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Transcript
Lecture 12: Evolution Biological Change Over Time Key terms: Microevolution Macroevolution Changes Change Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by: – Natural selection – Gene flow from one species to another Undefined mechanism Interpretation of: species Well defined mechanism Easily observed Based on selection Reading: Ch16: Microevolution Ch17:Speciation Ch18:Macroevolution Microevolutionary Processes with in – Cladistics – Fossil record – Geological data Microevolution Genetics Microevolution changes a population not individuals Traits in a population vary among individuals Microevolution is change in frequency of traits Natural Selection Reproductive success for winning phenotypes Acts directly on phenotypes and indirectly on genotypes The first changed individual has no advantage – Genetic drift The Gene Pool All of the genes in the population Genetic resource that is shared (in theory) by all members of population Phenotype Variation Two copies of each gene (2 alleles) Inherit different allele combinations Different combinations= different phenotypes Inherit genotype, NOT phenotypes Variation is inherited Genotypes, Phenotypes and Environmental Effects Himalayan rabbit experiment Pluck hare 2. Grow hair with cold pack Rabbits share genotype but phenotype is dependent on environmental conditions 1. Fig. 10.18, p. 166 1 Genetic Equilibrium Allele frequencies at a locus are not changing 5 Rules for Equilibrium 1. 2. 3. 4. 5. No mutation No immigration/ emigration Gene doesn’t affect survival or reproduction Large population Random mating Interpreted What happens when the rules are broken? No Variation No Variation No selection No selection No selection Rule #1 No Mutation Variation in the gene pool? Recombination 1. Biological information changes Each gene has own mutation rate – What determines rates? • Rule #2 No Immigration Immigration from a separate, segregated populations – New variation Alleles Mutations • 5. Meiosis II (haploid germ cells) Fertilization 3. 4. Crossing over at meiosis I Independent assortment 2. Effect of mutations on selection – Lethal – Neutral – Advantageous • Haploid + haploid = diploid Changes in chromosome number or structure Mutations Changing Information Gene Flow Effects of immigration – Shifts allele frequency – Introduces new mutations through breeding Reorganizing Information Physical flow of alleles into a population Tends to keep the gene pools of populations similar Counters the differences between two populations that result from mutation, natural selection, and genetic drift 2 3. Survival advantage or Reproductive advantage 4. 5. Basics of Natural Selection Basics of Natural Selection Capacity and Competition Capacity and Competition All populations have the capacity to increase in numbers No population can increase indefinitely Eventually, the individuals of a population will end up competing for resources – Increased adaptation to a specific environment Results of Natural Selection Three possible outcomes: Directional selection – Decreases variation in favor of an extreme. Stabilizing selection – Selects most average/ common form of a trait Disruptive selection – Selects against intermediate forms The alleles that produce the most successful phenotypes will increase in the population Less successful alleles will become less common Change leads to increased fitness Directional Selection Allele frequencies shift in one direction Number of individuals in the population 2. Individuals of all populations have the capacity to produce more offspring than the environment is able to support, so individuals must compete for resources. Individuals of a population vary in size, form, and other traits. The variant forms of a trait may be more or less adaptive under prevailing conditions. When a form of a trait is adaptive under prevailing conditions, and when it has a heritable basis, its bearers tend to survive and reproduce more frequently than individuals with less adaptive forms of the trait. Over generations, the adaptive version becomes more common in the population. Natural selection is the result of differences in survival and reproduction among individuals of a population that differ from one another in one or more traits. Natural selection results in modifications of traits within a line of descent. Over time, it may bring about the evolution of a new species, with an array of traits uniquely its own. Range of values for the trait at time 1 Number of individuals in the population What does selection do for a population? Pillars of Natural Selection 1. Range of values for the trait at time 2 Number of individuals in the population Rule #3 Survival or Reproductive Advantage Range of values for the trait at time 3 3 Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Example: Pesticide Resistance Resistance Antibiotic Resistance Bacteria Antiviral Resistance HIV Pesticide Resistance Insects Forms at both ends of the range of variation are favored Intermediate forms are selected against Number of individuals Number of individuals in the population in the population Intermediate forms are favored and extremes are eliminated Number of individuals in the population Disruptive Selection Number of individuals in the population Stabilizing Selection Chemical kills susceptible individuals Resistant individuals survive If resistance is heritable, following generations exhibit the same trait. Evolution in Action The DDT Paradigm Preadapted to survive 99% Non-resistant die Spray with an Insecticide Second generation Spray Pesticide 100% resistant survive Second generation survivors 4 Spray with an Insecticide Third generation Mutation rate = 1 x 10-4 Third generation survivors 100 butterflies or 1 in 10,000 Insects Evolve at a High Rate 1 million butterflies Beneficial mutation = 1 x 10-9 or 1 in 1,000,000,000 Breeding “super-bugs” in the home? African Finches Sexual Selection Selection favors birds with very large or very small bills Birds with intermediatesized bill are less effective feeders Number of individuals 60 50 40 30 20 Selection favors certain secondary sexual characteristics Through nonrandom mating, alleles for preferred traits increase Leads to increased sexual dimorphism 10 10 12.8 15.7 18.5 Widest part of lower bill (millimeters) 5 Sickle-Cell Trait: Heterozygote Advantage Balanced Polymorphism Polymorphism - “having many forms” Occurs when two or more alleles are maintained at frequencies greater than 1 percent Allele HbS causes sickle-cell anemia when heterozygous Heterozygotes are more resistant to malaria than homozygotes Malaria case Sickle cell trait less than 1 in 1,600 1 in 400-1,600 1 in 180-400 1 in 100-180 1 in 64-100 more than 1 in 64 Rule #4 Large Population What happens if the population or allele frequency gets wacked? Genetic Drift Small number of individuals start a new population Low probability that allele frequencies are the same as original population Effect is pronounced on isolated islands Bottleneck A severe reduction in population size Causes pronounced drift Results – All progeny will be very similar. – Gene pool very shallow Large Population Simulation 100% Gene Frequency Founder Effect Random change in allele frequencies Most pronounced in small populations Sampling error - Fewer times an event occurs, greater the variance in outcome Fixation: one allele is established in a population 50% allele A neither lost nor fixed 0 1 5 10 15 20 25 30 35 40 45 50 Generation (500 stoneflies at the start of each) 6 Bottleneck Simulation 100% Gene Frequency AA in five populations Rule #5 Random Mating 50% allele A lost from four populations 0 1 5 10 15 20 25 30 35 40 45 50 Generation (25 stoneflies at the start of each) Genetic Equilibrium Inbreeding Allele frequencies at a locus are not changing 5 Rules for Equilibrium Nonrandom mating between related individuals Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed 1. 2. 3. 4. 5. Macroevolution and Speciation 1. 2. Biological evolution is the theory that all living things are modified descendants of a common ancestor that lived in the distant past, or “descent with modification.” Evolution simply means change over time. Descent with modification occurs because all organisms within a single species are related through descent with modification No mutation No immigration/ emigration Gene doesn’t affect survival or reproduction Large population Random mating Interpreted No Variation No Variation No selection No selection No selection Biological Species Concept “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” Ernst Mayr 7 Morphology & Species Variable Morphology Grown in water Morphological traits may not be useful in distinguishing species Grown on land – Members of same species may appear different because of environmental conditions – Morphology can vary with age and sex – Different species can appear identical Reproductive Isolation Isolation and Divergence Reproductive Isolation Cornerstone of the biological species concept Speciation is the attainment of reproductive isolation Reproductive isolation arises as a by-product of genetic change Genetic Divergence Gradual accumulation of differences in the gene pools of populations Natural selection, genetic drift, and mutation can contribute to divergence Gene flow counters divergence Can’t allow gene flow Prezygotic Isolation Ecological Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Mortality Zygote is a fertilized egg Speciation Allopatric Different lands, (physical barrier) Sympatric Same lands (no physical or ecological barrier Parapatric Same border (small hybrid zone) Postzygotic Isolation Zygotic mortality Hybrid inviability Hybrid sterility Allopatric Effect Speciation in geographically isolated populations Probably most common mechanism Some sort of barrier arises and prevents gene flow Effectiveness of barrier varies with species 8 Extensive Divergence Prevents Inbreeding Hawaiian Islands Species separated by geographic barriers will diverge genetically Volcanic origins, variety of habitats Adaptive radiations: – Honeycreepers - In absence of other bird species, they radiated to fill numerous niches If divergence is great enough it will prevent inbreeding even if the barrier later disappears – Fruit flies (Drosophila) - 40% of fruit fly species are found in Hawaii Reproductive Isolation Hawaiian Honeycreepers Can’t allow gene flow Prezygotic Isolation Ecological Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Mortality Postzygotic Isolation Zygotic mortality Hybrid inviability Hybrid sterility Zygote is a fertilized egg FOUNDER SPECIES Speciation without a Barrier Sympatric speciation – Species forms within the home range of the parent species Speciation by Polyploidy Parapatric speciation – Neighboring populations become distinct species while maintaining contact along a common border Change in chromosome number (3n, 4n, etc.) Offspring with altered chromosome number cannot breed with parent population Common mechanism of speciation in flowering plants 9 Possible Evolution of Wheat Unknown species of wild wheat Triticum monococcum (einkorn) 14AA X 14BB T. turgidum (wild emmer) CROSS-FERTILIZATION, FOLLOWED BY A SPONTANEOUS CHROMOSOME DOUBLING 14AB 28AABB X Parapatric Speciation T. tauschii (a wild relative) 14DD T. aestivum (one of the common bread wheats) 42AABBDD Adjacent populations evolve into distinct species while maintaining contact along a common border BULLOCK’S ORIOLE BALTIMORE ORIOLE HYBRID ZONE Patterns of Change in a Lineage Are We All Related? Are all species are related by descent? Do we share genetic connections that extend back in time to the first prototypical cell? Evolutionary Trees Cladogenesis – Branching pattern – Lineage splits, isolated populations diverge – Homology and morphology Anagenesis – No branching – Changes occur within single lineage – Gene flow throughout process Gradual Model extinction (branch ended before present) new species branch point (a time of divergence, speciation) a single lineage branch point (a time of divergence, speciation) a new species a single lineage dashed line (only sketchy evidence of presumed evolutionary relationship) Speciation model in which species emerge through many small morphological changes that accumulate over a long time period Fits well with evidence from certain lineages in fossil record 10 Punctuation Model Adaptive Radiation Speciation model in which most changes in morphology are compressed into brief period near onset of divergence Supported by fossil evidence in some lineages Adaptive Radiation Extinction Who Survives? Species survival is to some extent random Asteroids have repeatedly struck Earth destroying many lineages Changes in global temperature favor lineages that are widely distributed Burst of divergence Single lineage gives rise to many new species New species fill vacant adaptive zone Adaptive zone is “way of life” Irrevocable loss of a species Mass extinctions have played a major role in evolutionary history Fossil record shows 20 or more large-scale extinctions Reduced diversity is followed by adaptive radiation Critics of Evolution 1. 2. 3. 4. Critics of Evolution do not propose any alternative hypotheses that can be tested by evidence. The critics selectively use evidence as the basis of their alternative hypotheses. Science is not democratic, the majority of the scientific community rejects the critics regardless of their evidence. There is no controversy 11 Jones vs. Smith Returning a cracked kettle extinction (branch ended before present) new species 1. 2. 3. 4. Smith never borrowed the kettle When Smith returned the kettle it wasn’t broken The kettle was already cracked when Smith borrowed it There is no kettle branch point (a time of divergence, speciation) a new species branch point (a time of divergence, speciation) dashed line (only sketchy evidence of presumed evolutionary relationship) a single lineage a single lineage Fig. 17.11 p. 268 Mechanism of Evolution Progeny Large Populations Genetic Variability Parental Generation Selection Genetic Variability Fig. 17.12 p. 269 Mechanism of Evolution Factors that cause change Mutations- new alleles Genetic Drift- unselected random change in allele frequencies – Genetic Bottlenecks Founder effect Inbreeding Gene Flow- moving alleles with mating Natural Selection Evolution changes allele frequencies in populations not individuals 12 Mechanism of Evolution Variation – Mutations- new alleles – Natural Selection – Genetic Drift – Gene Flow Selection – Directional Selection – Stabilizing Selection – Disruptive Selection Survival – Selective forces Abiotic- weather, nature Biotic- diseases Competition Reproduction – Advantageous traits must be passed to progeny – Ability to pass on the genotype to the next generation is the measure of success 13