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Evolution Evolution= change in a species, populations, or groups of species over time • Populations vary by the frequency of inheritable traits from one generation to the next • Traits are represented by alleles • Alleles may modify morphology (form or structure), physiology, or behavior • Evolution = changes in allele frequency over time Two areas of Evolutionary Study exist: • MICROEVOLUTION:describes how populations change from one generation to the next • describes how new species originate • focus is on individuals • MACROEVOLUTION: describes changes in groups of related species • covers broad periods of time • determines relationships between species Jean Baptiste Lamarck: early theory of evolution (1809) • 3 important ideas: • Use and Disuse: body parts of organisms can develop with increased usage, while unused parts weaken • Inheritance of Acquired Characteristics: body features acquired during the lifetime of an organism (muscle bulk) could be passed on to offspring. THIS IS NOT CORRECT! • Natural Transformation of Species: offspring produced with changes, future generations will be different Charles Darwin • Charles Darwin (1860) published “The Origin of Species”; proposed the idea of Natural Selection = “Survival of the Fittest” • Studied as a Naturalist aboard the H.M.S. Beagle • Studied extensively in the Galapagos Islands • Darwinism = Natural selection is the driving force • NEO–Darwinism = today’s view, incorporates genetics Natural Selection • the differences in survival and reproduction among individuals in a population due to interaction with the environment • some genotypes provide better traits for coping with the environment • More successful individuals produce more offspring • Superior traits = Adaptations increase fitness • Favorable traits are adaptive (selection acts for the trait) • Unfavorable traits are maladaptive (selection acts against the trait) Darwin’s arguments for Natural Selection: • Populations possess an enormous reproductive potential (Darwin calculated that 2 elephants would produce a population of 19 million after 750 years if all offspring survive!) • Population sizes remain stable • Resources are limited • Individuals compete for survival • There is variation among the members of a species • Variation is heritable • Only the most fit individuals survive • Evolution occurs as advantageous traits accumulate • Result = small changes accumulate over large periods of time • Evidence for evolution does exist and can be observed. • The mechanisms by which evolution occurs have been the cause for debate through the ages! Some of the other beliefs for the origin of life are not based upon science: • Extraterrestrial origin • “Intelligent Design” – religion • “Big Bang” theory • Others? KINDS OF SELECTION: Natural Selection may act upon a population in a variety of ways… • 1. Stabilizing Selection= eliminates individuals that have extreme or unusual traits • Individuals with the most common traits are the best adapted • Frequency of common traits is maintained • Other trait variations are selected against • 2. Directional Selection= favors traits that are at one extreme of a range of traits • Traits of the opposite extreme are selected against • Favored traits become more frequent • Examples: Insecticide resistance, Industrial Melanism (peppered moth) • 3. Disruptive Selection (Diversifying Selection) = occurs when the environment favors extreme or unusual traits, selects against common ones • Examples: weed in Nature are tall, in lawns short • 4. Sexual Selection = the differential mating of males (sometimes females) in a population • Females choose superior males for mates (Female Choice) • Male Competition: strongest males reproduce • Leads to sexual dimorphism = differences in the appearance of males and females • This can lead to disruptive selection SOURCES OF VARIATION • In order for Evolution to occur variation must be present • CONSIDERABLE variation exists in all populations • Variation arises from the following mechanisms: Sources of Variation 1. Mutations = provide the raw material, can invent alleles that never existed before Sources of Variation 2. Sexual Reproduction = new combinations of alleles, genetic recombination • Crossing Over – exchange of DNA between non sister chromatids • Independent Assortment of homologous pairs • Random joining of gametes Sources of Variation 3. Diploidy = copies of each chromosome in a cell; recessive alleles can be hidden Sources of Variation 4. Outbreeding = mating of unrelated partners Sources of Variation 5. Balanced Polymorphism = maintaining different phenotypes in a population • Heterozygote Advantage: Sickle Cell Anemia • Hybrid Vigor: superior quality of offspring resulting from 2 different strains of inbred plants • Frequency Dependent Selection (Minority Advantage): when the least common phenotypes have the biggest advantage Important things to remember: • Not all variation has selective value! • Much is Neutral Variation i.e. fingerprints in humans • The environment often determines whether a variation is neutral or whether it has selective value. Causes of Changes in Allele Frequencies • Natural Selection was Darwin’s mechanism for evolution • With the understanding of genetics, the picture becomes more clear • Other factors can change allele frequency other than natural selection Causes of Changes in Allele Frequencies 1. Natural Selection: increase or decrease in allele frequency due to the impact of the environment 2. Mutations: Introduce new alleles that may provide a selective advantage; most however are deleterious (harmful) 3. Gene Flow:alleles introduced by emigration (leave) vs. immigration (enter) Causes of Changes in Allele Frequencies 4. Genetic Drift: random increase or decrease of alleles; the effect is strongest in small populations • The Founder Effect: Occurs when the allele frequencies in a group of migrating individuals are, by chance, not the same as their population of origin (Amish & Polydactyly) • Bottleneck: occurs when a population undergoes a dramatic decrease in size; the small group left becomes vulnerable to genetic drift (environmental catastrophes) Causes of Changes in Allele Frequencies 5. Nonrandom Mating: mates chosen based on particular traits similar to their own, choose nearby individuals • Inbreeding: individuals mate with relatives • Sexual Selection: females choose males based upon attractive appearance, behavior etc. GENETIC EQUILIBRIUM • Occurs when allele frequencies remain constant from generation to generation • This is known as The Hardy-Weinburg Principle • AT GENETIC EQUILIBIUM THERE IS NO EVOLUTION! In order for this to occur the following conditions must be met: • No Natural Selection • Mutations do not Occur • The population must be isolated from other populations (No gene flow) • The Population is large (No Genetic Drift) • Mating is Random IF ANY OF THESE CONDITIONS IS NOT MET, EVOLUTION WILL TAKE PLACE! Hardy-Weinburg Equations • Genetic Equilibrium is determined by evaluating the following values: • Allele frequencies for each allele (p, q) • Frequencies of homozygotes (p , q ) • Frequency of heterozygotes (pq + qp = 2pq) 2 2 – Also the following two equations hold: ● p + q = 1 (all alleles sum to 100%) ● p + 2pq + q = 1 (all individuals sum to 100%) 2 2 Example Problem • Suppose a plant population consists of 84% plants with red flowers and 16% with white flowers. The red allele (R) is dominant over the white recessive allele (r). • q = 0.16 = White flowered plants (rr trait) 2 • p + 2pq = 0.84 = red flowered plants (RR and Rr traits) 2 • To determine the allele frequency of the white flower allele, calculate q by finding the square root of q q = .016 = 0.4 2 Since p + q = 1, p must equal 0.6 • You can also figure out the percentages of individuals with the homozygous dominant and heterozygous condition 2pq = (2)(0.6)(.4) = 0.48 or 48% = heterozygotes p = (0.6)(0.6) = 0.36 or 36% = homozygous dominant 2 Speciation: the formation of a new species, can occur by way of 3 processes • 1. Allopatric Speciation= a population is divided by some geographic barrier preventing interbreeding • Gene frequencies in each population diverge • If gene pools diverge enough, interbreeding will not take place if the barrier is removed • As a result, new species form Speciation • 2. Sympatric Speciation= formation of a new species without the presence of a geographic barrier • Balanced Polymorphism: ex. Camouflaged insects • Hybridization: two distinctly different forms of a species mate and produce offspring along a geographic boundary; the hybrid eventually diverges from both parent populations Speciation • 3. Adaptive Radiation = relatively rapid evolution of many species from a single ancestor (Darwin’s Finches 14 varieties) Maintaining Reproductive Isolation: mechanisms at work if no physical barrier exists Prezygotic Isolating Mechanisms = mechanisms that prevent fertilization • Habitat isolation: species don’t encounter each other • Temporal isolation: species mate at different seasons or at different times of the day • Behavioral isolation: when a species does not recognize a potential mate because it does not perform the correct mating ritual, display the proper visual signals etc. • Mechanical isolation: male and female genitalia are structurally incompatible • Gametic isolation: male gametes don’t survive in the environment of the female gamete Postzygotic Isolation Mechanisms = prevent the formation of fertile offspring • Hybrid inviability: zygote fails to develop properly and is aborted • Hybrid sterility: Functional adults but they are reproductively sterile (mule) • Hybrid breakdown: offspring have reduced viability or fertility Patterns of Evolution • 1. Divergent Evolution: 2 or more species originating from a common ancestor • 2. Convergent Evolution: 2 unrelated species that share similar traits; occur because each species has independently adapted to similar ecological conditions (not due to shared ancestry) • 3. Parallel Evolution: 2 related species that have made similar evolutionary changes after their divergence from a common ancestor • 4. Coevolution: evolution of one species in response to another (plants & insects, predator & prey) Divergent Evolution Convergent Evolution Parallel Evolution Coevolution Patterns of Evolution 5. Macroevolution: patterns of evolution occurring over extended periods of geologic time • Gradualism: gradual accumulation of small changes • Punctuated Equilibrium: long periods of “status quo” followed by short periods of rapid evolution Evidence for Evolution • 1. Comparative Anatomy = similar anatomy in different organisms •Homologous Structures = similar structures shared due to inheritance from a common ancestor (limb structure) •Vestigial Structures = traces of a structure that no longer serves any useful purpose (hip bones in snakes) •Analogous Structures = similar in purpose but not derived from a common ancestor (wings in birds vs. insects) Homologous Structures Vestigial Structures Analogous Structures Evidence for Evolution • 2. Comparative Embryology = embryos of related organisms develop in similar ways as embryos (vertebrates - gill pouches) • “Ontogeny recapitulates phylogeny”. This means that during an organisms embryological development they will go through stages that resemble the stages they went through during their evolutionary development. Evidence for Evolution 3. Comparative Biochemistry = chemical processes (metabolism) • -genetic code, organic compounds, dependence on water • - 98% of DNA is shared between humans and chimps Evidence for Evolution 4. Paleontology: Transitional fossil forms = show intermediate stages of change (horses) Evidence for Evolution 5. Genetics provides variation - mutations 6. Environmental changes can cause change (Industrial Revolution - Peppered Moth) Evidence for Evolution 7. Normal Distribution = bell shaped curve showing variation in a species , few extremes (rabbit color ranges from light to dark) • Directional Selection = shift in one direction (dark coats) • Disruptive Selection = acts on middle of curve (predation of medium coats) • Stabilizing Selection = selection against the extremes in stable conditions Directional Selection Disruptive Selection Stabilizing Selection Evidence for Evolution 8. Biogeography: unrelated species living in similar environments but in different geographic areas look alike