INTRODUCTORY BIOLOGY II
INTRODUCTORY BIOLOGY II BIOL 112
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Date Created: 10/21/15
Ch 23 The Evolution of Populations 1272010 113900 PM In 1977 a population of medium ground finches were decimated by a drought and the few surviving finches tended to have larger deeper beaks because the only available food was large hard seeds As a result subsequent generations of the finches had deeper larger beaks The finch population evolved through natural selection not the individual finch o Microevolution Evolutionary change below the species level change in the allele frequencies in a population over generations 231 Mutation and sexual reproduction produce the genetic variation that makes evolution possible Genetic Variation 0 People have unique genotypes and therefore unique phenotypes 0 Variation within a population discrete characters are those which either are or aren t like color of O a flower quantitative characters vary along a continuum within a population 0 Average heterozygosity the average percent of loci that are 0 heterozygous estimated by surveying the protein products of genes using gelelectrophoresis 0 Variations between populations 0 Geographic variation differences in the genetic composition of separate populations 0 Cline A graded change in a character along a geographic axis some produced by gradation by an environmental variable Mutations o Mutation A change in the nucleotide sequence of an organism s DNA ultimately creating genetic diversity Mutations also can occur in the DNA of RNA of a virus 0 It cannot be predicted which segments of DNA will be effected or how 0 Most mutations occur in somatic cells and cannot be passed on to offspring 0 Point Mutations 0 Change of as little as one base in a gene a point mutation can have a significant impact on a phenotype 0 Most mutations do not impact the phenotype of an organism at all o Rarely does a mutation make an organism better suited to their environment 0 Mutations That Alter Gene Number or Sequence 0 Chromosomal changes that delete disrupt or rearrange many loci at once are almost certain to be harmful 0 An important source of variation begins when genes are duplicated due to errors in meiosis slippage during DNA replication or the activities of transposable elements 0 Ex The remote ancestors of mammals carried a single gene for detecting odors that have been duplicated many times resulting in heightened senses of mammals o Mutation Rates 0 Mutation rates are low in plants and animals and ever lower in prokaryotes but because prokaryotes have shorter generations so a mutation causes greater genetic variation Sexual Reproduction 0 Most of the genetic variation that occurs in populations that reproduce sexually occurs due the unique combination of alleles that an individual receives from their parents 232 The HardyWeinberg equation can be used to test whether a population is evolving 1272010 113900 PM Gene Pools and Allele Frequency 0 A population is a group of individuals of the same species that lie in the same area and interbreed producing fertile offspring 0 Different populations of the same species may be isolated o Geographically and exchange genetic material rarely o A population s genetics are described by its gene pool all of the alleles for all the loci in all individuals of the population The Hardy Weinberg Principle 0 HardyWeinberg Equilibrium o This principle states that the frequencies of alleles and genotypes in a population will remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work 0 Conditions for HardyWeinberg Equilibrium 1 No mutations 2 Random mating 3 No natural selection 4 Extremely large population size 5 No gene flow no immigration or emigration 0 Applying the HardyWeinberg Principle 0 HW is often used to check if evolution is occurring in a population 233 Natural selection genetic drift and gene flow can alter allele frequencies in a population 1272010 113900 PM Natural Selection Organisms with bettersuited traits for the environment produce more offspring Proportions of alleles change when selection occurs Ex Drosophila melanogaster has an allele that causes an individual to be resistant to pesticides Flies from the 1930 s have a 0 allele frequency Strains collected after DDT was introduced have an allele frequency of 37 DDT is a strong selective force in exposed fruit fly populations Adaptive evolution evolution that results in a better match between organisms and their environment Genetic Drift Genetic drift a process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next Effects of genetic drift are most pronounced in small populations The Founder Effect When few individuals become isolated from a larger population this smaller group may establish a new population whose gene pool differs from the source population 0 For example the founder effect occurs when a few members of a population are blown onto a new island and genetic drift occurs when the alleles are altered by chance 0 Ex 15 British colonists founded a settlement on Tristan da Cunha one of whom carried a recessive allele for retinitis pigmentosa a progressive form of blindness A few generations later the allele s frequency was ten times higher on Tristan da Cunha that in the founders home populations The Bottleneck Effect Genetic drift that occurs when the size of a population is reduced as by a natural disaster or human actions Typically the surviving population is no longer genetically representative of the original population 0 It is important to understand the bottleneck effect because human actions sometimes create severe bottlenecks for other species 0 Ex Impact of Genetic Drift on the Greater Prairie Chicken Millions of Greater Prairie Chickens once lived in Illinois but when the prairies were converted to farmland the population dropped to about 50 birds The surviving birds had low genetic variation and had fewer eggs hatch compared to hatching rates on larger populations To see if there really was a loss of genetic variation scientists examined DNA from specimens from the 19305 and 19605 they observed that the modern chickens had lost nine alleles present in the older specimens showing that genetic drift had reduced the genetic variation Effects of Genetic Drift A Summary 1 Genetic drift is significant in small populations 2 Genetic drift can cause 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 as in the prairie chicken example Gene Flow Gene flow The transfer of alleles from one population to another resulting from the movement of fertile individuals or their gametes Gene flow tends to reduce the genetic differences between populations When neighboring populations live in different environments alleles transferred by gene flow may prevent a population from fully adapting to its environment 0 Ex Bent grass populations growing next to copper mines live in soil that that has high concentrations of copper There is an allele for copper tolerance that most plants that live in the copper soil possess but when the coppertolerant plants live in copperfree environments they reproduce poorly Sometimes beneficial alleles are transferred widely Pesticide resistant mosquitoes Gene flow like mutation can introduce new alleles into a population But because it can occur at a higher rate than mutation gene flow is more likely than mutation to alter frequencies directly 234 Natural selection is the only mechanism that consistently causes adaptive evolution 1272010 113900 PM A Closer Look at Natural Selection 0 Relative Fitness 0 Struggle for survival does not always mean a literal battle For example a barnacle that is more efficient at collecting food than its neighbors may have greater stores of energy and hence be able to produce a larger number of eggs 0 Relative fitness the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals 0 Natural selection acts more directly on the phenotype not the underlying genotype it acts on the genotype indirectly o Directional Disruptive and Stabilizing Selection 0 Directional selection natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals Example Fossil evidence indicates that the average size of black bears in Europe increased during each frigid glacial period and decreased during warmer weather 0 Disruptive selection occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes Ex There is a population of blackbellied seedcracker finches whose members display two distinctly different beak sizes Smallbilled birds feed mainly on soft seeds and largebilled birds crack hard seeds Birds with intermediatesized bills are inefficient at cracking wither type of seed and relatively low fitness 0 Stabilizing selection acts against both extreme phenotypes and favors intermediate variants Ex The birth weights of most human lie in the 34 kg range and babies that are much heavier or lighter suffer higher rates of mortality The Key Role of Natural Selection in Adaptive Evolution 0 Adaptations can arise gradually over time as natueal selection increases the frequencies of alleles that enhance survival and reproduction 0 Adaptive evolution occurs when the match between a species and its environment improve due to selection 0 Adaptive evolution is a dynamic continuous process Sexual Selection 0 Sexual selection a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates 0 Sexual dimorphism marked differences between the two sexes in secondary sexual characteristics which are not directly associated with reproductive or survival 0 These distinctions include differences in size color ornamentation and behavior 0 Intrasexual selection A direct competition among individuals of one sex usually the males in vertebrates for mates of the opposite sex 0 The selected trait are often disadvantageous o A hypothesis is that females prefer male traits that are correlated with good genes 0 Sometimes selected traits are related to overall male health The Preservation of Genetic Variation 0 Diploidy o Recessive alleles are a way to maintain unfavorable traits to eliminate the potential problem of natural selection getting rid of all unfavorable traits o Heterozygote protection maintains a huge pool of alleles that might not be favored under present conditions but which could bring new benefits if the environment changes 0 Balancing Selection 0 Balancing selection natural selection that maintains two or more phenotypic forms in a population 0 includes heterozygote advantage and frequencydependent selection 0 Heterozygote advantage Greater reproductive success of heterozygous individuals compared with homoozygotes tends to preserve variation in a gene pool defined by genotype not phenotype Ex sicklecell disease and malaria o Frequencydependant selection A decline in the reproductive success of individuals that have a phenotype that has become too common in a population Left and rightmouthed scaleeating fish frequency oscillates over time and the traits alternate in dominance Selects for least common trait 0 Neutral Variation 0 Neutral variation Genetic variation that does not appear to provide a selective advantage or disadvantage 0 Over time the frequencies of alleles that are not affected by natural selection may increase or decrease as a result of genetic drift 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 0 Evolution cannot craft perfect organisms There is evidence that evolution causes many imperfections in organisms Ch 22 Descent with Modification A Darwinian View of Life 1262010 427 00 AM A species of beetle that lives in the Namib desert adapts to an environment where the only precipitation is fog by tilting itself headdownward so that the water droplets accumulate on the beetle s body and run into its mouth There are over 350000 species of beetles on earth each adapted to its different environment Evolution descent with modification 221 The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species Scala Naturae and Classification of Species Aristotle viewed species as unchanging Concluded that life forms could be organized on a ladder of increasing complexity No extinction Geology in the 1700 s Earth was thought to be about 6000 yrs old and does not change except for sudden catastrophes Linnaeus developed a binomial system of naming species that is still used today Lamark was the first to suggest evolution Ideas About Change Over Time Fossils the remains or traces if organisms from the past Darwin drew many of his conclusions from his observations of fossils Strata layers of rock Paleontology the study of fossils Developed by French Scientist Georges Cuvier Cuvier opposed the idea of evolution Instead advocated catastophism the principle that events in the past occurred suddenly and were caused by mechanisms different from those operating in the present James Hutton proposed that Earth s geographic features could be explained by gradual mechanisms still operating Charles Lyell incorporated Hutton s thinking into his principle of uniforitarianism which stated that mechanisms of change are constant over time Lamark s Hypothesis of Evolution Use and Disuse the idea that parts of the body that are used extensively become larger and stronger while those that are not used deteriorate 0 Ex giraffe s neck Inheritance of Acquired Characteristics stated that an organism could pass these modifications to its offspring Disproven 222 Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life 1262010 42700 AM Darwin s research o The Voyage of the Beagle 0 Many organisms completely different than in Europe No one had catalogued organisms from S America 0 Fossils in S America resembled S American species not old world species 0 Darwin found marine fossils on the Andes Mts o Organisms highly adapted to their environments o Darwin s Focus on Adaptation o Adaptations characteristics of organisms that enhance their survival and reproduction in specific environments 0 Natural Selection a process in which individuals with certain inherited traits leave more offspring than individuals with other traits 0 Darwin was reluctant to publish his work because he was aware of the uproar it would cause 0 Darwin decided to publish his work because someone else found the same hypothesis 0 Darwin was different because he was the first to propose a plausible mechanism foe evolution not the first to propose evolution The Origin of Species o Descent with Modification 0 Darwin perceived unity in life which he attributed to the descent of all organisms from an ancestor that lived in the distant past 0 He proposed the history of life was like a tree with multiple branches from a common trunk The twigs representing the diversity of currently living organisms The forks representing an ancestor of all the branches that connect 0 Linnaeus s system of nomenclature meshed well with Darwin s hypothesis o Artificial Selection Natural Selection and Adaptation Artificial Selection humans modifying other species over many generations by selecting and breeding individuals that possess desired traits Artificial selection results in species that look very dissimilar to their wild ancestor Darwin s Observations Members of a population often vary greatly in their traits Traits are inherited from parents to offspring All species are capable of producing more offspring than their environment can support Owing to lack of food or other resources many of these offspring do not survive o Darwin s Inferences Individuals whose inherited traits gove them a higher probability of surviving and reproducing in a goven environment tend to leave more offspring than other individuals This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations 0 Organisms with favorable traits go on to produce more offspring with that trait while unfavorable traits disappear over generations o Natural Selection A Summary 1 Natural selection is a process in which individuals that have certain heritable characteristics survive and reproduce at a higher rate than other individuals 2 Over time natural selection can increase the match between organisms and their environment 3 If an environment changes or of individuals move to a new environment natural selection may result in adaptation to these new 0 O O O O 0 conditions sometimes giving rise to new species in the process Individuals do not evolve populations do Natural selection can only amplify or diminish heritable traits Traits that are favorable in one environment may not be in others Which trait is favored depends on the environment 223 Evolution is supported by an overwhelming amount of scientific evidence 1262010 427 00 AM Direct Observation Predation and Coloration of Guppies O O O 0 Male guppy color patterns vary so much that no two guppies look alike Females are attracted to brightly colored males Brightly colored guppies are more conspicuous to predators In nature in was observed that the color patterns of males corresponded to the intensity of predation in the area In pools with high numbers of predators colors tended to be more drab over time In pools with a predator that preyed on young guppies that have not yet displayed their adult colors had guppies that grew up to be more brightly colored over time The Evolution of DrugResistant HIV 0 Individual bacterium or viruses that are resistant to drugs can increase in number rapidly A few drugresistant viruses may be present by chance at the beginning of treatment As all the other viruses are killed by the medication the drug resistant forms reproduce rapidly Artificial Selection 0 O 0 domestic dogs varieties of cabbage Novel traits Example Peppered moths Speciation O O O O A species is a group whose members possess similar anatomical characteristics and can produce viable offspring Complete speciation physical reproductive isolation takes a long time and can vary Ex Cichlid fishes in Lake Victoria Horseshoe crabs Observing speciation requires short generation times Drosophila Polyploidy and speciation Wheat some species are diploid tetraploid etc cannot reproduce Biogeography the geographic distributions of species o Continental drift the slow movement of Earth s continents over time o Islands 0 Species similar to mainland not other islands 0 Rapid diversification and convergent evolution 0 endemic species that are found nowhere else in the world o Galapagos finches o Came from common ancestor o Diverges rapidly on islands 0 Would require coevolution with flowering plants Homology Similarity resulting from common ancestry o Anatomical and Molecular Homologies o Homologous structures structures in different species that are similar because of common ancestry Closely related species have similar anatomical features Homology everywhere All organisms use the same genetic code humans and bacteria share genes 0 Vestigial structures remnants of features that served important functions in the organisms ancestors Example Whales have vestiges of a pelvis and hind leg bones of walking ancestors o Homologies and Tree Thinking o Tetrapod limbs o evolutionary tree a diagram that reflects evolutionary relationships among groups of organisms o Convergent Evolution 0 Convergent evolution the independent evolution of similar features in different lineages 0 different groups of organisms have similar adaptations because they live in similar environments 0 flight evolves independently in bats birds and insects O O example of both convergent evolution and homologous structures Bats didn t evolve from insects Bat and bird wings both based on tetrapod limb but both evolved separately Made from same bones Fossil Record c For many taxa almost all intermediate forms found o The fossil record shows how the limb structure of oceanmammals changed over time eventually the loss of hind limbs and development of flippers Microevolution Evolution within a species Macroevolution Everything older than speciation Speciation An evolutionary process in which one species splits into two or more species
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