Lab 10 - Evolution

Lab 10 - Evolution 27.1 Theory of Evolution 12 Darwin’s theory of natural selection. • Individual organisms within a population differ in terms of their reproductive success. • Some individuals in a population have favorable traits that enable them to better compete for limited resources. • Can devote more energy to reproduction and have more offspring. 27.1 Theory of Evolution 13 Darwin’s theory of natural selection. • Organisms become adapted to conditions as the environment changes. • Natural selection through differential reproductive success shapes traits. • The result of this descent with modification is adaptation. • Adaptation—any evolved trait that helps an organism be more suited to its environment. 27.2 Evidence of Evolution 2 Descent with modification. • All living things share the same fundamental characteristics. • Made of cells. • Take chemicals and energy from the environment. • Respond to stimuli. • Reproduce. • Life’s diversity is due to organisms adapting to different environments. • Features that enable them to survive in each environment vary greatly. Fossil Evidence 1 Fossils are remains and traces of past life or any other direct evidence of past life. • Hard body parts most often preserved: shells, bones, teeth. • Soft parts can be buried quickly so decomposition is never completed. • Traces include footprints, burrows, casts. • Majority of fossils embedded in sedimentary rock. • Deposited in layers called strata. • Any stratum is older than the one above it and younger than the one below it. Fossil Evidence 2 Some fossils serve as transitional links between groups. • Fossils of Archaeopteryx that lived 165 million years ago serve as an example. • They display an intermediate form between dinosaurs and birds. • Dinosaur features—jaws with teeth, a long, jointed tail. • Bird features—feathers and wings found in modern birds. • The fossils of Archaeopteryx are similar to other transitional fossils in that they have some traits like their ancestors and others like their descendants, rather than intermediate traits. Transitional Fossils (a): ©Jason Edwards/Getty Images; (b): ©Joe Tucciarone Fossil Evidence 3 Fossils of Ambulocetus natans suggest that whales had a terrestrial ancestor. • It was the size of a large sea lion, with broad, webbed feet on its forelimbs. • Its hindlimbs enabled walking and swimming. • It also had tiny hooves on its toes. • It had the primitive skull and teeth of early whales. Fossil Evidence 4 Modern whales still have remnants of a hindlimb with fewer bones and bones reduced in size. As ancestors to whales adapted to an increasing aquatic lifestyle, the location of the nasal opening transitioned. • Tip of snout (Ambulocetus). • Midway between tip of snout and the skull (Basilosaurus). • Now at the very top of the head in modern whales. Geological Timescale 1 History of Earth is divided into eras, then periods, and then epochs. • Fossil record has helped determine the dates. Two methods are used: • Relative dating method. • Determines the relative order of fossils and strata but not the actual date. • Absolute method. • Radioactive dating techniques are used to assign an actual date to a fossil. • Technique is based on the half-life of radioactive isotopes. Geological Timescale 2 The Geological Timescale: Major Divisions of Geological Time and Some of the Major Evolutionary Events of Each Time Period Era Period Epoch Million Years Ago (mya) Plant Life Animal Life Cenozoic Quaternary Holocene current Humans influence plant life. Age of Homo sapiens Significant Extinction Event Underway Cenozoic Quaternary Pleistocene 0.01 Herbaceous plants spread and diversify. Neogene Pliocene 2.6 Neogene Miocene 5.3 Herbaceous angiosperms flourish. Grasslands spread as forests contract. Neogene Oligocene 23.0 Many modern families of flowering plants evolve; appearance of grasses. Presence of ice age mammals. Modern humans appear. First hominids appear. Apelike mammals and grazing mammals flourish; insects flourish. Browsing mammals and monkeylike primates appear. Geological Timescale Cenozoic 3 Paleogene Eocene 33.9 Subtropical forests with heavy rainfall thrive. All modern orders of mammals are represented. Paleogene Paleocene 55.8 Flowering plants continue to diversify. Ancestral primates, herbivores, carnivores, and insectivores appear. Mass Extinction: 50% of all species, dinosaurs and most reptiles Mesozoic Cretaceous N/A 65.5 Flowering plants spread; conifers persist. Placental mammals appear; modern insect groups appear. Jurassic N/A 145.5 Flowering plants appear. Dinosaurs flourish; birds appear. Mass Extinction: 48% of all species, including corals and ferns Mesozoic Triassic N/A 199.6 Forests of conifers and cycads dominate. First mammals appear; first dinosaurs appear; corals and molluscs dominate seas. Mass Extinction (“The Great Dying”): 83% of all species on land and sea Geological Timescale Paleozoic 4 Permian N/A 251.0 Gymnosperms diversify. Reptiles diversify; amphibians decline. Carboniferous N/A 299.0 Age of great coalforming forests: ferns, club mosses, and horsetails flourish. Amphibians diversify; first reptiles appear; first great radiation of insects. Mass Extinction: Over 50% of coastal marine species, corals Paleozoic Devonian N/A 359.2 Flowering plants spread; conifers persist. First insects and first amphibians appear on land. Silurian N/A 416.0 Seedless vascular plants appear. Jawed fishes diversify and dominate the seas. Mass Extinction: Over 57% of marine species Paleozoic Ordovician N/A 443.7 Nonvascular land plants appear. Invertebrates spread and diversify; first jawless and then jawed fishes appear. Geological Timescale Paleozoic Cambrian N/A 488.3 Marine algae flourish. N/A 630 N/A 1,000 Protists diversify. N/A 2,100 First eukaryotic cells evolve. N/A 2,700 O2 accumulates in atmosphere. N/A 3,500 First prokaryotic cells evolve. N/A 4,570 Earth forms. 5 All invertebrate phyla present; first chordates appear. First soft-bodied invertebrates evolve. Biogeographical Evidence 1 The first cells evolved in water. Organisms are composed of 70% to 90% water. Water is a polar molecule. Water molecules form hydrogen bonds, which cause them to cling to one another. • Water is liquid at temperatures typical of the Earth’s surface due to hydrogen bonding. Biogeographical Evidence 2 Continental Drift. • The positions of continents and oceans have shifted through time. • The distribution of fossils and existing species provides evidence of former positions of continents. Mass Extinctions 1 Extinction—death of every member of a species. • Large numbers of species become extinct in a short period of time in a mass extinction. • The remaining species may spread out and fill habitats left vacant. • Five major extinctions have occurred. • They occurred at the end of Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. • Earth may currently be experiencing a sixth mass extinction due to human activities. Mass Extinctions 2 Cretaceous mass extinction. • Likely due to large meteorite striking Earth. • Dust cloud blocked out sun, killing plants. • All dinosaurs went extinct. Ordovician mass extinction. • Continental drift of Gondwana to South Pole contributed. Devonian mass extinction. • End of 70% of marine invertebrates. • May have been due to movement of Gondwana back to South Pole. Anatomical Evidence 1 Common descent offers explanation for anatomical similarities among organisms. • Homologous structures. • Anatomically similar because they are inherited from a common ancestor. • Example: human arm and whale forelimb. • Vestigial structures. • Anatomical structures fully functional in one group and reduced or nonfunctional in another. • Example: snakes have no use for hindlimbs, and yet some have remnants of hindlimbs in pelvic girdle and leg. Anatomical Evidence 2 Embryological development. • At some time in development, all vertebrates have a tail and paired pharyngeal pouches. • Reflects common ancestry. Exception: Analogous structures. • Serve the same function but not constructed similarly and do not share a common ancestry. • Example: wing of bird and wing of an insect. Anatomical Evidence 3 Similarities in Vertebrate Development Biochemical Evidence Almost all organisms use the same basic biochemical molecules, including DNA, ATP, and many enzymes. • Organisms use the same DNA triplet code for the same 20 amino acids in their proteins. • Humans share many genes with much simpler organisms. • The diversity of life is due to only a slight difference in many of the same genes. • The more similar the DNA sequences, the more closely related the organisms are. Evolution is a theory, supported by a large number of observations and experiments. Significance of Biochemical Differences Humans as Agents of Evolution 1 Humans can artificially modify desired traits in plants and animals by selecting to breed individuals with preferred traits. Artificial selection is a type of human-controlled breeding to increase the frequency of desired traits. Artificial selection, like natural selection, is possible because the original population exhibits a variety of characteristics, allowing humans to select preferred traits. Humans as Agents of Evolution 2 Artificial selection has led to the existence of many dog breeds. • All breeds are descendants of the wolf. Darwin concluded that if humans could create such variety of organisms by artificial selection, then natural selection could also produce diversity. • In natural selection, the environment is the force selecting for particular traits. Humans as Agents of Evolution 3 Evolution in Natural Populations 1 The Galápagos finches have beaks adapted to the food they eat, with different species of finches on each island. One study of finches from the island called Daphne Major began in 1973. • Revealed that the beak depth of the medium ground finch (Geospiza fortis) changed between wet and dry years. • During wet years, finches eat small, tender seeds. • During dry years, they must eat larger, drier seeds that are more difficult to crush. Evolution in Natural Populations 2 During dry years, the majority of finches died from starvation because their beaks were not well-equipped to feed on harder seeds. The finches with deeper beaks survived and reproduced. • In the next generation of G. fortis birds, the average beak was deeper than the previous generation. Conclusion—evolutionary change can sometimes be observed within the time frame of a human life span. Evolution in Natural Populations 3 27.3 Microevolution Evolution is change in heritable traits, not change in traits in an individual’s lifetime. Darwin observed that populations evolve, not individuals. • Could not explain how traits change over time. • Now known that genes interact with the environment to determine traits. • Evolution is really about genetic change, because genes and traits are linked. Microevolution is the change in allele frequencies in a population over time. Allele Frequencies 1 Gene pool—the sum total of all alleles of all genes in a population. Sample study. • 25 peppered moths collected from a population. • • • • Some are dark and some are light. A total of 50 alleles are present. 10 alleles are D and 40 alleles are d. The frequency of the D and d alleles is • • for D. for d. • Allele frequency: the proportion of each allele in a population’s gene pool. Allele Frequencies 2 The frequencies of D and d add up to 1. • This relationship is true of the sum of allele frequencies in a population for any gene of any diploid organism. • The relationship is described by the expression: • p+q=1 • p is the frequency of D. • q is the frequency of d. • In the moth study, the allele frequencies did not change over three generations, implying no evolution. Hardy-Weinberg Equilibrium 1 Genetic equilibrium describes a population in which allele frequencies do not change over time (also referred to as Hardy–Weinberg equilibrium)—a stable nonevolving state. Hardy and Weinberg used the equation: p2 + 2pq + q2 to describe the genotype and allele frequencies in a population. • p2 = frequency of DD genotype. • 2pq = frequency of Dd genotype. • q2 = frequency of dd genotype. Hardy-Weinberg Equilibrium 3 The Hardy–Weinberg principle. • Allele frequencies in a gene pool will remain at equilibrium, thus constant, in each generation of a large, sexually reproducing population as long as the following five conditions are met: • No mutations. • No genetic drift. • No gene flow. • Random mating. • No selection. Hardy-Weinberg Equilibrium 4 Although possible in theory, the Hardy–Weinberg equilibrium is never achieved in wild populations. • All five conditions are never met in the real world. • Populations are constantly evolving. The Hardy–Weinberg principle is an important tool because the violation of one or more of the conditions causes allele and genotype frequencies to change in predictable ways. Hardy-Weinberg Equilibrium 5 Hardy-Weinberg Proportions Can Be Used to Determine If Evolution Has Occurred Hardy-Weinberg Condition Deviation from Condition Effect of Deviation on Population Expected Deviation from HWE Evolution Occurred? Random mating Nonrandom mating Allelles do not assort randomly Change in genotype frequencies No No selection Selection Certain allelles are selected for or against Change in allelle frequencies Yes No mutation Mutation Addition of new allelles Change in allelle frequencies Yes No migration Immigration or emigration Individuals carry allelles into, or out of, the population Change in allelle frequencies Yes Large population (no genetic drift) Small population (genetic drift) 1. bottleneck effect 2. founder effect Loss of allelle diversity; some allelles may disappear Change in allelle frequencies Yes 27.4 Processes of Evolution Five Agents of Evolutionary Change. • There are five theoretical conditions for a population to be in genetic equilibrium. • The opposite of these conditions can cause evolutionary change. • They include mutations, genetic drift, gene flow, nonrandom mating, and natural selection. Mutations New mutations can cause allele frequencies in a population to change. Mutations are genetic changes that are the only source of new variation in a population. • Without mutations, there would be no new inheritable genetic diversity. • Mutations are random events. • A mutation does not arise because the organism “needs” one. • Some are beneficial (provide a survival advantage), and some are harmful. Genetic Drift 1 Genetic drift is a change in allele frequencies of a gene pool due to random meeting of gametes in fertilization. • The random selection and assortment of gametes in a population causes allele frequencies to shift in each generation. Genetic drift has greater effects in smaller populations due to fewer gametes assorting. • Removal of gametes from a smaller population can affect allele frequencies in the next generation. • Example: the death of one individual in a population of 10 could change the frequency of an allele by 10% or cause its loss altogether. Genetic Drift 3 In nature, the bottleneck effect and founder effect occur when populations are drastically reduced in size. • The bottleneck effect occurs following a natural disaster that kills a large proportion of a population. • A severe reduction in the gene pool that can affect allele frequencies. • The founder effect occurs when a few individuals form a new colony and their collective genes represent only a fraction of the original gene pool. Gene Flow Gene flow is the movement of alleles between populations. • Individuals or their gametes migrate from one population to another and breed in the new population. Gene flow mixes genetic diversity. Gene flow also keeps the gene pools of two or more populations similar. Nonrandom Mating Nonrandom mating occurs when individuals are selective about choosing a mate with a preferred trait. Random mating is never observed in natural populations, because most sexually reproducing organisms select mates based on some trait. Inbreeding, mating between close relatives, is an example of nonrandom mating. • Increases proportion of homozygotes. • Increases frequency of recessive abnormalities in humans. Natural Selection 1 Natural selection is a process that allows some individuals with an advantage over others to produce more offspring. • Charles Darwin explained evolution through natural selection. • Evolution by natural selection requires the following: • Individual variation. • Inheritance. • Overproduction. • Differential reproductive success. Natural Selection 2 Fitness is measured by the number of fertile offspring produced by an individual throughout its lifetime. • Variations that can contribute to fitness can arise from: • Mutation. • Crossing-over. • Independent assortment. • Random fertilization. Most traits on which natural selection acts are polygenic, controlled by more than one gene. • Range of phenotypes follows a bell-shaped curve. Natural Selection 3 Three main types of natural selection. • Stabilizing selection. • Directional selection. • Disruptive selection. Natural Selection 4 Stabilizing selection. • Extreme phenotypes are selected against. • Individuals near average phenotype are favored. • Stabilizing selection improves adaptation of the population to aspects of the environment that remain constant. • Example: Human infants born with an intermediate birth weight (3 to 4 kilograms) have a better chance of survival than those of either extreme. • Small babies do not have fully functional systems. • Large babies experience difficult deliveries. Natural Selection 5 Directional selection. • One extreme phenotype is favored. • The distribution curve shifts in that direction. • Directional selection can occur when the population is adjusting to a changing environment. • Example: When bacteria are exposed to an antibiotic, most are susceptible and die, but some are resistant and survive to reproduce. • In subsequent generations, antibiotic-resistant bacteria continue to reproduce, and the antibiotic resistance allele becomes more prevalent. Directional Selection Natural Selection 6 Disruptive selection. • Two or more extreme phenotypes are favored. • Two different habitats could result in two different phenotypes in the population. • British land snails with dark shells are more prevalent in forested areas. • Snails with light, banded shells are more common in areas of low-lying vegetation. Disruptive Selection (b) (left): ©NHPA/Superstock; (right): ©W. Layer/ Blickwinkel/age 27.5 Macroevolution and Speciation 1 Macroevolution—evolution on a large scale. • Macroevolution includes the history of life on Earth. • Macroevolution involves speciation (splitting of one species into two or more). • Speciation is governed by the same microevolutionary agents that are in play in populations. • Thus, microevolution and macroevolution are the results of the same agents, differing only in scale. • Macroevolution is caused by the accumulation of microevolutionary change that results in new species. 27.5 Macroevolution and Speciation 2 Species. • A species is defined as a group of organisms capable of interbreeding and are isolated reproductively from other species. • Under the biological species concept, if organisms cannot mate and produce fertile offspring in nature, they are defined as different species. • Mechanisms exist to reproductively isolate similar species. 27.5 Macroevolution and Speciation 3 Reproductive isolating mechanisms. • Prezygotic isolating mechanisms. • In place before fertilization. • No attempt at reproduction. • Postzygotic isolating mechanisms. • In place after fertilization. • Reproduction may take place, but no fertile offspring produced.