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CP biology - chapter 10 Evolution Early ideas about evolution Early scientists proposed ideas bout evolution Evolution is the process of biological change by which descendants come to differ from their ancestors. Much of today’s understanding of evolution is based on Charles Darwin’s work in the 1800s. But Darwin did not come up with the idea of evolution himself. Many other scientists contributed important ideas to the study of evolution. Several historical ideas – about species, geology, and the mechanisms of evolution – are described below. Like all science, the modern understanding of biological evolution builds on hundreds of years of study and research. Species In the 1700s, a botanist named Carolus Linneaus came up with a system to organize and name all of the different types of organisms, or species. A species is a group of organisms that can reproduce and have fertile offspring. Linneaus’ classificiation system grouped organisms according to physical similarities. His system also shows evolutionary relationships, and is still in use today. In Linneaus’ time, there was a common belief that the organisms that lived on earth were fixed, or that species did not change. Linneaus proposed that species could change. For example, he observed in experiments that two different plant species could cross, and make a new type of plant. In the 1700s, it was also a common belief that Earth was only 6000 years old. But several geologists began to challenge this idea. The geologists believed that they had evidence that Earth was much older. The evidence of the old age of Earth was important to Darwin’s development of his idea. Mechanisms of evolution Many scientists in the 1700s did not believe that species could go extinct. But many scientists thought species could change, or evolve,. There were many different ideas, however, about the mechanism of evolution, or how evolution happens. For example, different scientists had different ideas about how environmental changes affect evolution, how changes get passed on to offspring, and what causes biological variation. Theories of geologic change set the stage for Darwin’s theory The study of fossils led some scientists to conclude that species do go extinct. Fossils are traces of organisms that existed in the past. The locations of fossils in different rock layers provide clues about Earth’s past. But how did those rock layers form? Geologists held different ideas about geologic change. Catastrophism is the idea that past natural disasters – like floods and volcanic eruptions – shaped landforms, and caused species to become extinct in the process. 1 Gradualism is the idea that landforms were shaped by very slow changes over a long period of time, and not by natural disasters. Uniformitarianism is the idea that the same processes that shaped landforms in the past also shape landforms today. In other words, the geologic processes that shape Earth are uniform, or the same, through time. Uniformitarianism became the favored theory of geologic change and played an important role in the development of Darwin’s theory. 10.2 Darwin’s observations Darwin observed differences among island species Darwin spent over 20 years researching biological evolution. He made important observations during his travels on a ship that sailed the coast of South America and the Pacific Islands. Darwin was amazed by the variation of traits among similar species that he saw in his travels. The word variation has many common uses, but in biology variation means the difference in physical traits among individuals in a group of organisms. 2 Darwin noticed that there was variation between species on different islands. This was especially noticeable in the Galapagos Islands, a chain of islands off the coast of Ecuador in South America. For example, he noticed that birds called finches had different kinds of beaks in areas with different food sources. He found finches with thick beaks in areas with large hard-shelled nuts, and finches with smaller beaks in areas where fruits were available. These differences seemed to match the environment and the diet of the finches. Darwin realized that species may somehow be able to adapt to their surroundings. An adaptation is a feature that allows an organism to better survive in its environment. Darwin observed fossil and geologic evidence supporting an ancient Earth During his travels, Darwin found that some fossils looked similar to living species. T his suggested that the living species might have some relationship to the fossil forms. In order for such changes to occur, Darwin figured that Earth must be much older than 2000 years. Darwin found much evidence supporting the ideas of uniformitarianism. For example, he found fossil shells of marine organisms high up in the Andes mountains. Later he experienced an earthquake and saw land that had been underwater get shifted up above the sea level. He put together observations like these, and concluded that daily geologic processes can add up to much bigger changes over a long period of time. 10.3 Theory of Natural Selection Several key insights led to Darwin’s idea for natural selection The variation of similar species among islands, fossil evidence, and geologic events convinced Darwin that evolution occurs But he still wondered how evolution occurs. Here, you will read about some of Darwin’s reasoning that led him to his idea for natural selection. Artificial selection Darwin noticed that plants and animals that are raised by humans had variations in traits that were not seen in their wild relatives. Think of all the different breeds of dogs that you have seen. In a process called artificial selection, humans select individuals with the traits they desire, and then breed them to produce more individuals with those traits. Heritability In order for artificial selection to occur, traits must be heritable. Heritable is the ability of a trait to be passed down from one generation to the next. Things that are acquired in an organism’s life, like a broken bone, are not heritable. Natural selection Darwin reasoned that a process similar to artificial selection could happen in nature. In artificial selection, humans are the source of selection. In natural selection, the environment is the source of selection. 3 Natural selection is a process in which individuals that have inherited beneficial adaptations produce more offspring than to other individuals. Struggle for survival Darwin was influenced by the work of an economist named Thomas Malthus. Malthus proposed that resources like food, water, and shelter were limits to human population growth. Darwin reasoned that a similar struggle happened in nature. Darwin saw great variation within populations of organisms. A population is all the individuals of a species that live in an area. He saw individuals with adaptations that matched their environment. Darwin proposed that these adaptations arose over many generations in a process he called “descent with modification.” Natural selection explains how evolution can occur Darwin was not the only scientist studying evolution during this time. Another scientist named Alfred Wallace independently developed an explanation of how evolution occurs. Wallace’s explanation was very similar to Darwin’s. In the late 1850’s, the idea of Darwin and Wallace were presented to the scientific community. There are four main principles to the theory of natural selection: variation, overproduction, adaptation, and descent with modification. Variation Individuals of a species differ due to genetic variation. Heritable differences are the basis for natural selection. Overproduction Organisms have more offspring than can survive. This results in competition among offspring for resources. Adaptation Some individuals have certain variations that allow them to survive better than other individuals in their environment. These individuals are “naturally selected” to live longer and produce more offspring that also have those adaptations. Descent with modification Over time, natural selection will result in species with adaptations that are beneficial for survival and reproduction in a particular environment. More individuals will have the trait in every following generation, as long as the environmental conditions stay the same. Let’s apply these four principles to an example of natural selection. About 11,000 years ago, jaguars faced a shortage of food due to a changing climate. There were fewer mammals to eat, and jaguars had to eat reptiles to survive. Variations in jaw and tooth size allowed some individuals to more easily eat shelled reptiles. 4 Natural selection acts on existing variation Natural selection cannot make new alleles. It can only work with variation that already exists. In other words, natural selection acts on phenotypes, or physical traits, and not on genetic material itself. Changing environments As an environment changes, different traits become beneficial. Think about the jaguars. When mammals were their main food source, small teeth and jaws were beneficial. But when the environment changed, larger teeth and jaws became beneficial to better eat reptiles. Because the environment constantly changes, a trait that is an advantage today may be a disadvantage in the future. Adaptations as compromises Adaptations can sometimes be thought of as compromises. One example of an adaptive compromise is the panda bear’s ‘thumb”. Modern pandas have five digits that are like your fingers and a sixth digit that functions like a thumb. This digit is actually a wrist bone. Ancestral pandas with bigger wrist bones had an advantage in holding and eating their food. Over time, this adaptation became typical of the species. 10.4 Evidence of evolution Evidence for evolution in Darwin’s time came from several sources Genetic inheritance was not known in Darwin’s time. But Darwin supported his ideas with evidence from many other sources – fossils, geography, embryology, and 5 anatomy. His evidence was very strong, and left no doubt in the minds of scientists that all organisms share a common ancestor. Today, the concept of evolution ties together all fields of biology. Fossils Before Darwin, scientists studying fossils knew that organisms changed over time. Fossilized organisms were different in different layers of rock. The bottom layers of rock are the oldest, and contain fossils of more ancient organisms. The upper layers of rock are the youngest, and contain fossils of more recent organism. Findings in the fossil record support Darwin’s idea of descent with modification. Geography During Darwin’s travels, he noticed that plants and animals on islands looked similar to species on the mainland, but not exactly the same. He hypothesized that organisms from the mainland had migrated to the islands. Different islands had different food sources, climates, and predators. The different environmental conditions favored different traits in these migrant organisms. For example, the finches on the different Galapagos islands have different shapes of beaks that evolved in response to the different island habitats. This is an important part of biogeography, the study of the distribution of organisms around the world. Embryology The study of embryos and their development. Darwin noticed that the embryos of different species may look very similar, although the adult species look very different. For example, fish, birds, reptiles, and mammals all have gill slits as embryos. These slits develop into gills in fish, but develop into ears and throats in mammals. The similar features of embryos in very different organisms suggest evolution from a distant common ancestor. Anatomy Some of Darwin’s best evidence came from comparing the body parts of different species. He found that some organisms have body parts that are similar in structure, but might be used differently. Homologous structures are features that are similar in structure, but appear in different organisms and may have different functions. For example, think of the front limbs of humans, bats, and moles. The front limb of these organisms share similar bone structure, but each organism uses these limbs differently. Homologous structure appear across man different species, and offer strong evidence for common descent Analogous structures are structures that perform a similar function, but are not similar in origin. For example, both birds and butterflies have wings. They both use their wings to fly, but their wings do not share a common origin. Bird wings have bones, and butterfly winds do not. 6 analogous structures vestigial structures Just because two organisms share similar structures does not mean they are closely related. Homologous structures show common ancestry, while analogous structures do not. Structural patterns are clues to the history of a species Vestigial structures are small leftover organs or structures that had a function in an early ancestor. For example, snakes and whales have tiny pelvic bones. Even though neither organism walks, their ancestors were four-legged animals. These pelvic bones are vestigial structures. The appendix is an example of a vestigial structure in humans. The appendix is a remnant of an organ that helped to digest certain plant material eaten by human ancestors. The human appendix has lost the ability to digest this material, and actually has no known function. Vestigial structures do not become smaller in one individual organism. These structures became smaller over many generations. Today, biologists consider vestigial structures among the most important examples demonstrating how evolution works. 1`0.5 Evolutionary biology today Fossils provide a record of evolution Paleontology is the study of fossils or extinct organisms. The fossil record is not complete. One reason for this is because most organisms do not form fossils after they die. Fossils form only in particular environmental conditions. However, fossil evidence that does not support evolution has never been found. Darwin wondered why he did not find fossils that showed transitions between different groups of organisms. Since Darwin’s time, many of these transitional fossils have been found. For example, fossils have been found of a transitional species in the evolution of whales. This organism had a whalelike body, but still had the limbs of land animals. Fossils continue to provide new information and evidence for current ideas about evolution. 7 whale evolution Molecular and genetic evidence support fossil and anatomical evidence The fields of genetics and molecular biology have added strong support to Darwin’s theory of natural selection. All living tings have DNA, share the same genetic code, and make most of the same proteins from the same 20 amino acids. Comparisons of DNA and protein sequences can be used to show evolutionary relationships between different organisms. The more related two organisms are, the more similar the sequences will be. Because there are thousands of genes in even simple organisms, DNA contains a huge amount of information on evolutionary history. Some particular genes are found in many organisms – from fruit flies to humans – and therefore give evidence of a very different common ancestor. For example, homeobox genes, which control development, are found in a wide range of organisms. These genes are even found in organisms that lived 600 million years ago. Evolution unites all fields of biology Scientists continue to actively study evolution through natural selection. The theory of natural selection combined with genetics is sometimes called the modern synthesis of evolutionary theory. New tools are providing more data than ever before. Modern tools from different fields of study add to what has been discovered through fossil evidence. For example, as shown below, the comparisons of milk protein genes support the fossil evidence for the relationship between whales and hippopotamuses. New discoveries are limited only by the time and resources of scientists. 8 molecular evidence The basic principles of evolution are used in all fields of science, including medicine, geology, geography, chemistry, and ecology. For example, the idea of common descent helps biologists understand where new diseases come from and how they might be best treated. As much as we know about life on Earth, there is so much more waiting to be discovered. As the great geneticist Theodosius Dobzhansky once noted, “Nothing in biology makes sense except in the light of evolution.” 9