College-Level Evolution
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TABLE OF CONTENTS Preface........................................................................................................ 1 Chapter One: Early Evolutionary Theories .................................................. 5 Early Evolutionists ........................................................................................................... 5 Timeline of Evolutionary Theories .................................................................................. 7 Evolutionary Thought in Ancient Times ......................................................................... 8 Christian Philosophies on Evolution ............................................................................... 9 Nineteenth Century Evolution ....................................................................................... 10 After Darwin and Natural Selection .............................................................................. 12 The Story of Darwin ....................................................................................................... 13 Key Takeaways ............................................................................................................... 15 Quiz ................................................................................................................................ 16 Chapter Two: Natural Selection ................................................................ 20 What is Natural Selection? ............................................................................................ 20 Adaptation, Fitness, and Reproductive Success............................................................ 25 Studying Natural Selection ............................................................................................ 29 Natural Selection and Complex Behaviors .................................................................... 29 Key Takeaways ............................................................................................................... 31 Quiz ................................................................................................................................ 32 Chapter Three: The Study of Taxonomy and Phylogeny ............................. 36 Taxonomy....................................................................................................................... 36 Phylogenic Trees ............................................................................................................ 39 Cladistics ........................................................................................................................ 42
Phylogenetics and Molecular Phylogenetics ................................................................. 44 Key Takeaways ............................................................................................................... 46 Quiz ................................................................................................................................ 47 Chapter Four: Genetics and Genetic Variation .......................................... 51 Introduction to Genetics ................................................................................................ 51 History of Genetics ........................................................................................................ 52 Genetic Processes ........................................................................................................... 54 Mendelian Genetics ....................................................................................................... 58 Genetic Variability and Mutation .................................................................................. 60 Mutations ....................................................................................................................... 61 Key Takeaways ............................................................................................................... 62 Quiz ................................................................................................................................ 63 Chapter Five: Origin of Life ....................................................................... 67 Early Earth Environment .............................................................................................. 67 Early Forms of Life ........................................................................................................ 73 Viral Evolution ............................................................................................................... 74 Prokaryotic Cell and Eukaryotic Cell Evolution ............................................................ 75 Key Takeaways ............................................................................................................... 79 Quiz ................................................................................................................................80 Origin of Eukaryotes ...................................................................................................... 84 Evolution of Multicellularity.......................................................................................... 86 Evolution of Individuality ..............................................................................................88 Transition to Group Living ............................................................................................ 89 Key Takeaways ............................................................................................................... 91
Quiz ................................................................................................................................ 92 Chapter Seven: Species and Speciation ..................................................... 96 Identifying Species ......................................................................................................... 96 Speciation and Modes of Speciation .............................................................................. 99 Genetics of Speciation ...................................................................................................101 Key Takeaways ............................................................................................................. 102 Quiz .............................................................................................................................. 103 Chapter Eight: Evolution of the Human Species ...................................... 107 Early Man ..................................................................................................................... 107 Human Evolution......................................................................................................... 109 Human Structural Changes ..........................................................................................110 Human Migration ......................................................................................................... 112 Evidence for Human Evolution .................................................................................... 113 Evolution before the Homo Genus ............................................................................... 114 Evolution of the Homo Genus ...................................................................................... 115 Tool Usage ..................................................................................................................... 117 Behavioral Changes....................................................................................................... 117 Modern Human Evolution............................................................................................ 118 Key Takeaways .............................................................................................................. 119 Quiz .............................................................................................................................. 120 Chapter Nine: Extinction ........................................................................ 124 Extinction Basics .......................................................................................................... 124 Background Extinction ................................................................................................ 128 Mass Extinction ........................................................................................................... 129
Key Takeaways ............................................................................................................. 133 Quiz .............................................................................................................................. 134 Chapter Ten: Evolution of Reproduction ................................................. 138 Asexual Reproduction .................................................................................................. 138 Evolution of Sexual Reproduction ............................................................................... 140 Advantages and Disadvantages of Asexual Reproduction .......................................... 143 Mating Systems ............................................................................................................ 144 Sex and Mate Selection ................................................................................................ 145 Key Takeaways ............................................................................................................. 147 Quiz .............................................................................................................................. 148 Chapter Eleven: Evolution of Sociality and Populations ...........................152 Population Evolution ................................................................................................... 152 Hardy-Weinberg Model ................................................................................................155 Cooperation in Populations ......................................................................................... 156 Group Living .................................................................................................................157 Social Evolution ........................................................................................................... 158 Evolution in Finite Populations ................................................................................... 158 Key Takeaways ............................................................................................................. 160 Quiz ............................................................................................................................... 161 Chapter Twelve: Coevolution .................................................................. 165 Coevolution Explained ................................................................................................. 165 Coevolution and Mutualism ........................................................................................ 168 Host-Parasite Coevolution ........................................................................................... 169 Antagonistic Coevolution ............................................................................................. 170
Mosaic Coevolution....................................................................................................... 171 Key Takeaways ..............................................................................................................172 Quiz ...............................................................................................................................173 Chapter Thirteen: Evolution and Disease ................................................. 177 Evolution of Disease Origins ........................................................................................ 177 Disease Susceptibility .................................................................................................. 179 Host and Pathogen Evolution ...................................................................................... 180 Evolution of Senescence ............................................................................................... 181 Key Takeaways ............................................................................................................. 184 Quiz .............................................................................................................................. 185 Chapter Fourteen: Future of Evolution ................................................... 188 Holocene Extinction .................................................................................................... 188 Ways Humans Might Evolve ........................................................................................ 191 Human Extinction ....................................................................................................... 192 Future of the Planet with Global Warming ................................................................. 193 Key Takeaways ............................................................................................................. 196 Quiz .............................................................................................................................. 197 Summary ................................................................................................ 201 Course Questions and Answers ............................................................... 205
PREFACE This course has been designed to teach the interested college-level student the fundamentals of evolution. Evolution is a field of science that is continually changing as more is understood about cell biology, genetics, and the study of the early earth environment. Evolution involves the adaptation of organisms to their environment and the ways in which organisms find their niche or surpass other organisms in the process of natural selection. The course talks about the origins of life and explains what we know about how life has evolved on earth throughout the ages. As you will learn from the course, evolution is not just a historic event but is a process that continues in today s time and will continue to be part of life on earth in the future. No study of evolution would be complete without a discussion of the history of evolutionary theories, which is the topic of chapter one in the course. We will discuss some of the early evolutionists who gave rise to what we currently believe about how evolution works. We then talk in more detail about evolutionary thought throughout time, including modern evolutionary thinking. The story of Charles Darwin is a good one and will help you understand how his major breakthroughs in the understanding of evolution as a naturalist in the Nineteenth Century helped to pave the way for modern evolutionary thought. The focus of chapter two is natural selection. It is a key evolution-related process involving the ability of different organisms in a population to adapt to its environment and to pass on this adaptability to their offspring. As you will see in this chapter, natural selection relates to fitness in a given environment and an organism s reproductive success. Examples of natural selection are given as well as a discussion of how natural selection relates to complex behaviors in higher-order animals—a phenomenon known as evolutionary psychology. Chapter three in the course talks about the evolutionary relationships between the different types of living things. It starts with a discussion of taxonomy, which is the naming convention used to describe all living things. Every form of life falls under one
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of three domains, Bacteria, Archaea, and Eukarya. There are other subdivisions that describe these types of arrangements. Exactly how to describe the relationships between life forms involves a discussion of phylogenetic trees. As you will see, newer findings in biology and microbiology have changed the way these phylogenetic trees are arranged. Chapter four discusses genetics and genetic variation. Genetics works on a small scale in the inheritance of certain traits by a descendant from a direct ancestor. It also works on a large scale because it is through a series of genetic mutations that new species are ultimately created. We will talk about Mendelian genetics, the science of mutations, and the advantages and disadvantages that come with certain genetic situations. Chapter five in the course introduces topics related to the origin of life on earth. Life on earth in the beginning of time was very different than it is now. This is partly due to the fact that the early conditions of earth as a planet were vastly different from that of present-day time. The chapter talks about the evolution of viruses and of prokaryotes, which were the first cells to represent life on this planet. There is more to be said about evolution than the evolution of single-celled organisms so this is the topic of chapter six. Eukaryotes are infinitely more complex than prokaryotes—even those that are unicellular. Many eukaryotic organisms are multicellular; for this reason, the evolution of multicellularity is discussed in this chapter. Because evolution happens to populations rather than to individuals, it is important to also talk about the evolution of individuality. There are advantages to evolving in a social environment, which is also covered in this chapter. The major topics of chapter seven in the course are species and speciation. Earlier chapters talk about evolution and its role in the diversity of species on earth. In this chapter, we talk about how species are defined and the different methods in which speciation or the formation of a different species occurs. Historically, species were defined by their similar characteristics but, in this chapter, we talk also about how the knowledge of genetics has changed the definition of what exactly is meant when referring to an organism being of a certain species in today s scientific terms.
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Chapter eight talks about the evolution of the human species. From an evolutionary perspective, humans have not been around very long. Even so, there have been many changes that have taken place over the course of about 400,000 years. As you will see in the chapter, there have been changes in brain size and gait, among other things, that have been a part of the processes necessary to turn ancient species into modern man. The topic of chapter nine in the course is extinction, which is the final termination of a specific organism type or species. This has occurred to 99 percent of all species that have ever lived on earth. We will talk about background extinction, which happens over a period of time for a variety of reasons, as well as extinction events that have occurred in the earth s history, leading to the mass extinction of many of the species on earth at roughly the same time. Chapter ten discusses issues related to the evolution of reproduction. There are basically two broad categories of reproduction, which are asexual reproduction and sexual reproduction. There are evolutionary advantages and disadvantages of both that will be compared in this chapter. With sexual reproduction, in particular, there are complex variables involved in mate selection, which will also be covered along with the evolutionary issues related to human sexuality and human sexual reproduction. The focuses of chapter eleven in the course are evolution within populations and the evolution of social behaviors. Anytime there is a group of individuals in a population, there will be issues of conflict and cooperation, which are discussed in the chapter. Social behaviors are complex but have genetic and evolutionary influences. Topics also included in the chapter are the Hardy-Weinberg Principle and the evolution involved in finite populations. Chapter twelve focuses on the subject of coevolution. Coevolution is a phenomenon that happens when two or more species affect each other s evolutionary processes. This can happen when two species have a mutualistic relationship or when there is a hostparasite relationship. There are two other types of coevolution discussed in this chapter, including antagonistic coevolution and mosaic coevolution, which involve specialized relationships between two or more species.
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The focus of chapter thirteen in the course is evolution and disease. Diseases affecting all species have been around since the beginning of time. The way in which diseases have originated is discussed in this chapter, including how some human diseases have crossed species to affect humans. Also covered in the chapter is the evolution of senescence or aging. There are several theories as to how and why humans age, which are discussed in the chapter.
Chapter fourteen looks at the future of evolution, particularly of humans and of the planet itself. We will talk about what s already happened with the Holocene extinction, often called the sixth mass extinction event on earth. Exactly how humans will evolve is unknown but scientists can make some speculations, which are discussed in the chapter. Human extinction is covered as a possibility as is the future of the planet with the progress of global warming, which will affect the earth itself and the humans on it.
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CHAPTER ONE: EARLY EVOLUTIONARY THEORIES No study of evolution would be complete without a discussion of the history of evolutionary theories, which is the topic of this chapter. We will discuss some of the early evolutionists who gave rise to what we currently believe about how evolution works. We then talk in more detail about evolutionary thought throughout time, including modern evolutionary thinking. The story of Charles Darwin is a good one and will help you understand how his major breakthroughs in the understanding of evolution as a naturalist in the Nineteenth Century helped to pave the way for modern evolutionary thought.
EARLY EVOLUTIONISTS The study of evolution did not miraculously begin with Charles Darwin. There have been ideas about evolution since the time of Ancient Greece. About 2600 years ago, in an area of the world now known as Turkey, Anaximander studied geology and astronomy. He came to understand that earth was a solitary entity that didn t actually rest on anything. He believed that early life began in a wet rather than a dry environment, that life gradually evolved into a drier environment, and that mankind did not exist on earth in their current form from the beginning. He believed that human ancestors were once fish and based his reasoning on the idea that, because human babies cannot look after themselves, there must have been an ancestor that could do this. Empedocles lived from about 490 BCE to 430 BCE. He was a scientist and Ancient Greek philosopher. He believed that everything was made from a combination of earth, water, fire, and air. He came up with the first ideas about natural selection but did not go as far as saying that natural selection led to different species. He thought that early earth was filled with monstrous creatures that had been selected out over time and had since become extinct so that more adaptable forms of life existed in modern times. The era of the Ancient Greeks set many of the ideas on evolution for many centuries. Carl Linnaeus was a natural scientist in the 1700s. He was the first to develop the scientific forms of we now understand as the genus and species names of living things. Homo sapiens is the scientific designation for humans; every other species on earth has a similar scientific name that defines it. He believed that God started all life on an island with just a few beginning species.
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In 1753, Comte de Buffon studied evolution and believed that all four-legged creatures were developed from a single common ancestor. He believed that four-legged animals were the perfect species above that of other animals. James Hutton was actually a geologist in the late 1700s. He furthered the ideas of natural selection, believing that, if an organism was not suitable or adapted in an environment so that it could not procreate, it was selected out so that forms of life that were more likely to procreate replaced it. Erasmus Darwin was the grandfather of Charles Darwin. He studied reproduction but believed that it was a filament that fathers contributed to the making of an embryo but that the mother did not contribute to any part of the embryo. Instead, the mother was the home to the embryo and provided it with the nutrients and oxygen necessary for growth. He believed that warmblooded animals came from some common ancestor. He believed that all plants and animals had a common ancestor. He felt that competition between males led to the survival of the fittest. Jean-Baptiste Lamarck developed a theory in 1800 that an animal could pass on certain traits to their offspring if they used the trait more often. As an example, if a man worked out and was muscular, his children would be born more muscular. While this was wrong, he did also believe that evolution was both slow and gradual. This is only partially true because it doesn t account for sudden mutations in an organism or its offspring. William Wells stated in 1813 that, when breeding animals, people could select out certain traits and breed animals with those traits, leading to the different domesticated animals. He furthered his theory to people but not to other species. This was basically an idea in support of natural selection. Geoffroy Saint-Hilaire was a researcher in the 1800s. He believed in evolution that could occur in large steps because of influences in the environment. He also believed that new species could arise in this fashion. Finally, he proposed the idea that birds evolved from reptiles, long before this was shown to be true archaeologically. Two contemporaries of Charles Darwin rejected Lamarckism, which was blatantly wrong, and felt that all forms of life could become other species. These were Robert Grant and Charles Lyell. They felt there was a common origin for modern plants and animals but did not understand how species transformation could take place. Edward Blyth was alive in the early 1800s. He understood about artificial selection, which is selection used by breeders of domestic animals. He also knew that there were variations within
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a species. He furthered his ideas by indicating that the variations in a species could be acted on in the natural environment through natural selection but didn t believe that new species could come from this. Instead, he believed that natural selection kept a species in its truest form. Robert Chambers published a book anonymously in 1844 that greatly opposed the religious views of his era. He wrote that the solar system developed from a nebula and that life sprung up on earth spontaneously. He didn t understand how evolution actually happened. The book was believed to have prepared the public for the work of Charles Darwin. Charles Darwin and Alfred Wallace wrote a joint paper in 1858 on evolution through natural selection. In 1959, The Origin of Species was published that showed evidence of natural selection. It left essentially no question in the minds of scientists that natural selection existed. This did not make it instantly popular, however. It was not fully accepted until the 1930s by most scientists.
TIMELINE OF EVOLUTIONARY THEORIES In this section, we will talk more about the timeline of evolutionary thought. There have been ideas related to evolution in the ancient cultures of the Romans, Greeks, Chinese, and Islam. There were two opposing thoughts that were discussed as part of the evolutionary debate. The first was essentialism”, which argued that every species on earth has unalterable characteristics. This was in line with the theology of the time. The second was evolutionary cosmology along with naturalism, which was the idea that the different species characteristics were variable. We ve already talked about early evolutionary thought; however, evolutionary thinking has changed in recent years with the knowledge about genetics and biology contributing to modern synthesis. This looks at genetic diversity in species populations and applies the study of paleontology and comparative anatomy to determine how the different species have evolved over time. Most recently, DNA sequencing has led to an understanding of phylogenetics from a molecular perspective. We will talk about the current three-domain system of all life, that was developed by Carl Woese, later in this course. For now, you should know that this is a relatively recent phenomenon in evolutionary thinking.
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EVOLUTIONARY THOUGHT IN ANCIENT TIMES Anaximander of Miletus lived from about 610 BCE to 546 BCE. He was a pre-Socratic philosopher. As mentioned, he believed that life originated in water, with creatures eventually spending more time on land than in the water over time. He believed that man must have been first the child of another type of animal, probably some type of fish, because modern man could not have survived early life on earth if he needed to be cared for as long as humans do before maturity. Philosophers at the time subscribed to essentialism, which was the belief in the unchanging essences of every living thing. In evolutionary thought, it involved the idea that species characteristics did not change over time but were fixed entities. Empedocles said that the birth and death experience simple mingled and separated the different elements that make up an organism. He believed that current animals and plants were derived from pieces and parts of older organisms that were mingled inside the embryo. Empedocles relative contemporaries did not believe this. In fact, Plato and Aristotle said that all things were fixed through divine design. Plato believed in essentialism and some type of creator of all things. He did not believe that species could transform. Aristotle believed in a great ladder of life” or chain of being” that was static over time. He believed that the final form of an organism perfectly served their function. He rejected Empedocles work. Zeno, about a century later, founded the Stoic school of philosophy and agreed with Aristotle. He believed in teleology, in which all features of nature showed evidence of having a purposeful design. The Roman Empire followed most of the Ancient Greek philosophers when it comes to timeline. Lucretius was one of the Romans who wrote on the development of the earth, humans, and other living things. His approach was purely naturalistic and did not reference any type of supernatural involvement. This work led to the beliefs that came during the Renaissance. Other theorists of the Roman era belonged to the Stoic school of thought. This included Seneca the Younger, Cicero, and Pliny the Elder. These philosophers influenced Christian thoughts on evolution because their thoughts were largely teleological and based on the theological origin of the different species. Origen of Alexandria was a third-century church father and Christian philosopher. While he supported the creationist beliefs in the Book of Genesis, he said it was all allegorical and should
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not be interpreted literally. He did not believe that people should truly expect that the universe was entirely made in seven days. After Origen of Alexandria, there was Augustine of Hippo, who also believed that the creationist story should not be accepted as literal fact. He felt that some creatures were derived from the decomposition of early life forms. It was stated in his book On the Literal Meaning of Genesis that living things were not perfect but had the potential to be so. He also believed that life gradually transformed over time. In later times, the Roman Empire fell and Islamic philosophers predominated in the 8th through 13th centuries. Al Jahiz said that stronger animals devoured weaker animals in a struggle for existence and that God disposed some creatures in favor of others. He also wrote about the food chain in living things.
CHRISTIAN PHILOSOPHIES ON EVOLUTION The time of the Middle Ages was when much of Ancient Greek teachings were lost to Westerners. These had, however, been preserved in the Islamic cultures and were translated into Latin by the twelfth century. There were Christian thinkers, such as Peter Abelard and Thomas Aquinas, who looked at Aristotle s and Plato s work about the goodness of God, believing in the perfection of all things and in the great chain of being, which looked at the organization of all living things, inanimate objects, and spiritual beings. According to the great chain of being, there was an order of things, ranging from lowest to highest. Hell existed at the bottom of the chain and God was listed at the top. Mankind was in the middle, while worms were thought to be the lowest form of animal life. No species could transform into another or change places in the linkages that made up the great chain of being. The Book of Genesis was revered and it was believed that the hierarchy involved in the great chain of being was unchangeable. Thomas Aquinas was a Christian theologian who also believed that the Book of Genesis shouldn t be interpreted literally because it conflicted with the way that natural philosophers had learned already about how nature works. He said that God basically set up nature to run on its own natural processes, believing there was no conflict between theology and the development of the universe through natural mechanisms. He did not believe Empedocles, who said that there was no purpose in how things have evolved. He believed that nature was a form of divine art.
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In the early 1600s, Rene Descartes developed mechanical philosophy, in which the universe was a sort of natural machine. There were others that followed him and indicated that all of the universe had developed without divine intervention. This was different from philosophers like Gottfried Leibniz and Johann Herder, who believed that evolution was a divine process. Pierre Maupertuis had a materialistic view of nature, indicating that reproduction created natural modifications that accrued over many generations to produce new species and new races of man. He was in support of natural selection and opposed taxonomists who felt that the qualities of species were unchanging. This idea of unchanging species was characteristic of natural theological thinking. In the late 1700s, theorists like James Burnett said that man probably developed from primates and that species evolved over time due to their overall responses to the environment. As mentioned, Erasmus Darwin was a part of this but said that a single living filament gave rise to all warm-blooded animals.
NINETEENTH CENTURY EVOLUTION The ideas and findings of paleontologists entered into evolutionary thinking by the early 19th century. George Cuvier looked at the structural differences between elephants of today, mastodons, and mammoths, which are extinct. He said they were distinct species and was the first to describe the probability that a species could become extinct. Fossils were increasingly looked at in rock layers, which helped to establish how old the earth was. Cuvier said that catastrophism in nature explained some extinction patterns. He also looked at fossil records to see how plant life has evolved. John Phillips, in 1841, identified three major eras in geology. These were the Paleozoic era, which involved the predominance of marine invertebrates, the Mesozoic era, which was predominated by reptiles, and the Cenozoic era, which was dominated by mammals. There was also the work of Adam Sedgwick and William Buckland, who also believed that catastrophic events led to mass extinction and the arrival of new species. Unlike more progressive theorists, researchers like William Buckland believed that the biblical flood was the last major catastrophe in terms of evolution and extinction. Charles Lyell, on the other hand, said that there were more gradual changes in geology that contributed to evolution rather than cataclysmic events.
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Jean-Baptiste Lamarck studied the transmutation of species. He did not think there was a common ancestor but that there were simple forms of life that were still being spontaneously generated. He felt that there was some type of life force that caused species to be more complex with time. This was related to the great chain of being. Remember too that he thought that changes in a species adaptation to the environment during its lifetime were later inherited by the offspring. The concept of transmutation of species was a more primitive explanation for evolution. It described the ways in which certain species transformed into other species. Certain idealists like Louis Agassiz and Richard Own believed that species of plants and animals had fixed characteristics that were both unchangeable and developed by the creator. They used embryological patterns of development could explain the relationships between the species. Each of these ideas made Charles Darwin more convinced that he needed proof and sound science to back his theories. As you have learned too, there were many ideas that predated Charles Darwin s theories on natural selection. Darwin himself looked at selective breeding and the ideas put forth by others indicating that harsh environmental conditions led to weeding out of the weakest individuals in a population. When Darwin wrote on natural selection, he named those early theorists that had anticipated natural selection before it was somewhat more proven. Also prior to Darwin was Patrick Matthew, who indicated that, if there was enough of an evolutionary change leading to a new species, the new species might not be able to reproduce with species it had evolved from. This is how he defined a new species. We will talk about Charles Darwin in a minute but suffice it to say, he did not believe in the fixity of species, writing secretly on the transmutation of species prior to his actual finished work on natural selection. By the time of Darwin in the 1850s, it was a hot topic of debate as to whether or not species evolved. Darwin s work changed all that by indicating that evolution existed by new species diverging from existing species. Thomas Henry Huxley was an anatomist who was convinced by Darwin s work on the origin of species. He displaced the ideas of natural theology with naturalism, which did not involve theological ideas. Huxley used paleontology to help explain aspects of evolution, which included the idea that birds evolved from reptiles. This was proven by the discovery of Archaeopteryx in Europe as well as North American ancient birds that had teeth. The evolution of horses was also uncovered through paleontological records.
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AFTER DARWIN AND NATURAL SELECTION Darwin largely ignored issues related to human evolution in his work because of the controversies associated with the idea that humans evolved from lower animals. It was first believed that humans existed on earth for just a few thousand years. Then, there were archaeological digs that stretched for many thousands of years, indicating that human ancestors existed for thousands of years before that. Java man was discovered in the 1890s, which served as a bridge between Neanderthals and modern man. Thomas Henry Huxley wrote about the similarities between gorillas and humans, including similarities in brain structure. He wrote a book on the subject in 1863. Lyell and Wallace felt that there were similarities between apes and humans but felt that there was a common ancestor for each of the species of primates, including humans. Darwin later wrote that the differences between primate thinking and human thinking were a matter of degree rather than substance. There were four major alternatives to the idea of natural selection that were espoused in the latter part of the 19th century. There was theistic evolution, orthogenesis, neo-Lamarckism, and saltationism. These are explained as follows: •
Theistic evolution—this basically tries to align science with modern religious beliefs and isn’t really a scientific theory at all.
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Neo-Lamarckism—this is the idea that use or disuse of a body structure allows it to be passed on or not passed on to the offspring.
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Orthogenesis—this involves the idea that there is a driving force that allows for evolution to go in a specific direction.
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Saltationism—this is the idea that a single mutation has the potential to change offspring into an entirely new species all in one step.
We will discuss Mendelian genetics in a few chapters. He discovered the laws of inheritance that became more popular around 1900. At the time, there were the Mendelians, who looked at variations in a species as they apply to the laws of inheritance. There were also the biometricians, who were more interested in the variation of specific characteristics within populations of organisms. These two camps opposed each other. The two camps came together with the study of population genetics, which argued that larger changes in a population could come about by the natural selection and change in the frequencies
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of a large number of genes in a population. This was believed to be how new species could be developed. We will talk about the evolution of moths in populations where being a certain color offered better protection against predators. This indicated that natural selection could actually happen quite quickly. Modern synthesis of evolutionary thought came about in the twentieth century. This combined natural selection, genetic variation, and Mendelian inheritance into modern evolutionary thinking. There was a shift away from pure natural selection to ideas related to genetic drift within a population. A species came to be defined as a population of organisms that could breed together and that were reproductively different and isolated from others in the population. In the middle of the twentieth century, there was a rise in molecular biology and the discovery of DNA and genetic codes. Biochemists Emile Zuckerkandl and Linus Pauling developed the molecular clock theory, which was that sequence differences between related proteins in different species could help define how far apart they were in terms of species divergence. By the late twentieth century, there came to be a more gene-centered view of evolution. It was believed that sexual reproduction helped to create better resistance among the offspring to parasites and other pathogenic organisms. Sexual reproduction creates genetic diversity, which is felt to be better for the species overall. Whether or not sexual reproduction is truly better for a species or not is still the subject of debate. Most of evolutionary thought was developed without the background of microbiology. Because of advances in the study of the genes in microbes, called microbial genomics, small microorganisms can be studied as to the differences in the species. Horizontal gene transfer was identified in 1959, which is the way that pieces of genetic material can be transferred between different bacterial species, which in many cases, leads to antibiotic resistance. This phenomenon is believed to play a role in creating new species of microorganisms.
THE STORY OF DARWIN As you have seen, Charles Darwin did not develop his theories on evolution and the origin of species in a vacuum. His weren t the first words on evolution in the world and they haven t been the last words on the subject. Even so, he did a remarkable job of studying certain populations and wrote a landmark book On the Origin of Species. Darwin gained a reputation as an important fossil collector and geologist. He studied naturalism rather than become a clergyman. He became convinced in the idea of the
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transmutation of species after learning about bird specimens gathered from the Galapagos Islands. He also studied orangutans in the zoo as to their facial expressions; he studied animal breeding and how that changed the fitness of subsequent litters of domesticated animals. He initially studied under Robert Edmund Grant and learned of Lamarck s ideas and the work of his grandfather Erasmus Darwin, showing a common ancestor to modern animals and plants. He collected beetles and took his voyage to other parts of the world on a ship called the Beagle. He did not develop his evolutionary ideas because of his work on the Beagle, however. While on the Beagle, he collected numerous fossils and zoological specimens. Afterward, he looked around for zoologists to help him catalog his specimens. It was actually ornithologist John Gould who took on the bird specimens Darwin had collected; it was Gould who recognized that the birds collected were actually about 12 different species of finches. Darwin was extremely concerned that his work met the accepted scientific methodology of the day. He did not want to simply present theories; he wanted facts to back them. There were many who believed in theistic evolution at the time and, while he was a religious man, he wanted to be very scientific about his findings. He came to understand that, while individual organisms did not change over time, their offspring could change because of the reproductive process. Those offspring that adapted better could bring forth offspring that also fared better when it came to adaptation. He felt that geographic isolation led to reproductive isolation of species that lived there. This would lead to divergence of species and to what he called geographic speciation. It was exactly these circumstances that existed on the Galapagos Islands. Darwin actually wrote about a number of things besides the origin of species. He wrote about barnacles, geological topics including earthquakes, and orangutans in the zoo. He was ill throughout most of his adult life but still kept a busy working life, ultimately writing his seminal work on the origin of species more than 20 years after his voyage on the Beagle.
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KEY TAKEAWAYS •
There have been ideas in support of evolutionary theory since the ancient times, first proposed by pre-Socratic scholars.
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Almost all of the early theories on evolution were made without the knowledge we currently have about microbiology and genetics.
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Natural selection involves the adaptation of an organism to its environment and the ability to pass on these adaptations to one’s offspring.
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Charles Darwin was not the first nor the last to talk about the origin of species and speciation but he spent a great deal of time studying the phenomenon in the wilds of the Galapagos Islands.
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