Lecture 12: Evolution Biological Change Over Time Key terms: Reading: Ch16: Microevolution Ch17:Speciation Ch18:Macroevolution Microevolution Changes with in species Well defined mechanism Easily observed Based on selection Macroevolution Change from one species to another Undefined mechanism Interpretation of: Cladistics Fossil record Geological data Microevolutionary Processes Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by: Natural selection Gene flow Genetic drift Microevolution Genetics Microevolution changes a population not individuals Traits in a population vary among individuals Microevolution is change in frequency of traits Natural Reproductive success for winning phenotypes Acts directly on phenotypes and indirectly on genotypes The first changed individual has no advantage 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 1. Pluck hare 2. Grow hair with cold pack Rabbits share genotype but phenotype is dependent on environmental conditions Fig. 10.18, p. 166
Genetic Equilibrium Allele frequencies at a locus are not changing 5 Rules for Equilibrium 1. No mutation 2. No immigration/ emigration 3. Gene doesn t affect survival or reproduction 4. Large population 5. Random mating Interpreted No Variation No Variation No selection No selection No selection What happens when the rules are broken? Rule #1 No Mutation Biological information changes Each gene has own mutation rate What determines rates? Effect of mutations on selection Lethal Neutral Advantageous Variation in the gene pool? 1. Recombination Crossing over at meiosis I 2. Independent assortment Meiosis II (haploid germ cells) 3. Fertilization Haploid + haploid = diploid 4. Changes in chromosome number or structure 5. Mutations Reorganizing Information Changing Information Rule #2 No Immigration Gene Flow Immigration from a separate, segregated populations New variation Alleles Mutations Effects of immigration Shifts allele frequency Introduces new mutations through breeding 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
Rule #3 Survival or Reproductive Advantage What does selection do for a population? Survival advantage or Reproductive advantage Pillars of Natural 1. Individuals of all populations have the capacity to produce more offspring than the environment is able to support, so individuals must compete for resources. 2. 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. 3. 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. 4. 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. 5. 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. Basics of Natural 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 Basics of Natural Capacity and Competition 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 Increased adaptation to a specific environment Results of Natural 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 Directional Allele frequencies shift in one direction Number of individuals Number of individuals Number of individuals 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 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 Intermediate forms are favored and extremes are eliminated Number of individuals Disruptive Number of individuals Stabilizing 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 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 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
Balanced Polymorphism Sickle-Cell Trait: Heterozygote Advantage Polymorphism - having many forms Occurs when two or more alleles are maintained at frequencies greater than 1 percent Allele Hb S 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 Genetic Drift What happens if the population or allele frequency gets wacked? 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 Founder Effect 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 Gene Frequency 100% 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)
Bottleneck Simulation Gene Frequency 100% 50% AA in five populations 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) Rule #5 Random Mating Inbreeding Nonrandom mating between related individuals Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed Genetic Equilibrium Allele frequencies at a locus are not changing 5 Rules for Equilibrium 1. No mutation 2. No immigration/ emigration 3. Gene doesn t affect survival or reproduction 4. Large population 5. Random mating Interpreted No Variation No Variation No selection No selection No selection Macroevolution and Speciation 1. 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. 2. Evolution simply means change over time. Descent with modification occurs because all organisms within a single species are related through descent with modification Biological Species Concept Species are groups of interbreeding natural populations that are reproductively isolated from other such groups. Ernst Mayr
Morphology & Species Variable Morphology Morphological traits may not be useful in distinguishing species Members of same species may appear different because of environmental conditions Morphology can vary with age and sex Different species can appear identical Grown in water Grown on land Isolation and Divergence Reproductive Isolation Can t allow gene flow 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 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 Speciation Allopatric Effect Allopatric Different lands, (physical barrier) Sympatric Same lands (no physical or ecological barrier Parapatric Same border (small hybrid zone) Speciation in geographically isolated populations Probably most common mechanism Some sort of barrier arises and prevents gene flow Effectiveness of barrier varies with species
Extensive Divergence Prevents Inbreeding Species separated by geographic barriers will diverge genetically If divergence is great enough it will prevent inbreeding even if the barrier later disappears Hawaiian Islands Volcanic origins, variety of habitats Adaptive radiations: Honeycreepers - In absence of other bird species, they radiated to fill numerous niches Fruit flies (Drosophila) - 40% of fruit fly species are found in Hawaii Hawaiian Honeycreepers Reproductive Isolation 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 FOUNDER SPECIES Zygote is a fertilized egg Speciation without a Barrier Sympatric speciation Species forms within the home range of the parent species Parapatric speciation Neighboring populations become distinct species while maintaining contact along a common border Speciation by Polyploidy Change in chromosome number (3n, 4n, etc.) Offspring with altered chromosome number cannot breed with parent population Common mechanism of speciation in flowering plants
Possible Evolution of Wheat Parapatric Speciation Triticum monococcum (einkorn) Unknown species of wild wheat CROSS-FERTILIZATION, FOLLOWED BY A SPONTANEOUS CHROMOSOME DOUBLING T. turgidum (wild emmer) T. tauschii (a wild relative) 14AA X 14BB 14AB 28AABB X 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 Are We All Related? Patterns of Change in a Lineage Are all species are related by descent? Do we share genetic connections that extend back in time to the first prototypical cell? Cladogenesis Branching pattern Lineage splits, isolated populations diverge Homology and morphology Anagenesis No branching Changes occur within single lineage Gene flow throughout process Evolutionary Trees Gradual Model 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 extinction (branch ended before present) 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
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 Burst of divergence Single lineage gives rise to many new species New species fill vacant adaptive zone Adaptive zone is way of life Adaptive Radiation Extinction 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 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 Critics of Evolution 1. Critics of Evolution do not propose any alternative hypotheses that can be tested by evidence. 2. The critics selectively use evidence as the basis of their alternative hypotheses. 3. Science is not democratic, the majority of the scientific community rejects the critics regardless of their evidence. 4. There is no controversy
Jones vs. Smith Returning a cracked kettle 1. Smith never borrowed the kettle 2. When Smith returned the kettle it wasn t broken 3. The kettle was already cracked when Smith borrowed it 4. There is no kettle 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 extinction (branch ended before present) dashed line (only sketchy evidence of presumed evolutionary relationship) Fig. 17.11 p. 268 Mechanism of Evolution Progeny Large Populations Genetic Variability Parental Generation 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 Evolution changes allele frequencies in populations not individuals
Mechanism of Evolution Variation Mutations- new alleles Natural Genetic Drift Gene Flow Directional Stabilizing Disruptive 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