Chap 25, 26, & 27
Chap 25, 26, & 27 BSC 1011C
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CHAPTER 25 – EVOLUTION BY NATURAL SELECTION What is the definition of evolution? Populations and species evolve, meaning that their heritable characteristics change through time. Evolution is change in allele frequencies over time. The Evolution of Evolutionary Thought • People often use the word revolutionary to describe the theory of evolution by natural selection. • Why was evolution a revolutionary concept? Plato & Typological Thinking • The Greek philosopher Plato claimed that every organism was an example of a perfect essence or type created by God and that these types were unchanging. • Today, philosophers and biologists refer to ideas like this as typological thinking. – Typological thinking is based on the idea that species are unchanging types and that variations within species are unimportant or even misleading. Aristotle and others proposed that species were organized into a sequence based on Humans increased size and complexity, with Live-bearing vertebrates humans at the top Egg-bearing vertebrates Aristotle & the Great Chain of Being Invertebrates Higher plants Lower plants Inanimate matter Aristotle • Aristotle ordered the types of organisms into a linear scheme called the great chain of being. – In this chain, species were organized into a fixed sequence the top. increasing size and complexity, with humans at • In the 1700s Aristotle’s ideas were still popular in scientific and religious circles. The central claims were that: 1.Species are fixed types. 2.Some species are higher—in the sense of being more complex or “better”—than others. • Lamarck – Change over time via the inheritance of acquired characteristics Charles Darwin 1809-1882 Voyage of the Beagle - 1831 Charles Darwin & Alfred Russel Wallace • Variations in populations •In 1858, Charles Darwin andAlfred Russel Wallace made the claim that evolution has occurred, that species have changed through time. Then they proposed natural selection as a process to explain the pattern of evolution. 3 Observations: -Unity of life -Diversity of life -Match between organisms & their environments The theory of evolution by natural selection was revolutionary for several reasons: 1.It overturned the idea that species are static and unchanging. 2.It replaced typological thinking with population thinking. 3.It was scientific. It proposed a mechanism that could account for change through time and made predictions that could be tested through observation and experimentation. Figure 25.12 Have species changed over time? Primary claims about the nature of species according to evolution • Species change through time. • Species are related by common ancestry. Is there evidence to support these claims? Evidence for Change over Time In the fossil record (a) 180-million-year-old (b) 210-million-year-old bird) 13,000-year-old giant ammonite shells tracks sloth dung 1 cm 5 cm 1 cm Earth is about 4.6 billion years old. Earliest sign of life are in rocks that formed 3.4-3.8 bya. Evidence for Change over Time -Extinct species 99% of all species That have ever lived Are now extinct Extinction has Occurred continuously Evidence for Change over Time – Transitional features Evidence for Change over Time – V estigial structures (a) The human tailbone is a vestigial trait(b) Goose bumps are a vestigial trait. Monkey for balance Human Erect hair on chimp and locomotion) coccyx (insulation, emotional display)ose bumps •Avestigial trait is a reduced or incompletely developed structure in an organism that has no function or reduced function, but is clearly similar to functioning organs or structures in closely related species. Evidence of Descent from a Common Ancestor Nesomimus Nesomimus Nesomimus Nesomimus parvulus trifasciatus melanotis macdonaldi (a) Pattern: Although the Galápagos mockingbirds are extremely similar, distinct species are found on different islands. There are often striking similarities among island species. For example, Darwin collected mockingbirds from the Galápagos islands. The mockingbirds were superficially similar, but different islands had distinct species. (b) Recent data support Darwin’s hypothesis that the Galápagos mockingbirds share a common ancestor. N. parvulus (Santa Cruz) Galápagos N. parvulus (Santiago) Genovesa islands Marchena N. parvulus (Marchena) Santiago N. parvulus (Isabela) Islands where species are found Santa Cruz Common N. trifasciatus (Floreana) ancestor San Cristobal N. melanotis (San Cristobal) Isabela N. parvulus (Genovesa) Floreana Española 50 km N. macdonaldi (Española) An ancestral population colonized the islands. Over time, the population diversified into Mockingbird species from the Caribbean and Gulf of Mexidistinct species on different islands. Darwin proposed that the mockingbirds were similar because they had descended from a common ancestor. The mockingbird species are part of a phylogeny, a family tree of populations or species. The relationship between different species can be shown on a phylogenetic tree. Evidence of Descent from a Common Ancestor - Homology Example of Developmental Homology Gill pouch Gill pouch Gill pouch Tail Tail Tail Chick Human House cat What is homology? Homology is a similarity that exists in species descended from a common ancestor. Humerus Radius and ulna Carpals Metacarpals Phalanges Turtle Human Horse Bird Bat Seal Example of Structural Homology Genetic homology is a similarity in the DNA sequences of different species. Fly eye For example, the eyeless gene in fruit flies and the Aniridia gene in humans are so similar that their protein produFly leg 90 percent identical in amino acid sequence. How does Natural Selection Work? Darwin’s 4 Postulates 1. Individuals in a population vary in their traits. 2. Some of these differences are heritable; they are passed on to offspring. 3. In each generation, many more offspring are produced than can survive; of these, only 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. Fitness & Adaptation • Biological fitness is the ability of an individual to produce offspring, relative to that ability in other individuals in the population. In biology, an adaptation is a heritable trait that increases an individual's Darwinian fitness in a particular environment relative to individuals lacking that trait. Adaptations increase fitness—the ability to produce offspring. Native Florida Plant Balloon Vine, Cardiospermum corindum Asian Goldenrain Tree, Koelreuteria elegans Example of Current Research ResearchArticle: Carroll, SP and Boyd, C. 1992. Host race radiation in the soapberry bug: Natural history with the history. Evolution 46:1052-1069. Recent Research Case Study 1: Bacterial Resistance • The bacterium Mycobacterium tuberculosis causes tuberculosis (TB), a disease that was once as great a public health issue as cancer is now. • Sanitation, nutrition, and antibiotics such as rifampin greatly reduced deaths due to TB in industrialized nations between 1950 and about 1990. • However, in the late 1980s, rates of TB started to surge due to the evolution of drug-resistant strains. – global health emergency. Organization declared TB a M. tuberculosis in lung tissue 1. A chance 2. Drug therapy kills 3. Mutant cells 4. Drug Therapy is mutation occurs. Mutant most bacteria without proliferate. ineffective against cell the mutation. mutant cells. rpoB gene with Normal rpoB gene CT mutation TCG rpoB gene codes for TTG part of RNA polymerase (mutated RNA polymerase has a shape change) Rifampin Transcription by Rifampin Transcription by binds RNA polymerase binds RNA polymerase tightly slows or stops loosely is efficient infections Vancomycin- resistant Vancomycin use S. aureus S. aureus begins here withesistant to vancomycin Percentage of ICU patients Year • Variation existed in the population. Due to mutation, both resistant and nonresistant strains of TB were present prior to administration of the drug. • The variation was heritable. The variation in the phenotypes of the two strains was due to variation in their genotypes. • There was variation in reproductive success. Only a tiny fraction of M. tuberculosis cells survived the first round of antibiotics long enough to reproduce. • Selection occurred. The cells with the drug-resistant allele had higher reproductive success. Recent Research Case Study 2: Galapagos Finches Peter & Rosemary Grant Princeton University •Peter and Rosemary Grant have done long-term research on the population of medium ground finches found on Isle Daphne Major of the Galápagos Islands. (a) Medium ground finches (Geospiza fortis) Male Female They found that beak form and body size are heritable in these birds. Overall body size has decreased Body size Beak size Beak size has decreased Beaks have Beak shape become more pointed Year Lower Bmp4 expression Higher Bmp4 expression (dark area) in embryo’s beak (dark area) in embryo’s beak 2 mm 2 mm Deep adult beak Shallow adult beak Geospiza fortis Geospiza magnirostris Misunderstandings • Although natural selection appears to be a simple process, research has shown that it is often misunderstood. Perhaps the most important point to clarify about natural selection is that during the process, individuals do not change—only the population does. Acclimation vs. Adapation • Acclimation occurs when an individual’s phenotype changes in response to changes in the environment, but an individual’s genotype remains fixed, so the changes are not passed on to offspring. • In contrast, adaptation occurs when the allele frequencies in a population change in response to natural selection. Bacteria Archaea Common ancestor of all species Flowering living today plants Mosses Tapeworms Vertebrates The branches on the tree represent the relatedness of populations. All of the species have evolved from Fungi a common ancestor. None is “higher” than any other Evolution is not goal-oriented. Individuals with favorable adaptations are successful. Adaptations do not occur because organisms want or need them. Limitations of Natural Selection • Not all traits are adaptive (vestigial traits) • Adaptations that organisms have are constrained in a variety of important ways. These include genetic constraints, fitness trade- offs, and historical constraints CHAPTER 26 EVOLUTIONARY PROCESSES Roadmap 26 EVOLUTION: In this chapter you will learn that Change in Four evolutionary processes change allele frequencies in populations allelic frequencies by starting with a over time Null hypothesis: The Hardy–Weinberg Principle26.1 which makes five assumptions Nonrandom no with respect to a particular gene . . . mating 26.2 no no no no Natural Genetic drift Gene flow Mutation selection 26.3 26.4 26.5 26.6 Four evolutionary processes There are four mechanisms that shift allele frequencies in populations: 1. Natural selection increases the frequency of those alleles that contribute to reproductive success in a particular environment 2. Genetic drift causes allele frequencies to change randomly 3. Gene flow occurs when individuals leave one population, join another, and breed 4. Mutation modifies allele frequencies by continually introducing new alleles Via: • point mutations • Chromosome level mutations • Lateral gene transfer Due to: Bottleneck events GENETIC DRIFT Founder’s effect MUTATION NO(INBREEDING or EVOLUTION SELF-FERTILIZATION) NATURAL SELECTION GENE SELECTION FORCES FLOW May be: • Directional • Stabilizing • Disruptive • Balancing • Sexual Directional selection changes the average value Normal distribution of a trait. Before selection Low High fitness fitness During selection Number of individuals Change in average After selection value Value of a trait (b) For example, directional selection caused average body size to increase in a cliff swallow population. Original population (N 2880) Survivors Change in (N 1027) average value Percentage of birds Body size class Normal distribution Before selection High fitness Low Low fitness fitness During selection Number of individuals Stabilizing selection Reduction reduces the amount of in variation After selection variation in a trait. Value of a trait (b) For example, very small and very large babies are the most likely to die, leaving a narrower distribution of birth weights. Mortality Heavy mortality on extremes Percentage of mortality Percentage of newborn population Birth weight (pounds) Normal distribution Before selection HighLow fitness fitness fitness During selection Disruptive selection Number of individuals increases the amount of variationin After selection Variation in a trait. Value of a trait (b) For example, only juvenile black-bellied seedcrackers that had very long or very short beaks survived long enough to breed. extremese Only the survived extremes survived Number of individuals Beak length (mm) Link to Animation Population size 4 Genetic drift 1 A In this population, the 1llele A drifted • Any change in allele to fixation in less than 20 generations frequencies in a population due to chance (sampling error) In this population, It causes allele Frequency of allelehe allel1 A was lost frequencies to drift up and down randomly Population size 400 over time A • Drift occurs in every population, in every generation It is especially prevalent in small populations Frequency of allele Generation Founder effect Homozygous for allele A 1 Homozygous for allele A 2 Heterozygous New population Immigrants is likely to have establish new population different allele frequencies than the source population, by chance Genetic bottleneck Bottlenecked High mortality population is likely strikes to have different individuals allele frequencies at random than original population, by chance Time 1 Time 2 PingelapAtoll Case Study (a) Bristle shape is a useful genetic marker in fruit flies. Normal bristles Forked (bent) bristles (b) Genetic drift reduced allelic diversity in most populations. At start of experiment, almost all populations have both alleles Both alleles present Only normal allele present (forked lost) Only forked allele present (normal lost) % of experimental populations Generation Inbreeding (b) Effect of extreme inbreeding (self-fertilization) over time A A A A A A Homozygote Heterozygote Homozygote Generation 1 The arrows represent the offspring genotypes 100% 25% 50% 25% 100% that are produced by each parental genotype Generation 2 when self-fertilization occurs (extreme 100% 25% 50% 25% 100% inbreeding) Generation 3 100% 100% Generation 4 Frequency of genotypes • Inbreeding depression • Heterozygote advantage Texas Florida panthers Florida (a component of fitness) Florida panthers Percent survivorship of offspring Age (yr) Gene Flow Homozygous for allele A 1 Time 1 Homozygous for allele A 2 At time 1, populations differ in allele frequencies Population 1 Population 2 Time 2 Gene Gene flow flow causes allele frequencies in the two Gene populations flow to be more alike Population 1 Population 2 (a) Gene flow from mainland to Vlieland More great tits in the eastern Vlieland Eastern population population have island-born Western population grandparents Gene flow is higher to the Gene flow western population Mainland Netherlands (b) In this case, survival rate is highest where gene flow is lowest. Survival rate Great tit (probability of surviving until next year) Number of island-born grandparents Figure 26.16a (a) Wild steelhead trout populations are declining Hood River Case Study - Oregon (b) Captive-bred trout reduce the fitness of wild populations Relative fitness Wild Captive Captive Wild Wild Captive Measuring Gene Flow between Populations • On average, compared to the fitness of fish with two wild parents: – Fish with one captive-bred parent have 16% lower fitness – Fish with two captive-bred parents have 38% lower fitness • This study and a similar study on salmon show that gene flow is occurring from captive-bred to wild populations – In both cases, this reduces the fitness of the wild population What is the role of sexual selection? (a) Male zebra finch (b) Effect of carotenoids Beak color (rank) ControlCarotenoid- supplemented Female kiwis lay a large egg relative to their body size Males compete for the opportunity to mate with females. Male Female BeetleDuring breeding season, males of the beetle Dynastes granti use their elongated horns to fight over femSexual Male Female Dimorphism LionMale lions are larger than female lions and have an elaborate ruff of fur called a mane. If directional selection on a given trait is occurring, you know the _____. • variation present in the trait will increase • variation present in the trait will decrease • mean value for the trait will change • mean value for the trait will stay the same When allele frequencies change because of genetic drift, you expect _____. • the average fitness of the population to increase • the average fitness of the population to decrease • the average fitness of the population to remain the same • The effect of drift on average fitness is not predictable Gene flow _____. • can introduce new alleles into a population • makes allele frequencies across populations more similar • reduces allele frequencies • can introduce new alleles into a population and make allele frequencies across populations more similar Nonrandom mating _____. • is a strong evolutionary force. • often leads to a loss of heterozygotes. • tends to increase the fitness of a population. • is a strong evolutionary force that often leads to a loss of heterozygotes. Hardy-Weinberg Equilibrium • To study how the four evolutionary processes affect populations, in 1908 G. H. Hardy and Wilhelm Weinberg developed a mathematical model to analyze the consequences of matings among all of the individuals in a population. • Hardy and Weinberg wanted to know what happened in an entire population, when all of the individuals—and thus all possible genotypes—bred. • The Hardy-Weinberg principle makes two fundamental claims: 1. If the frequencies of allele1 A and 2 in a population are given by p and q, then the frequencies of genotypes 1 1 , 1 2 , and 2 2 will be given by p , 2pq, and q for generation after generation. 2. When alleles are transmitted via meiosis and random combination of gametes, their frequencies do not change over time. For evolution to occur, some other factor or factors must come into play. For a population to conform to the Hardy- Weinberg principle there are five assumptions that must be met: 1. No natural selection. 2. No genetic drift or random allele frequency changes. 3. No gene flow. 4. No mutation. 5. Random mating. Allele frequencies in parental generation: A1 p = 0.7 A 2 q = 0.3 If q = .4, what is the frequency of A A ? 1 1 © 2011 Pearson Education, Inc. Chapter27 Speciation The Nature of Species Speciation: the process by which new species arise, either by transformation of one species into another, or by the splitting of one ancestral species into two descendant species The Nature of Species Population: any group of individuals, usually of a single species, occupying a given area at the same time Exhibit geographic variation Subspecies: within a single species, individuals in populations that occur in different areas may be distinct Polymorphic species: Cryptic species: Defining a species: Biological Species Concept - One or more populations whose members are capable of interbreeding in nature to produce fertile offspring and do not interbreed with members of another species. Defining a species: Phylogentic Species Concept– For a population to be declared a separate species, it must be the smallest monophyletic group on the phylogenetic tree. Defining a species: Morphological Species Concept – Separating species based on structural differences. Dusky Seaside Sparrow How do new species arise from old ones? Speciation (3 steps) 1. A population becomes reproductively isolated 2. Genetic exchange stops 3. The separated gene pools diverge Reproductive isolating mechanisms Prevent gene flow between species 1. Prezygotic barriers Prevent mating or fertilization 2. Postzygotic barriers Reproductive failure after fertilization 1. Prezygotic Barriers Habitat Isolation Asian lions and tigers are ecologically isolated Behavioral Isolation The male satin Behavioral (sexual) bowerbird isolation builds a bower Required courtship of twigs behaviors to attract females Mating Rituals of birds More Dances Temporal isolation Mating at different times of year Mating at different times of day Gametic isolation Incompatible egg and sperm Molecular recognition on the surface of the cells Mechanical isolation Incompatible genital organs Only large bees brush against the stamens of the white sage 2. Postzygotic barriers Hybrid inviability Hybrid embryos die when genetic regulation fails during development 2. Postzygotic barriers Hybrid sterility Problems during meiosis cause abnormal gametes •Female horse (2n = 64) •Male donkey (2n = 62) •Mule (2n = 63) A. Allopatric speciation Geographically separated populations Most common form of speciation A. harrisi A. leucurus Allopatric speciation in antelope squirrel species separated by the Grand Canyon Sympatric Speciation B. Sympatric speciation Two species of maggot flies are sympatric in the northern half of one fly’s range B. Sympatric speciation Divergence of two populations in the same geographic region Reproductive isolating mechanisms develop and start the speciation process Sympatric speciation in animals Population occupies a new ecological niche No gene flow even though species live in the same area 2 MECHANISMS: • 1) DISRUPTIVE SELECTION • 2) POLYPLOIDIZATION POLYPLOIDY IN PLANTS -Mutation that leads to individuals with multiple sets of chromosomes -Tetraploid & Diploid individuals rarely produce fertile offspring - - reproductive isolation -Common in plants due to unique properties of plants (self- fertilization, hybridization, change of somatic cells to reproductive cells) What happens at the ‘Family Reunions’? • Fusion of the population • Reinforcement of the divergence • Hybridization • Extinction • Creation of a new species Sarracenia minor Sarracenia psittacina Sarracenia formosa (S. minor x psittacina)