CHAPTER 22 Evolution by Natural Selection- The Evidence ∙ Fossils is the first evidence o Fossils: traces of organisms that lived in the past ∙ All of the fossils that are found and identified are collectively called a fossil record ∙ Fossils give a good idea of what organisms looked like ∙ Sometimes they just give a trace that animals were tWe also discuss several other topics like why do shortages develop under a binding price ceiling?
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here in general ∙ Most fossils found are in sedimentary rocks (sand or mud) ∙ Layers give a time scale that is based on fossil content within each layer (geological time scale) ∙ Earth is older than 6000 years ∙ Layering of rocks is called stratification o Bottom layers are older, top layers are younger rock layers o Where you find specific fossils in rock layers will help you figure out when the species existed ∙ Researches use radioactive isotopes to assign absolute ages to the geologic time scale ∙ Fossils provide us with evidence that there are extinct species (species that are not living) ∙ Darwin noticed that species living on Earth change over time ∙ There is continuous extinction throughout time ∙ Extinct fossils species are followed by living species that are similar in the same area (law of succession) o Evidence that extinct and living forms are related and represent ancestors and descendants ∙ Transitional forms were discovered with traits that are intermediate between ancestral and derived species o Provide evidence that there is in fact change throughout time ∙ Can see ancestor and current species but can also see steps between those two stages ∙ Consistent with predictions of the theory of evolution∙ If the traits all evolved, then all of the transitional would in theory occur ∙ There are just small modifications between each stage of transition, but the roughly contain the same bones/ligaments ∙ Vestigial traits: something that was used in an ancestor that is still there in reduced form but doesn’t have any function in the species today o Gives evidence that there are ancestors/that the characteristics of species have changed over time o Ex: human tailbone, goosebumps o Ex: snakes and whales with nonfunctional hip bones, reduced wings in flightless birds, eye sockets in eyeless fish, ∙ Other examples: bacteria have evolved resistance to drugs, insects have evolved resistance to pesticides, weedy plants have evolved resistant to herbicides, bird migration, insect emergence and blooming of flowering plants have evolved in response to climate change ∙ Species are dynamic & always change – NOT static, unchanging forms ∙ Relationships between different species can be shown on a phylogenetic tree (a diagram that illustrates the ancestor descendant relationships ∙ More evidence comes from homology: similarity that’s due to descendants from a common ancestor o 3 levels of homology: 1. Genetic 2. Developmental 3. Structural ∙ Genetic homology: similarity in the DNA sequences of different species ∙ Developmental homology: is seen in embryos of different species ∙ Structural homology: similarity in adult morphology ∙ The three levels of homology interact: o Genetic homologies cause the developmental homologies observed in embryoso Developmental homologies lead to the structural homologies recognized in adults ∙ The most fundamental homology is the genetic code ∙ Homologies help out in modern medicine ∙ The theory of evolution by natural selection predicts that homologies will occur o If species were created independently of one another, these similarities would not occur ∙ Speciation: process that results in a single species splitting into 2+ descendant species ∙ Supports the claim that all organisms are related by a common ancestor ∙ Internal consistency: observation of data from independent sources agree in supporting predictions made by a theory ∙ Since this theory has been controversial, there has been an abundance of evidence to prove that species have descended with modification from a common ancestor ∙ Several lines of evidence support the hypothesis that whales evolved from a terrestrial ancestor Evolution by Natural Selection – How it works ∙ While many researchers had proposed the fact of evolution, Darwin didn’t just explain the process of modification but he was also able to describe the pattern by which it happened – natural selection ∙ Darwin’s Four Postulates: 1. Individuals vary in their traits 2. Some of these differences are heritable; they are passed to their offspring 3. In each generation, many more offspring are produced then can survive; of these, some will survive long enough to reproduce and some will produce more offspring than others 4. Individuals with certain heritable traits are more likely to survive and reproduce. Natural selection occurs when individuals with certain traits produce more offspring than do individuals without those traits.∙ The selected traits will increase in frequency in the population from one generation to the next causing evolution – a change in the genetic characteristics of a population over time ∙ Some traits are selected over others which leads to evolution ∙ Evolution by natural selection occurs when: o Heritable variation leads to differential success in survival and reproduction ∙ Fitness: ability to survive and reproduce ∙ Adaptation: heritable trait that increases an individual’s fitness in a particular environment ∙ Adaptations increase fitness – the ability to produce offspring ∙ The theory of evolution by natural selection is both observable & testable o Resistance to antibiotics: M. tuberculosis o The bacterium Mycobacterium tuberculosis causes tuberculosis o Eventually TB rates started to surge due to the evolution of drug-resistant strains o DNA from rifampin-resistant bacteria was found to have a single point mutation in a gene called rpoB o Rifampin works by interfering with RNA polymerase and transcription, but the mutation prevents rifampin from binding o Usually mutant forms of RNA polymerase do not work as well as the normal form, but cells with normal RNA polymerase grow slower or die while those with mutant RNA polymerase proliferate ∙ Testing Darwin’s Postulates Using TB example 1. Variation existed in the population. Due to mutation, both resistant and nonresistant strains of TB were present prior to administration of the drug. 2. The variation was heritable. The variation in the phenotypes of the two strains was due to variation in their genotypes. 3. There was variation in reproductive success. Only a tiny fraction of tuberculosis cells survived the first round of antibiotics long enough to reproduce. 4. Selection occurred. The cells with the drug-resistant allele had higher reproductive success. ∙ When drug therapy occurs, the drug kills off most of the bacteria without the mutation, therefore the mutant cells proliferate and then you have a bunch of mutant cells so then drug therapy becomes ineffective against the mutant cells ∙ Natural selection acts on the individuals (the human), because individuals experience differential success ∙ Populations (the bacteria) evolve, as allele frequencies change in populations, not individuals ∙ Drug resistant populations form because evolution has occurred because individuals with the heritable ability to resist some chemical compound were present in the original population. Evolution by Natural Selection – Misconceptions and Limitations ∙ Individuals do not change (evolve) – only the population does ∙ Natural selection sorts out existing variants – it doesn’t change them o Individuals do not change; they just simply produce more surviving offspring than other individuals Giraffes with long necks; the ones with long necks survived therefore passing that trait to their offspring ∙ When individuals change, it’s not in a genetic sense (DNA isn’t affected) o Acclimation: personal advantage that is not genetic ∙ Adaptation: genetic advantage (allele frequencies change in a population) ∙ Evolution by natural selection is not goal directed, it just favors individuals that happen to be better adapted to the environment at the time o Adaptations do not occur because organisms want or need them ∙ Evolution is driven by fitness to an environment ∙ Selection is choosing the better mutation∙ Evolution is not progressive, meaning producing “better” or more complex organisms ∙ Complex traits are routinely lost or simplified over time because of evolution by natural selection ∙ Species are related by a common ancestry and all have evolved equally through time o As evolution continues, species may become simpler or more complex, depending on which traits are favored by the environment ∙ There is no such thing as “higher” or “lower” organisms o Populations have evolved based on their ability to gather resources and produce offspring ∙ All organisms are adapted to their environment and are related by common ancestry ∙ Organisms do not act for the good of the species o Individuals with self-sacrificing alleles die and do not produce offspring (therefore do not evolve) o Individuals with selfish, cheater alleles survive and produce offspring (therefore selfish alleles increase in frequency) ∙ Not all traits are adaptive, and the adaptations that organisms have are constrained in a variety of important ways. (genetic constraints, fitness trade-offs, and historical constraints) o Genetic constraints o Genetic correlation: selection favors one allele but then causes another negative change o Genetic correlations occur because of the pleiotropy in which a single allele affects multiple traits o Lack of genetic variation can constrain evolution because natural selection can work only on existing variation in a population o Fitness Trade-Offs o Compromise between traits in terms of how those traits are adapted for the environment o Because selection acts on many traits at once, every adaptation is a compromiseo Ex: bright color to attract mates might also have a tendency to attract predators o Historical Constraints o Traits evolve from previously existing traits o Not all traits are adaptive, and even adaptive traits are constrained by genetic and historical factors CHAPTER 23 Evolutionary Processes – Types of Selection ∙ Genetic variation: number & relative frequency of alleles that are present in a particular population o Maintaining variation is important because lack of variation can make populations less able to respond successfully to changes in the environment ∙ Directional selection: pattern of selection that increases frequency of one allele o Reduction in genetic diversity over time o If it continues then favored alleles become fixed (100% frequency) while disadvantageous alleles will be lost and will reach a frequency of 0. o When disadvantageous alleles decline in frequency purifying selection is said to occur ∙ Stabilizing selection: occurs when individuals with intermediate traits reproduce more than others o No change in the average value of a trait over time o Genetic variation in the population is reduced o Ex. % Newborns with various weights vs mortality rates; those with weights in the middle were most likely to survive ∙ Disruptive Selection (opposite of stabilizing): intermediate phenotypes are selected against and extreme phenotypes are favored o Maintains genetic variation but does not change the mean value of a trait o Can cause speciation (the formation of a new species) if individuals with one extreme of a trait start mating with individuals that have the same trait ∙ Heterozygote advantage: heterozygous individuals have higher fitness than homozygous individuals do, thus maintain genetic variation in the population o Responsible for a balancing selection, in which no single allele has a distinct advantage ∙ Balancing selection occurs when: o The environment varies over time or in different areas such that certain alleles are favored by natural selection at different times or in different places. This results in maintenance or increase in genetic variation. o Certain alleles are favored when they are rare, but not when they are common – known as frequency dependent selection. As a result, overall genetic variation in the population is maintained or increased. ∙ Sexual selection: occurs when individuals within a population differ in their ability to attract mates, it favors individuals with heritable traits that enhance their ability to obtain mates o Two types: female choice and male-male competition o Females should be choosy about their mates while males will have to compete with each other for mates∙ The fundamental Asymmetry of Sex: results from the fact that, in most species, females usually invest more in their offspring than males do ∙ Female choice: females may choose mates on the basis of physical characteristics that signal male genetic quality ∙ Female choice for paternal care: females prefer to mate with males that care for young or that provide the resources required to produce eggs ∙ Male- Male competition o Establish territories, areas they defend and can use exclusively o Males with larger territories father more offspring, and their alleles rapidly increase in the population ∙ Sexual dimorphism refers to any trait that differs between males and females of the same species o Ex. Lion’s maneCHAPTER 24 Speciation: Species Concepts ∙ If gene flow ends, allele frequencies in isolated populations are free to diverge – meaning that the populations begin to evolve independently of each other o Divergence may occur as a result of mutation, natural selection or genetic drift ∙ This divergence may eventually lead to speciation – the creation of a new species ∙ Speciation creates two or more distinct species from a single ancestral group ∙ A species is defined as an evolutionary independent population or group of populations How to identify a species? A. Biological species ∙ Biological species concept considers populations to be evolutionarily independent if they are reproductively isolated from each other i.e. they do not interbreed or they fail to produce viable, fertile offspring (therefore there is no gene flow occurring between those populations) ∙ Disadvantages: the criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually ∙ It can only be applied to populations that overlap geographically ∙ The mechanisms that stop gene flow between populations are either prezygotic or postzygotic ∙ Prezygotic isolation: when individuals of different species are prevented from mating ∙ Postzygotic isolation: when individuals mate but their offspring have low fitness and may not survive or reproduce Prezygotic Barriers 1. Habitat isolation: two species encounter each other rarely or not at all because they occupy different habitats even though not isolated by physical barriers2. Temporal isolation: species that breed at different times of the day, different seasons or different years cannot mix their gametes 3. Behavioral isolation: courtship rituals and other behaviors unique to a species are barriers 4. Gametic isolation: sperm of one species may not be able to fertilize eggs of another species 5. Mechanical isolation: physical differences in body shape cause a prevention in mating Postzygotic Barriers 1. Reduced hybrid viability: genes of the different parent’s species may interact & impair the hybrids development 2. Reduced hybrid fertility: even if the hybrids are vigorous they may be sterile or have reduced fertility 3. Hybrid breakdown: some 1st generation hybrids are fertile but when they mate with another species or with either parent’s species, offspring of the next generation are feeble and sterile B. Morphospecies ∙ Morphospecies concept says that there are evolutionarily independent lineages by differences in morphological features ∙ idea that distinguishing features are most likely to arise if populations are independent and isolated from gene flow ∙ disadvantages: can’t identify cryptic species that differ in non-morphological traits C. Phylogenetic species ∙ Phylogenetic species concept is based on reconstructing the evolutionary history of populations ∙ On a phylogenetic tree, an ancestral population plus all of its descendants is called a monophyletic group ∙ A monophyletic group are identified by synapomorphies, homologous traits inherited from a common ancestor that are unique to certain populations or lineages∙ a species is defined as the smallest monophyletic group on the tree ∙ disadvantages: phylogenies are currently available for only a tiny (though growing) subset of populations on the tree of life ∙ subspecies are populations that: live in discrete geographic areas, have distinguishing features, are not distinct enough to be considered a species, can interbreed if geographical barriers are removed Chapter 24: Allopatry and Sympatry Isolation and Divergence in Allopatry ∙ Genetic isolation happens routinely when populations become physically separated. Physical isolation, in turn, occurs in one of two ways: dispersal or vicariance. ∙ Dispersal: occurs when a population moves to a new habitat, colonizes it, and forms a new population. ∙ Vicariance: occurs when a physical barrier splits a widespread population into subgroups that are physically isolated from each other ∙ Speciation that begins with physical isolation via either dispersal or vicariance is known as allopatric speciation ∙ Populations that live in different areas are said to be in allopatry ∙ Biogeography—the study of how species and populations are distributed geographically—can tell us how colonization and range-splitting events occur Dispersal and Colonization Isolate Populations ∙ Colonization events often cause speciation because the physical separation reduces gene flow, and genetic drift via the founder effect causes the old and new populations to diverge rapidly. o Peter and Rosemary Grant compared parents and offspring from large ground finches that remained on the island of Daphne Major (colonists) with those from the home island (migrants) ∙ Subsequent natural selection may cause divergence if the newly colonized environment is different from the original habitat Vicariance Isolates Populations ∙ Vicariance events are thought to be responsible for the origin of many modern species ∙ Physical isolation of populations via dispersal or vicariance produces genetic isolation, the first requirement of speciation ∙ When genetic isolation is accompanied by genetic divergence due to mutation, natural selection, and genetic drift, speciation results. Isolation and Divergence in Sympatry ∙ Populations or species that live in the same geographic region (close enough to mate) live in sympratry ∙ Researchers traditionally believed that speciation could not occur among sympatric populations because of gene flow o The prediction was the gene flow would easily overwhelm any differences among populations created by genetic drift and natural selection Sympatric Speciation ∙ Under certain circumstances natural selection can overcome gene flow and cause sympatric speciation ∙ Speciation may occur because even though populations are not physically isolated, they may be isolated by preferences for different habitats Isolation and Divergence can be initiated by two types of events: 1. External events a. Example: disruptive selection based on different ecological niches or mate preferences i. A niche is the range of ecological resources that a species can use and the range of conditions it can tolerate2. Internal events a. Example: chromosomal mutations Sympatric Speciation in Apple Maggot Flies ∙ Apple maggot flies feed and mate on apple fruits, and hawthorne flies feed and mate on hawthorne fruits o Experiments show that each species responds most strongly to its own fruit’s scent ∙ Even though the two species are often found sympatrically, they do not generally interbreed ∙ Although they are not yet separate species on the basis of any species concept, apple flies and hawthorne flies are diverging. They are currently in the process of becoming distinct species How Can Polyploidy Lead to Speciation? ∙ If populations become isolated, it is unlikely that mutation alone could cause them to diverge appreciably o However, a mutation that results in polyploidy—the condition of having more than two sets of chromosomes —can cause speciation, particularly in plants ∙ Tetraploid individuals are genetically isolated from wild-type individuals because they produce diploid gametes rather than haploid gametes o If the two gametes combined, the resulting zygote would be triploid. Triploid individuals produce gametes with a dysfunctional set of chromosomes *slide 16 explains this more ∙ Tetraploid and diploid individuals rarely produce fertile offspring when they mate. As a result, tetraploid and diploid populations are reproductively isolated ∙ Diploid makes haploid gametes Tetraploid makes diploid gametes haploid and diploid gametes make triploid zygotes ∙ Mutations that result in a doubling of chromosome number produce autopolyploid individuals o In these individuals, the chromosomes all come from the same specieso Less common than alloploidy ∙ Allopolyploid individuals are created when parents that belong to different species produce an offspring in which chromosome number doubles Why is Speciation by Polyploidy is Common in Plants? 1. In plants, somatic cells that have undergone many rounds of mitosis can undergo meiosis and produce gametes. The multiple rounds of mitosis increase the likelihood of tetraploid daughter cells 2. The ability of some plant species to self-fertilize makes it possible for diploid gametes to fuse and create genetically isolated tetraploid populations 3. Hybridization between plant species is common, creating opportunities for speciation via formation of allopolypoids ∙ To summarize, speciation by polyploidization is driven by chromosome-level mutations and occurs in sympatry ∙ Compared to the gradual process of speciation by geographic isolation or by disruptive selection in sympatry, speciation by polyploidy is virtually instantaneous. It is fast, sympatric, and commonChapter 24: When Isolated Populations Come in Contact When Isolated Populations Come into Contact ∙ What happens when isolated populations of related species come back into contact depends on many factors, but most importantly on whether the populations have diverged genetically or not o Closely related sympatric species will seldom mate with one another in the lab o Allopatric species are often willing to mate with one another in the lab ∙ If two populations have diverged and if divergence has affected when, where, or how individuals in the populations mate, prezygotic isolation exists ∙ In cases such as this, mating between the populations is rare, gene flow is minimal, and the populations continue to diverge ∙ When prezygotic isolation does not exist, populations may successfully interbreed o Gene flow then occurs and may erase distinctions between the two populations ∙ Other possible outcomes include reinforcement, development of hybrid zones, and speciation by hybridizationReinforcement ∙ If two populations have diverged extensively and are distinct genetically, it is reasonable to expect that their hybrid offspring will have lower fitness than their parents o The logic here is that if populations are well-adapted to different habitats, then hybrid offspring will not be well adapted to either habitat ∙ When postzygotic isolation occurs, there is strong natural selection against interbreeding ∙ Selection for traits that isolate populations reproductively is called reinforcement Hybrid Zones ∙ Sometimes the hybrid offspring of related species possess traits that are intermediate between the two parental populations, and they are healthy and capable of breeding ∙ A geographic area where interbreeding between two populations occurs and hybrid offspring are common is called a hybrid zone o Example of this is seen with warblers ∙ Hybridization often leads to extinction but sometimes leads to the origination of a new species