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Charles Darwin • 1859 – “Origin of Species” published 1. Argued from evidence that species inhabiting Earth today descended from ancestral species 2. Proposed a mechanism for evolution Natural Selection • Many scientists helped pave the way for Darwin’s Theory Fig. 22-2 Linnaeus (classification) Hutton (gradual geologic change) Lamarck (species can change) Malthus (population limits) Cuvier (fossils, extinction) Lyell (modern geology) Darwin (evolution, natural selection) Wallace (evolution, natural selection) American Revolution French Revolution U.S. Civil War 1800 1900 1750 1850 1795 Hutton proposes his theory of gradualism. 1798 Malthus publishes “Essay on the Principle of Population.” 1809 Lamarck publishes his hypothesis of evolution. 1830 Lyell publishes Principles of Geology. 1831–1836 Darwin travels around the world on HMS Beagle. 1837 Darwin begins his notebooks. 1844 Darwin writes essay on descent with modification. 1858 Wallace sends his hypothesis to Darwin. 1859 The Origin of Species is published. Ideas About Change over Time • The study of fossils helped to lay the groundwork for Darwin’s ideas • Fossils are remains or traces of organisms from the past, usually found in sedimentary rock, which appears in layers or strata Lamarck’s Hypothesis of Evolution • Lamarck hypothesized that species evolve through use and disuse of body parts and the inheritance of acquired characteristics • The mechanisms he proposed are unsupported by evidence •Charles Darwin had a consuming interest in nature •First studied medicine (unsuccessfully), and then theology at Cambridge University •After graduating, he took an unpaid position as naturalist and companion to Captain Robert FitzRoy for a 5-year around the world voyage on the Beagle GREAT BRITAIN EUROPE NORTH AMERICA ATLANTIC OCEAN The Galápagos Islands AFRICA Pinta Genovesa Equator Marchena Santiago Fernandina Isabela Daphne Islands Pinzón Santa Santa Cruz Fe Florenza SOUTH AMERICA AUSTRALIA PACIFIC OCEAN San Cristobal Cape of Good Hope Tasmania Española Cape Horn Tierra del Fuego New Zealand •During his travels on the Beagle, Darwin collected specimens of South American plants and animals •He observed adaptations of plants and animals that inhabited many diverse environments • The most influential stop on the voyage was to the Galápagos Islands • Darwin noticed that many of the birds and reptiles were unique to specific islands in the Galápagos archipelago – These included tortoises… Fig. 22-6 (a) Cactus-eater (c) Seed-eater (b) Insect-eater • In 1844, Darwin wrote an essay on the origin of species and natural selection but did not introduce his theory publicly, anticipating an uproar • In June 1858, Darwin received a manuscript from Alfred Russell Wallace, who had developed a theory of natural selection similar to Darwin’s • Darwin quickly finished The Origin of Species and published it the next year Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Origin of Species • Darwin developed two main ideas: – Descent with modification explains life’s unity and diversity • Darwin never used the word evolution in the first edition of The Origin of Species • He called it descent with modification all organisms are related through descent from an ancestor that lived in the remote past – Natural selection is a cause of adaptive evolution • In the Darwinian view, the history of life is like a tree with branches representing life’s diversity • Darwin’s theory meshed well with the hierarchy of Linnaeus Artificial Selection, Natural Selection, and Adaptation • Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits, a process called artificial selection • Darwin then described four observations of nature and from these drew two inferences Fig. 22-9 Terminal bud Lateral buds Cabbage Brussels sprouts Flower clusters Leaves Kale Cauliflower Stem Wild mustard Flowers and stems Broccoli Kohlrabi • Observation #1: Members of a population often vary greatly in their traits • Observation #2: Traits are inherited from parents to offspring • Observation #3: All species are capable of producing more offspring than the environment can support • Observation #4: Owing to lack of food or other resources, many of these offspring do not survive • Inference #1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals • Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations An example of the process of evolution: • Some populations of head lice won’t be killed by the chemical permethrin, a common treatment • Studies have shown that this evolution occurred after only about 30 months (40 generations of lice) The Process of Evolution • Individuals DO NOT evolve – One louse did not, all of a sudden, have the ability to stay alive when permethrin was in the environment – The resistance that developed was genetic, passed on from one generation to the next Fig. 22-12 (a) A flower mantid in Malaysia (b) A stick mantid in Africa Direct Observations of Evolutionary Change • New discoveries continue to fill the gaps identified by Darwin in The Origin of Species • Examples provide evidence for natural selection: – the effect of differential predation on guppy populations – evolution of drug-resistant HIV – Antibiotic resistance in bacteria – Pesticide resistance in insects Fig. 22-13 EXPERIMENT Predator: Killifish; preys mainly on juvenile guppies (which do not express the color genes) Experimental transplant of guppies Pools with killifish, but no guppies prior to transplant Guppies: Adult males have brighter colors than those in “pike-cichlid pools” Predator: Pike-cichlid; preys mainly on adult guppies Guppies: Adult males are more drab in color than those in “killifish pools” RESULTS 12 Number of colored spots 12 10 8 6 4 2 0 Source population Transplanted population 10 8 6 4 2 0 Source population Transplanted population Fig. 22-14 100 Patient No. 1 Patient No. 2 75 50 Patient No. 3 25 0 0 2 4 6 Weeks 8 10 12 Fig. 22-UN2 Insecticide Resistance • Natural selection does not create new traits, but edits or selects for traits already present in the population • The local environment determines which traits will be selected for or selected against in any specific population There are 5 major pieces of evidence that supports the theory of evolution 1. The Fossil Record (a) Pakicetus (terrestrial) • The fossil record provides evidence of the extinction of species, the origin of new groups, and changes within groups over time (b) Rhodocetus (predominantly aquatic) (c) Dorudon (fully aquatic) (d) Balaena (recent whale ancestor) Fig. 22-8 Hyracoidea (Hyraxes) Sirenia (Manatees and relatives) Moeritherium Barytherium Deinotherium Mammut Platybelodon Stegodon Mammuthus Elephas maximus (Asia) Loxodonta africana (Africa) Loxodonta cyclotis (Africa) 34 24 Millions of years ago 5.5 2 104 0 Years ago Fig. 22-17 Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor 2. Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat Homologous Structures • 3. Biochemical Evidence – Molecular level genes shared among organisms inherited from a common ancestor Fig. 22-18 Pharyngeal pouches Post-anal tail Chick embryo (LM) Human embryo 4. Comparative embryology reveals anatomical homologies not visible in adult organisms 5. Vestigial structures are remnants of features that served important functions in the organism’s ancestors Homologies and “Tree Thinking” • The Darwinian concept of an evolutionary tree of life can explain homologies • Evolutionary trees are hypotheses about the relationships among different groups • Evolutionary trees can be made using different types of data, for example, anatomical and DNA sequence data Fig. 22-19 Branch point (common ancestor) Lungfishes Amphibians 1 Mammals 2 Tetrapod limbs Amnion Lizards and snakes 3 4 Homologous characteristic Crocodiles Ostriches 6 Feathers Hawks and other birds Birds 5 Convergent Evolution • Convergent evolution is the evolution of similar, or analogous, features in distantly related groups • Analogous traits arise when groups independently adapt to similar environments in similar ways • Convergent evolution does not provide information about ancestry Sugar glider Flying squirrel • Earth’s continents were formerly united in a single large continent called Pangaea, but have since separated by continental drift • An understanding of continent movement and modern distribution of species allows us to predict when and where different groups evolved Population Genetics • The genetic structure of a population is defined by its allele and genotype frequencies • Hardy Weinberg Theorem describes non-evolving populations • The 5 causes of microevolution are: – Genetic drift – Gene flow – Mutation – Nonrandom mating – Natural selection Genetic drift • Changes in the gene pool of a small population due to chance • The larger the population = less drift • 2 situations which result in populations small enough for genetic drift to be important are: – Bottleneck effect (population is reduced by natural disasters) (ex. Cheetah) – Founder effect ( few individuals colonize new habitat) (ex. Amish) The Bottleneck Effect • The bottleneck effect is a sudden reduction in population size due to a change in the environment • The resulting gene pool may no longer be reflective of the original population’s gene pool • If the population remains small, it may be further affected by genetic drift The Founder Effect • The founder effect occurs when a few individuals become isolated from a larger population • Allele frequencies in the small founder population can be different from those in the larger parent population Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Pre-bottleneck Post-bottleneck (Illinois, 1820) (Illinois, 1993) Range of greater prairie chicken (a) Location Population size Percentage Number of alleles of eggs per locus hatched Illinois 1,000–25,000 5.2 93 <50 3.7 <50 Kansas, 1998 (no bottleneck) 750,000 5.8 99 Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 Minnesota, 1998 (no bottleneck) 4,000 5.3 85 1930–1960s 1993 (b) Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck The results showed a loss of alleles at several loci Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90% Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations 2. Genetic drift causes allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed Gene Flow • Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) • Gene flow tends to reduce differences between populations over time • Gene flow is more likely than mutation to alter allele frequencies directly • Effects of GMO’s on environment? • • • • Gene flow can decrease the fitness of a population In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils Windblown pollen moves these alleles between populations The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment 70 60 MINE SOIL NONMINE SOIL NONMINE SOIL 50 Prevailing wind direction 40 30 20 10 0 20 0 20 0 20 40 60 80 Distance from mine edge (meters) 100 120 140 160 • Gene flow can increase the fitness of a population • Insecticides have been used to target mosquitoes that carry West Nile virus and malaria • Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes • The flow of insecticide resistance alleles into a population can cause an increase in fitness Mutations • A new mutation that is transmitted in gametes can change the gene pool by substituting one allele for another • Does not have a quantitative effect • Is the original source of variation Nonrandom mating • Increases # of homozygous loci in a population, but does not alter frequencies of allele’s in gene pool • 2 kinds: – Inbreeding (results in higher frequency of homo rec. individuals) – Assortive mating (individuals prefer similar phenotypic characters) (ex. Size, color) Natural selection • Due to selection, alleles are passed on to the next generation in a disproportionate numbers relative to their frequencies in the present generation • The only agent of microevolution that is adaptive, since it accumulates and maintains favorable genotypes Natural selection is the only mechanism that consistently causes adaptive evolution • Only natural selection consistently results in adaptive evolution • Natural selection brings about adaptive evolution by acting on an organism’s phenotype • The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals • Reproductive success is generally more subtle and depends on many factors Fig. 23-13 Three modes of selection Original population Original Evolved population population (a) Directional selection Phenotypes (fur color) (b) Disruptive selection (c) Stabilizing selection If the Siberian Husky had heavier muscles, it would sink deeper into the snow, so they would move slower or would sink and get stuck in the snow. Yet if the Siberian Husky had lighter muscles, it would not be strong enough to pull sleds and equipment, so the dog would have little value as a working dog Peppered Moth: Biston betularia • Industrial melanism study by Kettlewell Genetic Drift Convergent evolution Convergent evolution Evolution of Camouflage • Mantis:mimicry http://www.pbs.org/wg bh/evolution/library/01/ 1/l_011_03.html Orchid and Bee: coevolution http://www.pbs.org/wgbh/evolution/library/01/1/l_011_02.html exampl e of mimicr y in which the orchid has evolve d to Crab spider: Sexual Selection • Sexual selection is natural selection for mating success • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics • Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex • Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates • Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact Loud call great for calling females but also great for calling predators The Biological Species Concept • The biological species concept states that a species is a group of populations whose (a) Similarity between different species members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations • Gene flow between populations holds the phenotype of a population together (b) Diversity within a species Reproductive Isolation • Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring • Hybrids are the offspring of crosses between different species • Reproductive isolation can be classified by whether factors act before or after fertilization Fig. 24-4a Prezygotic barriers Habitat Isolation Temporal Isolation Individuals of different species (a) Mating attempt (c) (d) (b) Mechanical Isolation Behavioral Isolation (e) (f) Fig. 24-4i Prezygotic barriers Gametic Isolation Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Viable, fertile offspring Fertilization (g) (h) (i) (j) (k) (l) Limitations of the Biological Species Concept • The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) Other Definitions of Species • Other species concepts emphasize the unity within a species rather than the separateness of different species • The morphological species concept defines a species by structural features – It applies to sexual and asexual species but relies on subjective criteria • The ecological species concept views a species in terms of its ecological niche – It applies to sexual and asexual species and emphasizes the role of disruptive selection • The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree – It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species Concept 24.2: Speciation can take place with or without geographic separation • Speciation can occur in two ways: – Allopatric speciation – Sympatric speciation Fig. 24-5 gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations (a) Allopatric speciation speciation takes place in geographically overlapping populations (b) Sympatric speciation Fig. 24-6 A. harrisi A. leucurus Regions with many geographic barriers typically have more species than do regions with fewer barriers Degree of reproductive isolation Reproductive isolation between populations generally increases as the distance between them increases 2.0 1.5 1.0 0.5 0 0 50 200 250 100 150 Geographic distance (km) 300 Sexual Selection • Sexual selection can drive sympatric speciation • Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria Fig. 24-12 EXPERIMENT Normal light P. pundamilia P. nyererei Monochromatic orange light Allopatric and Sympatric Speciation: A Review • In allopatric speciation, geographic isolation restricts gene flow between populations • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations • Even if contact is restored between populations, interbreeding is prevented • In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species • Sympatric speciation can result from polyploidy, natural selection, or sexual selection Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes • Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species The Time Course of Speciation • Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data Patterns in the Fossil Record • The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear • Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change • The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence Fig. 24-17 (a) Punctuated pattern Time (b) Gradual pattern Speciation Rates • The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid • The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years