish past events or their causes. There are nonetheless scientific methods of establishing beyond reasonable doubt what happened in some cases. How do these methods compare to those used by historians to reconstruct the past? 1 Chapter 22 Evidence for Evolution “All truths are easy to understand once they are discovered; the point is to discover them.”--Galileo Essential Idea There is overwhelming evidence for the evolution of life on Earth. 3 Charles Darwin The man responsible for brining the idea of evolution and natural selection to the forefront of science. His book, The Origin of Species, presented his idea of descent with modification. Although parts were wrong, or erroneous, the majority of it is correct. The errors were due mostly to the limitations of science at the time. Prior to Darwin Prior to Darwin, there were a number of explanations that sought to explain life, how it arose, and it did/did not change. 5 Development of the Theory There were many scientists who studied the history of the earth and the life that inhabits our planet. Cuvier Hutton Lyell Lamarck Wallace Darwin 6 Georges Cuvier (1769-1832) Developed the science of paleontology while he studied layers of rock around Paris (1790’s). He noted that the deeper layers of rock contained fossils that were more dissimilar than current life. From one stratum to the next, many characteristics disappeared and others formed. Thus, he concluded, extinctions must have been common throughout the history of life. Georges Cuvier Cuvier was staunchly opposed to the idea of gradualism. Rather, he advocated catastrophism-the idea that a local catastrophe such as a flood or drought was responsible for the changes observed in species-their extinction. As these organisms were wiped out, new species repopulated the region. James Hutton (1726-1797) In contrast to catastrophism, Hutton proposed the idea of gradualism--slow, cumulative, continuous change results in profound change. His examination of the geological features of Earth allowed him to propose parallels between the Earth’s history and life (1740’s-50’s). Charles Lyell (1797-1875) Lyell, a great geologist, incorporated Hutton’s ideas into a theory called uniformitarianism--which states that the same processes of the past are taking place today at the same rate. Hutton was a horrible writer. Lyell wrote The Principles of Geology (~1830). The Effect on Darwin Lyell and Hutton had a great impact on Darwin. Darwin felt that if these two were right, then the Earth is much older than 6,000 years as theologians argued. Also, he presumed, if these process could act on the Earth, then they could also act on living organisms producing slow and gradual change. Jean Baptist de Lamarck (1744-1829) Lamarck is often remembered for his erroneous interpretation for how evolution occurs--inheritance of acquired characteristics. However, he has made many useful contributions to science. For example, he was the first scientist to propose the idea that evolutionary change can explain the fossil record (early 1800’s). In 1861, Darwin said of Lamarck... “Lamarck was the first man whose conclusions on the subject excited much attention. This justly celebrated naturalist first published his views in 1801. . . he first did the eminent service of arousing attention to the probability of all changes in the organic, as well as in the inorganic world, being the result of law, and not of miraculous interposition.” 13 In 1801, Lamarck Wrote... “. . . time and favorable conditions are the two principal means which nature has employed in giving existence to all her productions. We know that for her time has no limit, and that consequently she always has it at her disposal.” 14 Jean Baptist de Lamarck Lamarck compared current species with fossil forms and chronicled an older to younger line of descent from fossil to living species. His 2 principles: Use and disuse-as determined by the environment. Inheritance of acquired characteristics. Jean Baptist de Lamarck Lamarck also held the belief that an innate drive to be more complex was responsible for the observed change in organisms. Jean Baptist de Lamarck Lamarckian inheritance, based upon the notion of use and disuse, was a perfectly reasonable hypothesis at the time. Many of his contemporaries, including Charles Darwin, subscribed to the principle of use and disuse. 17 Jean Baptist de Lamarck With the rediscovery of Mendel’s work in the early 1900‘s which largely explained the mechanism of inheritance, Lamarck’s ideas were largely rejected by science in favor of Darwin’s theory of natural selection. To be clear, you should understand that characteristics acquired during one’s lifetime are not heritable. 18 Darwin’s 2 Key Points 1. He presented evidence that many species of organisms on Earth are descendants of ancestral species which were different from modern species. These descendants are chronicled in the fossil record. 2. He proposed a mechanism for this change--natural selection. The crux of natural selection is that populations will change over time as individuals become more fit for their environment. Our Current Understanding Our current definition of evolution is referred to as a change over time in the genetic composition (heritable characteristics) of a population. Eventually, enough change will accumulate in a population that it will constitute a new species. The Uproar What made the idea of Darwinism so abrasive at the time is that it challenged traditional beliefs of Western culture: 1. The Earth is only a few thousand years old. 2. All species which now exist have been here since the beginning. Darwinism challenges worldview. Darwin’s Ride on the HMS Beagle Darwin was deeply impressed by the work of Charles Lyell. His belief in Lyell’s principle of uniformitarianism was solidified when he witnessed a huge earthquake while in S. America. This firsthand observation of the Chilean coastline rising several feet as a result of the quake got him thinking. Darwin’s Ride on the HMS Beagle Further exploration and his finding of marine fossils high up in the Andes mountains enabled him to understand how similar earthquakes over a long period of time could result in such change. He was now realizing that the physical evidence wasn’t supporting traditional view. Darwin’s Ride on the HMS Beagle These ideas really sparked Darwin’s interest and when he arrived at the Galapagos he was intrigued by what he saw. Many of the species of animals he observed existed no where else in the world. He did not notice the significance of any of these observations until he returned home to England. Back Home in England Darwin started to notice changes as he examined his specimens after the trip. For instance, he noticed that many of the birds of the Galapagos resembled birds from the mainland of S. America. His hypothesis was that somehow a bird from the mainland must have colonized the Galapagos, and through gradual change diversified into the many forms now seen on the island. Back Home in England Darwin also began outlining the major features of the theory of natural selection. He was reluctant to publish these because of the outrage he knew they would cause. Lyell urged him to publish because he was afraid someone else working on the same thing would beat him to it. Alfred Russel Wallace (1823-1913) A young British naturalist named Alfred Russel Wallace did beat him to it (~1858). While studying in the East Indies (1854-1862), Wallace sent a paper to Darwin for review and the paper basically said the same thing as Darwin’s did. Fortunately for Darwin, Wallace greatly admired him and gave him full credit for the development of the theory of natural selection. Charles Darwin Unlike his predecessors, Darwin was able to convince the biological community his theory is correct because of his reasoning, logic, and the overwhelming amount of supporting evidence. Darwin’s publication presented 2 main ideas: 1. Evolution explains the unity and diversity of life. 2. Natural selection is the cause of adaptive evolution. Natural Selection The main ideas of natural selection are: 1. Natural selection is the differential reproductive success of individuals and their traits. 2. Over time, natural selection increases the adaptations of organisms to their environment. 3. If an environment changes over time, or an individual moves, natural selection may result in adaptations to new conditions and Main Points of Natural Selection 1. Natural selection occurs through individuals and their environment. 2. Individuals don’t evolve. 3. A population is the smallest unit that can evolve. 4. Evolution is measured only as changes in the relative proportions of heritable variation in a population over multiple generations. 5. Natural selection only amplifies inheritable traits, not acquired characteristics. 6. Natural selection is always operating, but which traits are favored depends on the environment. Domesticated Animals Using your device, find explain 2 examples of the evolution of domesticated animals. 32 Rock Pocket Mouse Example Here is another example that illustrates how the success and traits of individuals vary with the environment in which they live. 33 Key Points Arising from Such Research: 1. Natural selection is an editing process rather than a creative mechanism. 2. Natural selection depends on time and place. It favors characteristics in a genetically variable population that increases the fitness in the current environment. Homologies Homologies and the tree of life are important in explaining ancestry. Homologies shared by a greater number of species are likely to have evolved earlier on in history and are further down on the tree of life. Homologies Homologies that have evolved more recently are seen on only small branches of the tree. Example: tetrapods--the vertebrate branch consisting of birds, amphibians, mammals and reptiles. They all have the same 5-digit limb structure which indicates these mammals all share a common ancestor. Homologies Similarities which result from common ancestry are known as homologies. Forelimbs of all mammals show the same arrangement of bones from shoulder to finger tip. These occur even if the structure is used for a completely different function. These anatomical differences would not have arisen in a new species if they didn’t share a common ancestor. Homologies Such structures are said to be homologous structures--which are variations on a common theme that was present in the common ancestor. Homologies Molecular homologies exist as well--the genetic machinery of DNA and RNA also points to a common ancestor; the existence of Hox genes in development.