Lecture 12 Evolution Key terms Reading Ch 16

Lecture 12: Evolution Key terms: Reading: Ch 16: Microevolution Ch 17: Speciation Ch 18: Macroevolution

Biological Change Over Time Microevolution Macroevolution n. Changes n. Change with in species n. Well defined mechanism n. Easily observed n. Based on selection from one species to another n. Undefined mechanism n. Interpretation of: – Cladistics – Fossil record – Geological data

Microevolutionary Processes n Drive a population away from genetic equilibrium n Small-scale changes in allele frequencies brought about by: – Natural selection – Gene flow – Genetic drift

Microevolution Genetics n n n Microevolution changes a population not individuals Traits in a population vary among individuals Microevolution is change in frequency of traits Natural Selection n Reproductive success for winning phenotypes Acts directly on phenotypes and indirectly on genotypes The first changed individual has no advantage

The Gene Pool n n All of the genes in the population Genetic resource that is shared (in theory) by all members of population Phenotype Variation n n 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

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 No Variation No selection

What happens when the rules are broken?

Rule #1 No Mutation Biological information changes n Each gene has own mutation rate n – What determines rates? n Effect of mutations on selection – Lethal – Neutral – Advantageous

Variation in the gene pool? 1. Recombination • 2. Independent assortment • 3. 5. Meiosis II (haploid germ cells) Fertilization • 4. Crossing over at meiosis I Reorganizing Information Haploid + haploid = diploid Changes in chromosome number or structure Mutations Changing Information

Rule #2 No Immigration n Immigration from a separate, segregated populations – New variation n Alleles n Mutations n Effects of immigration – Shifts allele frequency – Introduces new mutations through breeding

Gene Flow Physical flow of alleles into a population n Tends to keep the gene pools of populations similar n Counters the differences between two populations that result from mutation, natural selection, and genetic drift n

Rule #3 Survival or Reproductive Advantage What does selection do for a population? Survival advantage or Reproductive advantage

Pillars of Natural Selection 1. 2. 3. 4. 5. 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.

Basics of Natural Selection Capacity and Competition All populations have the capacity to increase in numbers n No population can increase indefinitely n Eventually, the individuals of a population will end up competing for resources n

Basics of Natural Selection Capacity and Competition The alleles that produce the most successful phenotypes will increase in the population n Less successful alleles will become less common n Change leads to increased fitness n – Increased adaptation to a specific environment

Results of Natural Selection Three possible outcomes: n Directional selection – Decreases variation in favor of an extreme. n Stabilizing selection – Selects most average/ common form of a trait n Disruptive selection – Selects against intermediate forms

Allele frequencies shift in one direction Number of individuals in the population n Range of values for the trait at time 1 Range of values for the trait at time 2 Number of individuals in the population Directional Selection Range of values for the trait at time 3

n Intermediate forms are favored and extremes are eliminated Number of individuals in the population Stabilizing Selection 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

Number of individuals in the population Forms at both ends of the range of variation are favored n Intermediate forms are selected against n Range of values for the trait at time 1 Range of values for the trait at time 2 Number of individuals in the population Disruptive Selection Range of values for the trait at time 3

Resistance Antibiotic Resistance Bacteria Antiviral Resistance HIV Pesticide Resistance Insects Chemical kills susceptible individuals Resistant individuals survive If resistance is heritable, following generations exhibit the same trait.

Example: Pesticide Resistance Evolution in Action The DDT Paradigm

Preadapted to survive 99% Non-resistant die Spray Pesticide 100% resistant survive

Spray with an Insecticide Second generation survivors

Spray with an Insecticide Third generation survivors

Mutation rate = 1 x 10 -4 100 butterflies or 1 in 10, 000

1 million butterflies Beneficial mutation = 1 x 10 -9 or 1 in 1, 000, 000

Insects Evolve at a High Rate Breeding “super-bugs” in the home?

Sexual Selection n Selection favors certain secondary sexual characteristics n Through nonrandom mating, alleles for preferred traits increase n Leads to increased sexual dimorphism

Balanced Polymorphism - “having many forms” n Occurs when two or more alleles are maintained at frequencies greater than 1 percent n

Sickle-Cell Trait: Heterozygote Advantage Hb. S Allele causes sickle -cell anemia when heterozygous n Heterozygotes are more resistant to malaria than homozygotes n 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 Random change in allele frequencies n Most pronounced in small populations n Sampling error - Fewer times an event occurs, greater the variance in outcome n Fixation: one allele is established in a population n

Founder Effect n n n 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 n n n 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 Generation (500 stoneflies at the start of each) 50

Bottleneck Simulation 100% Gene Frequency AA in five populations 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)

Rule #5 Random Mating

Inbreeding Nonrandom mating between related individuals n Leads to increased homozygosity n Can lower fitness when deleterious recessive alleles are expressed n

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 No Variation No selection

Macroevolution and Speciation 1. Biological evolution is 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 n 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

Variable Morphology Grown in water Grown on land

Isolation and Divergence Reproductive Isolation n 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 n n n Gradual accumulation of differences in the gene pools of populations Natural selection, genetic drift, and mutation can contribute to divergence Gene flow counters divergence

Reproductive Isolation Can’t allow gene flow Prezygotic Isolation n Ecological Isolation n Temporal Isolation n Behavioral Isolation n Mechanical Isolation n Gametic Mortality Postzygotic Isolation n Zygotic mortality n Hybrid inviability n Hybrid sterility 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)

Allopatric Effect n Speciation in geographically isolated populations n Probably most common mechanism n Some sort of barrier arises and prevents gene flow n Effectiveness of barrier varies with species

Extensive Divergence Prevents Inbreeding n Species separated by geographic barriers will diverge genetically n If divergence is great enough it will prevent inbreeding even if the barrier later disappears

Hawaiian Islands n Volcanic origins, variety of habitats n 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 FOUNDER SPECIES

Reproductive Isolation Can’t allow gene flow Prezygotic Isolation n Ecological Isolation n Temporal Isolation n Behavioral Isolation n Mechanical Isolation n Gametic Mortality Postzygotic Isolation n Zygotic mortality n Hybrid inviability n Hybrid sterility Zygote is a fertilized egg

Speciation without a Barrier n Sympatric speciation – Species forms within the home range of the parent species n Parapatric speciation – Neighboring populations become distinct species while maintaining contact along a common border

Speciation by Polyploidy n Change in chromosome number (3 n, 4 n, etc. ) n Offspring with altered chromosome number cannot breed with parent population n Common mechanism of speciation in flowering plants

Possible Evolution of Wheat Triticum monococcum (einkorn) 14 AA Unknown species of wild wheat X 14 BB T. turgidum (wild emmer) CROSS-FERTILIZATION, FOLLOWED BY A SPONTANEOUS CHROMOSOME DOUBLING 14 AB 28 AABB X T. tauschii (a wild relative) 14 DD T. aestivum (one of the common bread wheats) 42 AABBDD

Parapatric Speciation Adjacent populations evolve into distinct species while maintaining contact along a common border BULLOCK’S ORIOLE BALTIMORE ORIOLE HYBRID ZONE

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?

Patterns of Change in a Lineage n Cladogenesis – Branching pattern – Lineage splits, isolated populations diverge – Homology and morphology n Anagenesis – No branching – Changes occur within single lineage – Gene flow throughout process

Evolutionary Trees 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)

Gradual Model n Speciation model in which species emerge through many small morphological changes that accumulate over a long time period n Fits well with evidence from certain lineages in fossil record

Punctuation Model n Speciation model in which most changes in morphology are compressed into brief period near onset of divergence n Supported by fossil evidence in some lineages

Adaptive Radiation Burst of divergence n Single lineage gives rise to many new species n New species fill vacant adaptive zone n Adaptive zone is “way of life” n

Adaptive Radiation

Extinction n Irrevocable loss of a species n Mass extinctions have played a major role in evolutionary history n Fossil record shows 20 or more large-scale extinctions n Reduced diversity is followed by adaptive radiation

Who Survives? Species survival is to some extent random n Asteroids have repeatedly struck Earth destroying many lineages n Changes in global temperature favor lineages that are widely distributed n

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

Jones vs. Smith Returning a cracked kettle 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

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) Fig. 17. 11 p. 268

Fig. 17. 12 p. 269

Mechanism of Evolution Progeny Large Populations Genetic Variability Parental Generation Selection Genetic Variability

Mechanism of Evolution

Factors that cause change Mutations- new alleles n Genetic Drift- unselected random change in allele frequencies n – Genetic Bottlenecks n Founder effect n Inbreeding Gene Flow- moving alleles with mating n Natural Selection n Evolution changes allele frequencies in populations not individuals

Mechanism of Evolution n Variation n – Mutations- new alleles – Natural Selection – Genetic Drift – Gene Flow n Selection – – – Directional Selection Stabilizing Selection Disruptive Selection Survival – Selective forces Abiotic- weather, nature Biotic- diseases Competition n Reproduction – Advantageous traits must be passed to progeny – Ability to pass on the genotype to the next generation is the measure of success