AP Bio Unit 7

Evolution Overview

• Unit 7 focuses on evolution, a central and potentially challenging topic in AP Biology.

• Covers billions of years of biological change.

• Key topics include:

  • Selection (natural, artificial, sexual).

  • Population genetics.

  • Hardy-Weinberg equilibrium.

  • Evidence for evolution.

  • Speciation.

  • Variation in populations.

  • Extinction (species-level and mass extinctions).

  • Phylogeny.

  • Origin of life.

Selection

• Most important concept for understanding evolution is natural selection (Charles Darwin).

Artificial Selection

• Also known as selective breeding.

• Breeders select for desired traits in plants/animals over generations.

• This creates a gene pool with individuals having genes for the desired trait.

• Examples:

  • Brassica oleraceae: Same species but different varieties (cauliflower, broccoli, Brussels sprouts, kale) bred for specific traits (flower clusters, flower buds, lateral buds, leaves).

  • Dogs: All breeds (German Shepherds, Schnauzers, Dachshunds) are varieties of Canus lupus (same species as wolves), bred for specific purposes.

Natural Selection

• Requires inherited variation (genes, recombination, mutation) within a population.

• Reproduction rate exceeds survival rate.

• Survivors possess a beneficial trait providing an advantage.

• Adaptations result from continued mutation and selection over generations.

• Adaptations:

  • Wing structure of a bat.

  • Bat sonar (behavioral adaptation).

  • Camouflage (Satanic leaf gecko).

  • Enzyme-substrate fit (molecular level adaptation).

Sexual Selection

• Selection for traits increasing reproductive success.

• Results in sexual dimorphism (different phenotypes in males/females).

• Types:

  • Intersexual Selection: One sex (typically females) chooses mates of the other sex (typically males).

    • Example: Female turkeys choosing males based on displays, tail feathers, and waddles.

    • Exaggerated in peacocks: colorful males, choosy females.

  • Intrasexual Selection: Competition between males for mates.

    • Example: Elephant seals, where large, aggressive males compete for harems.

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Types of Selection and Phenotype Distribution

• Bell curve represents continuous variation in a population.

• Types of selection:

  • Directional Selection: Selects against one extreme.

  • Stabilizing Selection: Selects against both extremes.

    • Example: Birth weight in babies (average size favored).

  • Disruptive Selection: Selects for both extremes, against the average phenotype.

Adaptive Melanism

• Darkening of the body in a population in response to a darkening environment.

• Evolutionary change at the gene level, not individual tanning.

• Selective pressure typically due to predation.

• Example: Rock pocket mice evolving dark coloration on dark substrates due to a mutation for increased melanin production.

Evolutionary Fitness

• Measured by the number of offspring and offspring's offspring that survive to reproduce, not by strength or speed.

• Fitness applies at every stage of the life cycle.

Peppered Moth Example

• Demonstrates directional selection and adaptive melanism due to environmental change.

• Two forms: peppered (light) and dark.

• Pre-Industrial Revolution: Light moths were predominant due to camouflage on light-colored, lichen-covered trees.

• Industrial Revolution: Soot darkened trees, killing lichens leading to selection for dark moths.

• Post-1960s: Clean Air Act reduced pollution, lichens returned, and light moths became more common again.

• Replicated by Michael Madras, confirming BD Kettlewell's work.

Population Genetics and Hardy-Weinberg

• Population genetics: study of gene distribution and changes in populations over time.

• Key measurement: allele frequency.

• Gene pool: all alleles of all genes in a population.

• Evolution: change in genetic makeup of a population over time (allele frequencies).

Allele Frequency and Dominance/Recessiveness

• Misconception: Dominant allele is always more common.

• Allele frequency is independent of dominance/recessiveness.

• Frequency is based on advantage/harm conferred by the allele or random historical factors.

• Example: Achondroplasia allele is rare but dominant.

Hardy-Weinberg Equations

• p + q = 1

• p^2 + 2pq + q^2 = 1

• p = frequency of dominant allele

• q = frequency of recessive allele

• p + q = 1: Frequency of dominant allele + frequency of recessive allele = all the alleles.

• p^2 + 2pq + q^2 = 1: frequency of homozygous dominant + frequency of heterozygotes + frequency of recessive individuals = all the individuals in the population

• Sample problem: 49% of mice have recessive trait (white fur).

  • q^2 = 0.49

  • q = ilda{0.7}

  • p = 0.3

  • 2pq = 0.42

  • p^2 = 0.09

Hardy-Weinberg Principle

• Allele frequencies in a gene pool remain constant unless certain conditions are not met.

• Hardy-Weinberg population (fictional) helps understand causes of evolutionary change in real populations.

• Conditions:

  1. Infinitely large population.

  2. No harmful or beneficial alleles.

  3. Random mating.

  4. No emigration or immigration.

  5. No net mutation.

• Violation of any condition leads to allele frequency changes (evolution).

• Factors causing evolution:

  • Small populations (genetic drift).

  • Natural selection (advantageous/harmful alleles).

  • Sexual selection (non-random mating).

  • Gene flow (migration).

  • Directional mutation.

Genetic Drift

• Random change in allele frequencies, often in small populations.

• Population bottleneck: a type of genetic drift due to biotic or abiotic factors wiping out large percentage of the population.

  • Cheetahs: Evidence of population bottleneck, resulting in low genetic diversity from a viral infection that wiped out the cheetahs down to a few individuals.

• Founder effect: Small number of individuals found a new population.

  • Allele frequencies in founders may differ from the parent population due to insufficient sampling.

Gene Flow

• Movement of alleles from one population to another (migration or gametes - for example, movement of individuals or pollen).

• Changes allele frequencies, reduces differences between adjacent populations.

Mutation

• Ultimate source of genetic variation.

• Directional mutation can change allele frequencies.

Sickle Cell Disease: Population Genetics Example

• Caused by recessive allele.

• High frequency in certain populations due to heterozygote advantage.

• Point mutation in hemoglobin gene: valine replaces glutamic acid.

• Heterozygotes experience mild sickling, resistant to malaria.

• Homozygous dominant (big S big S) selected against because of malaria.

• Heterozygous dominant (big S little s) selected for due to no sickle cell disease but protection against the malaria parasite.

• Homozygous recessive (little s little s) selected against due to sickle cell disease.

• Map shows correlation of sickle cell allele and malaria intensity.

Evidence of Evolution

Homologous Traits

• Share common underlying structure and embryological origin.

• Demonstrate descent with modification from a common ancestor.

• Example: Forelimbs of humans, dogs, birds, and whales.

• Result from adaptive radiation (one parent species producing diverse descendants).
  *Homologous structures may have different functions.

Vestigial Structures

• Type of homology providing evidence for evolution.

• No apparent function but inherited from an ancestor where it had a function.

• Examples:

  • Whale pelvis.

  • Human coccyx (tailbone).

Homology vs. Analogy

• Analogous features: Similar function, different underlying structure.

• Arise from convergent evolution, not adaptive radiation.

• Example: Hydrodynamic form of sharks, ichthyosaurs, and dolphins (convergent solution to swimming).

• Wings of birds and bats are analogous as wings, due to convergent evolution of flying, but the forelimbs are homologous as a bone structure.

Molecular Homologies

• Homologous features at the molecular level.

• Molecules indicate a common ancestry based on structure and monomer sequence.

• Example: Hemoglobin in vertebrates has similar structure.

• Closer morphologies correlate to closer sequence

  • Humans and gorillas have a one amino acid difference in hemoglobin.

  • Rhesus monkeys and humans have eight difference in hemoglobin.

  • Mice and humans have 27 difference in hemoglobin.

  • Chickens and humans have 45 difference in hemoglobin.

  • Frogs and humans have 62 difference in hemoglobin.

Pseudogenes

• Non-functional genes, variants of functional genes in related species.

• Example: GLO pseudogene (non-functional vitamin C synthesis gene) in humans and primates.

• Molecular vestigial feature.

• GLO Pseudogene also found in guinea pigs and bats, however they all have different mutations.

• Convergent features that are analogous and not homologous.

Universal Molecular Homologies

• Show that all living things are cousins.

• Include:

  • DNA as genetic material.

  • ATP for energy coupling.

  • Universal genetic code.

  • Ribosomes for protein synthesis.

  • Shared metabolic pathways (glycolysis, Krebs cycle, electron transport chain, chemiosmosis).

• Go back three point eight billion years to the origin of life.

Eukaryotic Molecular Homologies

• Go back one point eight billion years.

• Include:

  • Nucleus.

  • Mitochondria.

  • Endomembrane system.

  • Genes with introns.

  • Linear chromosomes.

  • Sexual reproduction.

Embryological Development

• Early embryos of vertebrates look similar.

• Differentiation leads to adult body forms.

• Similarity indicates common ancestry.

• Descent with modification leads to elaboration of form.

• Embryos show Vestigial features, tail in humans, the pharyngeal gill slit in humans

Shared Genes for Animal Development

• Shared genes control processes in animals separated by millions of years.

• Example: Eyeless gene (master switch for eye development) in arthropods and vertebrates.

• Homeotic genes (determine body plan) are common across diverse animals.

Biogeography

• Study of geographic distribution of species.

• Distribution patterns fit the spread of populations from one area to adjacent ones.

• Marsupials primarily in Australia due to its isolation.

• Parallel evolution: marsupials filling niches typically occupied by placental mammals elsewhere.

Fossils

• Petrified remains of living things, evidence of change over time.

• Transitional forms show descent with modification.

• Relative dating: based on superposition (younger layers on top of older layers).

• Absolute dating: based on the decay of radioactive isotopes (half-life).

  • Carbon-14 decays to nitrogen-14 (half-life of 5,730 years).

Evolution of Resistance to DDT in Mosquitoes

• Observed example of continuing evolution.

• Mosquitoes develop resistance to DDT over time due to genetic changes.

• Also observed in antibiotic resistance in bacteria, herbicide resistance in weeds, and chemotherapy drug resistance.

Speciation and Extinction

Biological Species Concept

• Species: group of organisms that can naturally interbreed to produce viable, fertile offspring and are reproductively isolated from other groups.

• Limitations:

  • Hybridization in closely related species.

  • Extinct or asexual species.

  • Prokaryotic species.

Reproductive Isolating Mechanisms

• Keep gene pools of closely related species separate.

• Prezygotic: prevent mating or fertilization:

  • Behavioral: different mating rituals.

  • Temporal: breeding at different times.

  • Mechanical: structural barriers.

  • Habitat: different habitats.

  • Gametic: egg and sperm incompatibility.

• Postzygotic: result in inviable or infertile hybrids.

  • Hybrid inviability: hybrid organisms don't develop.

  • Hybrid sterility: hybrid offspring are healthy but can't reproduce (e.g., mules).

  • Hybrid breakdown: F2 generation of hybrids are inviolable or infertile.

Allopatric vs. Sympatric Speciation

• Allopatric: geographic barrier leads to genetic differentiation and reproductive isolation.

• Sympatric: occurs without a geographic barrier.

  • In plants: polyploidy (change in chromosome number).

  • In animals: sexual selection (e.g., cichlids in Lake Victoria); adaptation to microhabitats (e.g., lice on different parts of birds).

Adaptive Radiation

• One parent species produces several descendant species with unique adaptations filling different niches.

• Galapagos finches: example of adaptive radiation from a single South American species.

• Phylogeny and homologous/vestigial traits reflect adaptive radiation.

Importance of Phenotypic Variation

• Essential for evolution; raw material for natural selection.

• Natural selection acts on phenotype NOT genotype.

• No variation = No selection = No adaptation = Extinction.

Examples of Variation

• Phospholipid Structure: saturated tails in extremities, unsaturated tails in bodycore.

• Hemoglobin: Fetal hemoglobin (higher oxygen affinity) vs. adult hemoglobin - Fetal form has gamma chains, adult has beta chains.

• Chlorophyll Types: chlorophyll A (red light) and chlorophyll B (blue light) which allow for the increase of total wavelength of sunlight absorbed.

Extinction

• Normal part of life's process.

• >99% of species that have ever lived are extinct.

• Extinction vortex: decline caused by change, a decline, or an extinction vortex

• Leads to genetic drift

• Leads to less variability

• Positive feedback loop causes decline to continue leading to extinction.

Mass Extinction

• Widespread, rapid decrease in biodiversity.

• Caused by geological / astronomical events.

• At least 5 major events; the Cretaceous extinction wiped out the dinosaurs.

• Mass extinction leaves niches vacant allowing for adaptive radiation.

Human Impact on Extinction

• Humans are causing a sixth extinction.

• Decline due to habitat destruction, overhunting, invasive species, etc.

Phylogeny

• Phylogeny: evolutionary history.

• Phylogenetic tree (evolutionary tree): branching diagram showing evolutionary relationships. Built using morphological, molecular, or genetic evidence.

• A claim based on evidence.

Clade

• Group of organisms with a common ancestor and all its descendants.

Shared Derived Character

• Trait distinguishing a clade, evolved in common ancestor.

Nodes and Sister Groups

• Node: where branches diverge, representing a common ancestor.

• Sister groups: descendants splitting from the same node.

Outgroup

• Distant group used to determine relationships among the in-group.

Biggest Mistake in Phylogenetic Analysis

• Misconception that vertical closeness indicates evolutionary closeness.

• Only recency of common ancestry matters.

• Nodes can rotate.

Ancestral Feature

• Trait shared by a clade and larger clades; doesn't define clade.

Evidence for Phylogenies

• Pre-1960s: morphological similarities.

• Post-1960s: nucleotide and amino acid sequences (DNA, RNA, proteins).

Molecular Clocks

• Rate of change (mutations) is constant over time.

• Calibrate change to fossil record.

• Use rate to determine when species split apart.

Origin of Life

• Key question: how did life emerge naturally?

• How did the first cells emerge without preexisting cells?

• First enzymes how do complex protiens get there with enzymes?

Key Steps for Origin of Life

1. Earth became habitable, more stable (4.5 billion years ago).

2. Abiotic synthesis of monomers (amino acids, nucleotides, fatty acids).

3. Abiotic synthesis of polymers from monomers; formation of vesicles.

4. Combine monomers and polymers into protocells.

5. Emergence of self-replicating cells (RNA). Origin of heridity

Miller-Urey Experiment

• Showed amino acids could be synthesized in an abiotic setup simulating early Earth environment.

• Setup: sterile apparatus with chambers and tubings; simulated early oceans and atmosphere (methane, ammonia, hydrogen, water vapor - NO oxygen); sparks simulated lightning.

• Found amino acids; proof of concept.

• Details may be wrong, but model for subsequent experiments producing monomers.

RNA World

• The widely believed the molecule of heredity was once RNA not DNA.

• RNA stores genetic infomation.

• Catalytic ability by the ribosome can stitch together monomers to polymers.

• Self-replicating RNA systems are subject to natural selection, become complex.

• RNA molecules complexs fold up and act on the world because of enzymatic proteins.

Key steps for RNA world

1. Inorganic precursor molecules (Miller-Urey experiment).

2. Abiotic processes to fomr monomers of RNA.

3. Other abiotic processes to form RNA polymers.

4. Complex shapes of RNA can start to have enzymatic properties.

5. Self-replicating systems of RNA leads system can repeat itself.

6. Those systems are encapsulated within lipid bilayers form proto cells.

7. Natural selection gives emergence to last universal common ancestor.

Last Universal Common Ancestor (LUCA)

• Population giving rise to archaea, bacteria, and eukaryotes.

• Features:

  • Lipid bilayer.

  • DNA as genetic material.

  • RNA for information transfer.

  • Ribosomes for protein synthesis.

  • Membrane channels for matter/energy flow.

  • Enzymes combining monomers to polymers.

  • ATP synthase.