Lesson 2 Chapter 25
Lesson 2 Chapter 25 05260
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Evolution by Natural Selection KEYCONCEPTS Populations and species evolve, meaning that their heritable characteristics change through time. Evolution is change in allele frequencies over time. Evolution by natural selection occurs when individuals with certain alleles produce the most surviving offspring in a population. An adaption is a genetically based trait that increases an individual’s ability to produce offspring in a particular environment. Evolution by natural selection is not progressive, and it does not change the characteristics of the individuals that are selected-it changes only the characteristics of the population. Animals do not do things for the good of the species, and not all traits are adaptive. All adaptations are constrained by tradeoffs and genetic and historical factors. 25.1 The Evolution of Evolutionary Thought Plato, Aristotle, and the Bible’s book of Genesis consider the species as unchanging types. This view is called typological thinking. Lamarck proposed a theory of evolution-that species are not static but change through time. He proposed that evolution occurs by the inheritance of acquired characteristics. Darwin and Wallace proposed that evolution occurs by natural selection. This was the beginning of population thinking, whereby variation among individuals is the key to understanding evolution You should be able to compare and contrast typological thinking and population thinking 25.2 The Pattern of Evolution: Have Species Changed, and Are They Related? Data on (1) the age of the Earth and the fact of extinction; (2) the resemblance of modern to fossil forms;(3) transitional features in fossils;(4) the presence of vestigial traits; and (5) change in contemporary populations show that species change through time. Data on (1) the geographic proximity of closely related species; (2) the existence of structural, developmental and genetic homologies; and (3) the contemporary formation of new species support the consensus that species are related by common ancestry. Evidence for evolution is internally consistent, meaning that data from several independent sources are mutually reinforcing. You should be able to cite examples in support of the statement that species have changed through time and are related by common ancestry. 25.3 The Process of Evolution: How Does Natural Selection Work? Darwin developed 4 postulates that outline the process of evolution by natural selection. These postulates can be summarized by the following statement: Heritable variation leads to differential reproductive success. Alleles or traits that increase the reproductive success of an individual are said to increase the individual’s fitness. A trait that leads to higher fitness, relative to individuals without the trait, is an adaption. If a particular allele increases fitness and leads to adaption, the allele will increase in frequency in the population. You should be able to explain the difference between the biological and everyday definitions of fitness. 25.4 Evolution In Action: Recent Research on Natural Selection Selection by drugs on the TB bacterium and changes in size and shape of finch beaks in the Galapagos as a result of the seed availability are well-studied examples of evolution by natural selection/ Both examples demonstrate that evolution can be observed and measured. Evolution by natural selection has been confirmed by a wide variety of studies and has long been considered to be the central organizing principle of biology. You should be able to predict how changes in Mycobacterium tuberculosis populations would be explained under special creation and under evolution by inheritance of acquired characters. 25.5 Common Misconceptions about Natural Selection and Adaptation Natural selection acts on individuals, but evolutionary change occurs in populations. Nonheritable changes that occur in individuals are not adaptations and do not result in evolution. Evolution is not goal directed and does not lead to perfection. There is no such thing as a higher or lower organism. Organisms do not act for the good of the species. Not all traits are adaptive, and even adaptive traits are limited by genetic and historical constraints. You should be able to discuss how adaptations such as the large brains of the homo sapiens and the ability of falcons to fly very fast are constrained. LEARNING OBJECTIVES Define evolution, fitness, and adaptation using the biological definitions. Describe the nature of the evidence regarding (1) whether species change through time and (2) whether they are related by common ancestry. Assess whether Darwin’s four postulates are true in any given example, explain to a friend why evolution must occur if all four are true, and explain whether evolution will occur if any of the four are not true. Identify common misconceptions about evolution, and give examples to illustrate why they are not true. (For example: Is evolution progressive? Do animals do things “for the good of the species”? Does evolution result in perfection?) 25.1 The Evolution of Evolutionary Thought The idea of evolution by natural selection overturned the idea that species were created supernaturally. Plato believed that species were created by god and were unchanging, which is known today as typological thinking. Typological thinking is based on the idea that species are unchanging types and that variations within species are unimportant or even misleading. This is almost always linked to a belief in a god that created all organisms. Aristotle proposed that species were organized into a sequence based on increased size and complexity, with humans at the top. He too believed that characteristics of a species were unchanging through time and in essence, “fixed”. He organized organisms into the Great Chain of being. In the 1700s Aristotle’s ideas were still popular in scientific and religious circles. The central claims were that (1) species are fixed types, and (2) some species are higher-in the sense of being more complex or “better”-than others. Lamarck proposed the formal theory of evolution where species are not static, but change through time. The pattern component was based on the great chain of being with species originating by spontaneous generation but later changing. Thus, Lamarckian evolution is progressive in the sense of always producing larger and more complex, or “better” species. Basically, if you cut your leg off, your children will be born without a leg. Darwin claimed that instead of being unimportant or an illusion, variation among individuals in a population was the key to understanding the nature of species. Biologists refer to this view as population thinking. The theory of evolution was revolutionary for several reasons: 1. It overturned the idea that species are static and unchanging. 2. It replaced typological thinking with population thinking. 3. It was scientific and could be tested through observation and experimentation. 25.2 The Pattern of Evolution: Have Species Changed, and Are They Related? In essence the pattern component of the theory of evolution by natural selection makes two predictions about the nature of species: 1. Species change through time. 2. Species are related by common ancestry. Evidence supported Darwin’s claims as the geological record indicated that the Earth was much, much older than the 6,000 years claimed by proponents of special creation. According to the data on radiometric dating, the Earth is about 4.6 billion years old and the earliest signs of life appear in rocks that formed 3.4-3.8 billion years ago. When they found fossils of the Irish elk, those who believed in special creation claimed it was animal that died in the flood and that’s why it became extinct. Darwin saw this evidence that species were going extinct as evidence that the species on Earth have changed through time. Recent analyses of the fossil record suggest that over 99% of all species that have ever lived are now extinct. Transitional features are traits found in fossil species that are intermediate between those of older ancestors and younger species today. For example, fin and limb bones in lungfish and tetrapods. How would observations of transitional features be explained under special creation? It’s not completely clear but likely would blame it on Noah’s flood. Theory evolution states that if the traits observed in more recent species evolved from traits in more ancient species, then transitional forms are expected to occur in the appropriate time sequence. Vestigial traits are evidence of change through time. A vestigial trait is a reduced or incompletely developed structure that has no function or reduced function, but is clearly similar to functioning organs or structures in closely related species. Some examples are whales with leg and hip bones, eyeless fish with eye sockets and ostriches with wings that do not allow them to fly. How would observations of vestigial traits be explained if evolution occurred via inheritance of acquired characters? If this were true, then eyeless fish would have eye sockets because a fish from an earlier form lost its eyes in some sort of battle, and then had children without eyes. Similarly, a monkey may have lost its tail in fight and its children would have reduced tail functioning until it no longer appeared. Overall, vestigial traits are inconsistent with the idea of special creation of perfect organisms and are evidence for change through time. The take home message is that species are dynamic-not static, unchanging and fixed types, as claimed by Plato, Aristotle, and advocates of special creation. Darwin observed finches and came to the conclusion that they were all similar yet different despite being in the same area because they had a common ancestor that dispersed into nearby islands and evolved in those habitats. When analyzing DNA, we see that those birds have almost identical amino acid sequences. Researches have used this information to place the birds onto a phylogenetic tree that explains their relationship to each other. Homology is a similarity between two or more species because they inherited a trait from a common ancestor. It can be studied at three different levels. 1. Genetic Homology: Similar or almost identical amino acid sequence from their DNA. 2. Developmental Homology: Similar forms in the womb like gill pouches in humans, chicks and cats. 3. Structural Homology: A similarity in morphology, or form. Limbs in vertebrates. The three levels of homology interact. Genetic homologies cause the developmental homologies in embryos, which then lead to structural homologies recognized in adults. The theory of evolution by natural selection predicts that homologies will occur. If species were created independently of one another, as special creation claims, these types of similarities would not occur. Perhaps the most powerful evidence for any scientific theory, including evolution by natural section, is what scientist call internal consistency. This is observation that data from independent sources agree in supporting predictions made by a theory. In the evolution of whales and dolphins it’s that the phylogeny is the same across all sources. Many independent lines of evidence converge on the same conclusion: Whales gradually evolved from a terrestrial ancestor about 50 million years ago. Data from many different sources are much more consistent with evolution than with special creation. Descent with modification is a successful and powerful scientific theory because it explains observations-such as vestigial traits and the close relationships among species on neighboring islands-that special creation does not. Determine what kind of evidence would support the hypothesis that birds evolved from dinosaurs. If birds evolved from dinosaurs, you would expect to find transitional fossil dinosaurs with feathers. Explain why the DNA of chimpanzees and humans are about 96% similar. The DNA sequences are so similar because we share a recent common ancestor. 25.3 The Process of Evolution: How Does Natural Selection Work? Darwin broke the process of evolution by natural selection into four postulates that form a sequence: 1. The individual organisms that make up a population must vary in the traits they possess. 2. Some of those trait differences must be heritable. 3. In each generation, more offspring are produced than can possibly survive. Some will reproduce more than others as only some will survive long enough to reproduce at all. 4. Those with certain heritable traits have an advantage in surviving and reproducing. Natural selection occurs when those with certain traits produce more offspring than others, they are selected naturally-meaning, by the environment. Today biologists condense these postulates into a two-part statement: Evolution by natural selection occurs when (1) heritable variation leads to (2) differential reproductive success. Fitness and adaptation are two key words to know when describing reproductive success. A. Fitness: the ability of an individual to produce surviving, fertile offspring relative to that ability in other individuals in the population. B. Adaptation: a heritable trait that increases the fitness of an individual in a particular environment relative to individuals lacking the trait. Adaptation increase fitness. 25.4 Evolution in Action: Recent Research on Natural Selection The theory of evolution by natural selection is testable. This means we can test the validity of each of Darwin’s postulates. Let’s look at mycobacterium tuberculosis. In the early 1990’s it receded in importance because: 1. Advances in nutrition made people better able to fight off infections. 2. Antibiotics made it so that physicians could stop even advanced infections. This all ended when they discovered an antibiotic resistant strain of the bacteria. A man ended up dying because: 1. By chance one or few of the bacterial cells had the rpob allele with a mutation that allowed the mutants to occur in low frequencies even as the overall normal bacteria grew. 2. Antibiotics were given to the man and the normal bacteria began to die off drastically so that the patient appeared to be completely cured. 3. The alleles with the mutation continued to increase after the treatment ended and the levels of overall bacteria came back to what it was prior, making the symptoms reappear. 4. Drug-resistant bacteria now dominated the population so another round did nothing. You should be able to explain why the relapse in step 3 occurred and whether a family member or worker who got TB from this patient later on would respond to drug therapy. Relapse occurred because the few resistant bacteria were not eliminated by the antibiotics and with a new empty environment, were able to grow freely and rapidly. Anybody who came into contact with them would not respond to therapy as the cells entering were already mostly resistant. Testing Darwin’s Postulates in Case Study #1: 1. Did variation exist in the population? Yes both resistant and nonresistant strains were found. 2. Was this variation heritable? Yes, the difference in resistance was due to differences in genotypes. 3. Was there variation in reproductive success? Yes, only a few actually survived to reproduce after the treatment. 4. Did selection occur? Yes, when the antibiotic is present the drug resistant alleles had higher reproductive success. Natural selection acts on individuals, because individuals experience differential reproductive success. But only populations evolve. Allele frequencies change in populations, not in individuals. If you understand that, then you should be able to explain why the over prescription of antibiotics by doctors can be a health risk. The over prescription leads to the killing of all bacteria in us, even the harmless and beneficial kind. When we use the treatments, the harmful bacteria will be able to grow more and multiply, reducing the ability of a treatment being effective in the future. The second case study involves the change in average beak size in finches after a large drought. They found that the average size shifted towards deeper beaks after they measured those who had survived. This was a natural experiment as they gathered information but did not control the conditions themselves. In only 1 generation, natural selection led to a measurable change in the characteristics of the population. Alleles that led to the development of deep beaks had increased in frequency in the population. You should be able to list Darwin’s 4 postulates in your own words and indicate which are related to heritable variation and which are related to reproductive success Answers will vary but do this in a way that makes it easy to remember! The first 2 postulates refer to heritable variation and the second 2 refer to differential reproductive success. Explain how data on beak size and shape of Galapagos finch populations provide examples of heritable variation and differential reproductive success. Beak size and shape are determined by an individual’s genes, making them heritable. When the drought occurred, those with deeper beaks survived and were able to mate, making them have higher reproductive success than the birds that had died. 25.5 Common Misconceptions about Natural Selection and Adaptation Individuals don’t change across time, populations do overall. While they each are born with different traits, those don’t change throughout their lifetime. The overall frequency of those traits is what changes as time goes by. Unlike what Lamarck stated in his theory of evolution by inheritance of acquired characters, phenotypic changes due to events in their lifetime are NOT passed on to offspring because no alleles have changed. Explain the difference between the biological and English definition of adaptation. Mutations don’t occur to solve problems, they just happen. Adaptations do not occur because organisms need or want them. Organisms don’t get “better” or more complex over time, in fact complex traits are often lost over time due to evolution by natural selection. Going back to “better” species, no such things exist. A human is no higher than a cow, or pig. Each is well adapted to its environment. Think about that next time you go out to lunch. No species is entirely self-sacrificing. Individuals that are selfish and cheaters almost always survive and produce offspring more than others than look like they’re doing things for the good of the species. Genetic correlation occurs because of pleiotropy, in which a single allele affects multiple traits. A fitness tradeoff is a compromise between traits. While one may help them survive initially, it may pose problems later on like the deep beaks in finches during the drought. Because selection acts on many traits at once, every adaption is a compromise.
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