Fluent Essays

Natural Selection Study Guide.  For populations to evolve by natural selection, there must be what three things?  What happens if a population is being selected against, but there is no variation? What happens if a trait is favored by natural selection, but that trait is not the result of genes? If a population is under selection for resistance to a fatal virus infection, does that mean that mutations for resistance will happen?  In animals (like Oldfield mice), an increase in the amounts of 𝛼α-MSH leads to... In animals (like Oldfield mice), a decrease in the amounts of 𝛼α-MSH leads to... In animals (like Oldfield mice), an increase in the amounts of ASP leads to... In animals (like Oldfield mice), a decrease in the amounts of ASP leads to... Why are inland populations of Oldfield mice more likely to have a darker coat color? Why are beach populations more likely to have a light coat color? Oldfield mice are found over a very wide geographic area, and over most of this area they have a dark coat color. Is that because dark color is more fit than light color in general?  ------------------------------------------------------------------ From the Evo-Ed exercise:  a. Does E. coli normally take in citrate under aerobic conditions?  b. What did one line of E. coli 'suddenly' do, and how was it first detected?  c. What was the exact genetic basis for this new mutation? Why did it give these E. coli a growth advantage? Don't just say it made them more fit. I want to know  why .  d. From the Evo Ed exercise for Oldfield mice, what happens to the frequency of light and dark mice between inland and beach populations when there is no predator? What happens to the respective populations when a predator is released?  How would predation effect the distribution of mice living on the beach if there was no mutation causing light mice? --------------------------------------------------------------------- From the Animation on Natural Selection in Guppies This is one of many experimental studies on natural selection in wild populations of guppies.  The guppies are in different streams and different areas of streams. What are the differences in selection pressure on the different guppy populations? What are the corresponding adaptations of the different guppy populations? In the study shown here, guppies from a high predation area were transferred to a low predation area.  What was observed in the years following this transfer?   -------------------------------------------------------------------- There are several constraints against natural selection. Among these...   -- why can't natural selection work well in small populations?    -- If a trait (a physical trait or an entirely genetic trait) is neutral to selection, then can natural selection act on trait?  Although natural selection changes the relative frequency of alleles, it cannot 'see' genotypes. What is 'seen' by natural selection? There are numerous harmful mutations in most any population. Most likely, these are recessive. So why does natural selection have an especially hard time eliminating harmful recessive mutations?  What is the 'norm of reaction'?  Why might we see identical twins grow to different heights? Can natural selection pull some strings and cause desirable mutations to happen?  Describe why a given genotype produces not one but a range of phenotypes. What causes variation in phenotypes for a single genotype? Suppose a population of bacteria come under exposure to an antibiotic. Resistance to this antibiotic could occur if, say, a single point mutation were to happen. Does this need for the mutation cause the mutation to happen? What if the needed mutation does not happen? What would happen then?  When we say "mutations are random", we technically mean that....    -- Natural selection can only react to selective agents. It cannot plan ahead for the future needs. Understand why this means that adaptations, however clever and extreme, are always based on what already exists. They are "contrivances". Cobbled together bits and pieces of spare parts, pressed into new use. This directly recalls the subject of exaptation from an earlier lecture.  Here are two common descriptions that are wrong. Describe what is technically wrong with them.  a. "In the past, the insecticide called DDT caused mosquitos to evolve so they were resistant to DDT."  b. "Eukaryotes especially have large amounts of DNA that has no known function. It must be that this DNA is present because some of it may one day become important in natural selection". [I kid you not. I've seen scientific papers make this claim. It is  impossible , but why? ] Explain why vertebrates are so far unable to evolve forms with more than 4 paired limbs. Describe in terms of interacting genes and periods of embryonic development. Be able to recognize other developmental constraints when I point them out.  Vertebrates can evolve to enormous size, but their limb bones must get dis-proportionally thicker to do so. Why is that? Be thorough, and consider changes in volume versus cross-section surface area.  Mass increases by a ...................... function, but cross-sectional surface increases by a .................. function. Suppose a city planner is working with engineers to build a building. The engineers design the building to be supported by pillars with a cross section area that is sufficient to hold the weight of the building. Then the city planner comes back and says 'I need the building to be twice as big. So you should just make everything proportionally twice as big.' What would the engineer say?  5 Evidence for Evolution Study Guide 5 Evidence for Evolution Study Guide The Modern Synthesis used to be called Neo-...........................  The Modern Synthesis of evolution emphasizes the inclusion of what 'new' branch of biology that developed in the early 20th century? You had a definition of evolution by natural selection. What is a definition of just evolution (this includes natural selection, + other processes) according to the Modern Synthesis?  What sorts of species are found on islands formed from continents that are not found on islands formed by volcanoes? Why are these species 'missing' on volcanic islands? Given a species of animal or plant. Where would we most likely find the closest relative of that species?   Squid and octopuses have well developed eyes, and likely evolved from a common cephalopod ancestor that also had eyes. So the eyes of squid and octopuses are ................................. structures.  Any two homologous structures share traits by common ancestry. These 'old' traits are said to be ................................... Any two homologous structures will also have specialized traits that evolved after species separate from each other. These 'specialized' traits are not 'primitive', but are rather called ................................... Moles have powerful forelimbs that are dedicated for digging. It is hard to imagine a better limb for that purpose. The limbs evolved from less specialized forelimbs of mammals. How is mole forelimb evolution an example of an exaptation (to use a term from a previous lecture)? (Strong hint: the walking limbs of the mole ancestor could dig a little).  Structures may evolve a superficial similarity because of similar selection pressures. This is called .................................. evolution.  Be sure to recognize examples of divergent evolution and convergent evolution. I know of lots of examples.    A dragonfly wing versus a bird wing: Homologous or analogous? Convergent or divergent evolution?    The front leg of a cat versus a wing of a bird: Homologous or analogous? Convergent or divergent evolution?     The cat leg and the bird wing has an upper bone called the humerus, with a proximal ball joint and a distal hinge joint. Are these shared features  primitive or derived?    Bird feathers likely evolved from dinosaur feathers, but early dinosaurs probably lacked feathers. So feathers are a trait that are .................................. in the dinsoaur lineage leading to birds.   For the following, fill in the blanks. The words to put in the blanks are listed.    Arachnids include spiders, scorpions, mites, and other critters. In many arachnids the first pair of appendages are  formed into pincers. In spiders, however, these appendages are sensory organs. Because these appendages correspond to appendages in a common arachnid ancestor, we say they are ................................... to each other. They became more and more different over time (becoming pincers in some, sensory appendages in others), and we called this ............................... evolution. Actually different groups of arachnids independently evolved the first limb into pincers. Evolution to a similar morphology due to similar selection pressures is ................................ evolution. All arachnids have simple eyes with a fixed lens that cannot look from side to side. But a group of spiders called jumping spiders can tilt their retina from side to side, so they can look up and down and sideways even though their lens cannot move!  The fixed lens is therefore a ......................... feature in spiders, but the more recent ability to move the retina in jumping spider eyes is a ....................... feature.  [The terms for these blanks are: convergent, divergent, derived, primitive, and homologous. Put the right terms in their blanks].  Organs that fall into disuse, such as eyes in cave animals, tend to evolve to small size and so are 'vestigial'. Why does natural selection "vestigialize" structures that are not used anymore?  Within a phylum, embryos pass through a stage where they look very much alike. All vertebrate embryos look similar to each other (during what is called the 'pharyngula' stage). Embryos of different species then start to look more and more different from each other (and more like what they will become) after this stage. Why do the earlier embryos look alike? Refer to genes and 'constraint'.  Tell me some structures that vertebrate embryos develop but then  'discard' later in development. These are structures that are also thought to develop in ancestors. We are accustomed to seeing primitive structures being modified with derived features, to be put to a new use. But several primitive structures that develop in embryos are not modified but are actually discarded and replaced with more derived structures. That is, where some organs die and are replaced by new organs! Why do embryos develop ancestral structures if they are only going to discard them later? Your answer should include statements about genes, but also about what cells need to do in embryos.  Give examples of vertebrates that are amniotes. Give examples of vertebrates that are not amniotes.  If you were able to dissect an 'Argentinosaurus', which was a huge dinosaur with a neck about 40 feet long, what would you likely find that is extraordinarily stupid about a branch of the vagus nerve?  Tell me what a transitional form represents.  Tetrapods (4-legged vertebrates) have a distinctive arrangement of limb bones. What is seen in the likely fish ancestors of tetrapods?  Tiktaalik is a transitional form between what two major groups?  Ardipithecus is a transitional form between what two major groups?  --------------------------------------------------------------- From the reading assignments on evolution of the mammalian ear:  A mammal has 3 middle ear bones that transmit sound vibrations. (These are the malleus, incus, and stapes.) A reptile has which of these middle ear bones?  A mammals' lower jaw is made from a single bone that articulates to the skull. How does this compare to the reptilian lower jaw?  Based on a series of fossils of the reptile ancestors of mammals, describe how the mammalian ear bones evolved from bones in their reptile ancestors. You do not need to recall the ancestral names of these bones.  ----------------------------------------------------------------- What is seen when comparing the chromosomes between related species? What is seeing when comparing chromosomes of less closely related species? Refer to the arrangement of genes in your answer.  What is seen when comparing genes between related species? What is seeing when comparing genes of less closely related species? Refer to base sequences in your answer.  Genes that are similar in sequence and similar in function between species would be shared by a common ancestor. These genes are                               to each other.  What is a pseudogene? When we compare pseudogenes between related species, what do we often find?   When we compare jumping gene insertions between related species, what do we often find?  B3 Natural Selection A result of natural selection: prey animals tend to have a color that helps them blend into their background. 1 1 The Components of Natural Selection Natural selection requires 3 conditions: a. Variation. b. Inheritance. c. Differences in reproduction. 2 2. The result of natural selection is adaptation. 2 3 In oldfield mice, (Peromyscus polionotus), different pigments create a coat color… Natural Selection & Coat Color in the Oldfield Mouse. 4 2. There is Variation: Some mice are dark. Others are light in color. 3. Coat color is controlled by inheritable gene products. a. aMSH  Eumelanin (dark pigment). 5 b. ASP Phaeomelanin (yellow pigment). 5 3. …Genetic differences cause the color variations... 6 6 7 4. There are potentially differences in reproduction: The mice are readily eaten by predators. 7 8 8 9 5. Natural selection for coat color is demonstrated in these mice: 9 10 10 11 A mere 1% advantage in fitness  frequency doubles every 70 generations. 11 Web assignments: Click on the link in Moodle: EvoEd: Cases for Evolution. b. Use the handout (available in moodle) to guide you through the assignments on natural selection in E. coli and in Oldfield mice (2 assignments here). 12 ________________________________________ c. A 3rd assignment: click on the link “Animation on Natural Selection in Guppies”. 12 Constraints on Natural Selection Natural selection can produce “endless forms most beautiful”… However… 13 Constraints on Natural Selection 14 … There are many constraints on natural selection. Natural selection cannot… 1. Work effectively in small populations. 2. Select for / against traits that are neutral to selection. 14 3. Natural selection cannot ‘see’ genotypes, it can only act on phenotypes. 15 aa Aa AA AA 15 4. A phenotype under selection might be influenced by the environment. This is called the norm of reaction. 16 16 17 5. Natural selection can only act on mutations that exist. It cannot make mutations happen. 17 18 This leads to a rather ’pedantic’ but still important point about mutations. When we say: “mutations are random” It technically does not mean what you think it means! What it really means is that mutations are “indifferent” to the needs of the population. 18 19 B/c natural sel. can only work on genetic variations that already exists, we get …. (!!!) 19 6. There are developmental constraints... Ok Ok Not Ok 20 20 One reason for developmental constraint: Interactions between genes in embryos. 21 Ok Ok 21 Natural selection can only evolve anatomies that are viable under our laws of physics. 7. There are physical constraints. 22 Ok Not Ok Volume (~ Wt.) Bone x.s. Surface Area if bone grows proportionally 23 Bone x.s. Surface Area sufficient to support weight Height 23 24 24 OK… 25 25 26 OK… Not OK! 27 … 28 29 As we have seen, the main scientific objections to Darwins’ theory of evolution began to be resolved over many decades. 30 Evolution requires mutations of genes (new alleles) and is the change in the frequency of alleles in populations. Evolutionary change is gradual. By the 1930’s - 1940’s a consensus was formed: Neo-Darwinism, known today as the Modern Synthesis. 31 Mechanisms of evolution include natural selection + genetic drift & other mechanisms. Speciation is through reproductive isolation. All life today shares a common ancestry. Mechanisms of evolution include natural selection + other mechanisms. Speciation is through reproductive isolation. All life today shares a common ancestry. By the 1930’s - 1940’s a consensus was formed: Neo-Darwinism, known today as the Modern Synthesis. It is good to pause here and take stock of 2 fundamental definitions: What is a definition of evolution by natural selection? 2. What is a definition of evolution according to the Modern Synthesis? 33 Some main areas of evidence for evolution: Biogeography Comparative anatomy Embryology Fossils Molecular Genetics 34 Biogeography In general, closely related species are found near each other… As predicted if species come from other species, when in isolation. Species B Species C Species A Species A Species A 35 Islands show speciation especially clearly. mammals, reptiles, birds, amphibians, f.w. fish… Some islands are derived from continents.. These have the same communities seen in the mainland. 36 Volcanic islands… mammals, reptiles, birds, amphibians, f.w. fish… tend to lack specific kinds of species – so they are clearly populated by immigration. 37 Volcanic islands have numerous unique species. 38 39 What is true for islands must be true in general. Islands cannot be be ‘special’, just simple. … 40 41 Comparative Anatomy 1. Branching evolution predicts a nested hierarchy of shared anatomy. 42 Some terms… 43 2. Corresponding structures due to common ancestry are homologous structures. 44 3. Homologous structures have a dual nature… 45 Bird a. Primitive (or conserved) features are shared b.c. of common ancestry. Reptile Mammal 3. Homologous structures have a dual nature… 46 b. Derived features are the special modifications that evolved after separating from the common ancestor. 3. Homologous structures have a dual nature… Bird Reptile Mammal 47 c. Accumulation of differences in different lineages is divergent evolution. 3. Homologous structures have a dual nature… Bird Reptile Mammal 48 4. Anatomy is like a palimpsest… 49 Palimpsests… Palimpsests &… …Palimpsests. 52 5. Organs that fall into disuse can become vestigial. Why? 53 6. Similar selection histories can  different lineages having a superficially similar anatomy. 54 7. This is convergent evolution. 55 Pill bug (crustacean) Pill millipede Earthworm (invertebrate) Caecilian (Amphibian!) 8. Structures made similar by convergent evolution need not be homologous. These are analogous structures. 57 Embryology 58 What are these embryos? 59 … 60 1. These embryos look alike b/c they look like embryos of their fish ancestors. Remember the developmental constraint on embryos at this time. 61 2. Constraint forces embryos to develop many ‘primitive’ structures, then discard them for derived structures! Human (a vertebrate chordate) Amphioxus (an invertebrate chordate) 62 Human (a vertebrate chordate) Amphioxus (an invertebrate chordate) 3. For example, mammals develop & discard: notochord, branchial arches & 2 pairs of kidneys (!!) 63 4. Amniotes develop an amniotic sac, yolk sac & allantois sac, but mammals have lost most use of them. 64 Developmental constraint has many aspects… a. We need the genes. b. A dense network of gene interactions. c. We need the cell cell interactions. 5. Why do embryos develop and discard organs? 65 6. Conserved development also means that anatomy has some dumb designs… 66 6. Conserved development also means that anatomy has some dumb designs… 67 TESTES 68 Fossils Record long-term macroevolution. 69 1. Most fossils are preserved in sedimentary rocks. Requires rapid burial, compression, & time. 70 2. Major problem: the fossil record is incomplete -- especially for soft and terrestrial species. Figure 3.04: Illustration of obvious evolutionary relationships among fossil species of the mollusk Paludina Paludina neumayri P. hoenisi 3. But gradual speciation is sometimes preserved. 72 Proterozoic Archean Mesozoic Cenozoic Phanerozoic First reptiles 1st chordates Jawed fishes Amphibians Mammals & Dinos Jawless fishes 542 488 443.7 416 299 199.6 145.5 1 Ordovician Permian Cretaceous Tertiary Quaternary Cambrian Jurassic Triassic Silurian Devonian Paleozoic Precambrian Hadean Carboniferous Eon Era Period m.y.a. Events 65.5 251 359.2 4550 3800 2500 1st animals Origin of earth 1st traces of life photosynthesis One feature of succession. Mammals take over Dinos go extinct 73 Proterozoic Archean Mesozoic Cenozoic Phanerozoic First reptiles 1st chordates Jawed fishes Amphibians Mammals & Dinos Jawless fishes 542 488 443.7 416 299 199.6 145.5 1 Ordovician Permian Cretaceous Tertiary Quaternary Cambrian Jurassic Triassic Silurian Devonian Paleozoic Precambrian Hadean Carboniferous Eon Era Period m.y.a. Events 65.5 251 359.2 4550 3800 2500 1st animals Origin of earth 1st traces of life photosynthesis 4. We predict (and observe) fossil transitional forms between new groups & their ancestors. 74 75 5. Lobe-finned fishes. 6. Older than any amphibian: ~ 385 mya: Lobe-finned fishes 77 78 ~ 375 mya Tiktaalik: 7. Before any amphibian: 79 80 8. Before any mammal: A variety of reptiles with different skull designs. Dimetrodon: an early synapsid reptile. 81 9. The ‘mammal-like reptiles’ slowly produced species with vertical legs, diverse teeth, and other signs of increased metabolic rate. Some descendants evolved  mammals. Their ears changed along the way … 82 10. Reading assignment: Read the two articles about the origin of the mammalian ear. 11. Before any human: Ardipithecus ramidus, 4.4. m.y.a. Molecular Genetics 85 1. Parents pass on chromosomes w/ their linear orders of genes + DNA sequences + mutations. 86 Human chromosomes Chimpanzee chromosomes 6 mya 87 Gorilla chromosomes 8 mya etc. Human chromosomes Mouse chromosomes Human chromosomes ~ 75 mya Cytochrome c 100,000’s of genes have been compared across species. They reveal the same relationships as trees based on anatomy and the fossil record. 2. Genes that are similar in sequence & similar in function are homologous genes. 90 -hemoglobin Hemoglobin genes. Similar in sequence and function. Therefore: homologous. 91 3. Pseudogenes (Ψ) are disabled genes in a species. b-hemoglobin 4. They are shared between related species. 92 6. Humans and most other primates cannot make this compound, and so we must get it from our diet as vitamin C. 5. Another example: A series of enzymes are needed to make ascorbic acid. Gulo X 7. The reason: One of the genes is disabled as a pseudogene 93 Note the several matched mutations. Note the several matched mutations. Rat 5. Jumping genes (a.k.a. transposable elements) insert randomly into DNA. There are two main classes of these ‘selfish genes’. RNA DNA 95 ‘Jumping genes’ 96 97 ‘There is grandeur to this view of life.’ -- C. Darwin 98