Lab Questions

Heritable variation is the raw material for natural selection. Artificial selection by humans mirrors natural selection: individuals with the desired trait are chosen to reproduce, shifting trait frequency over generations.

Seed mass, bristle number, leaf width, or other quantifiable trait measured before and after multiple rounds of selection. Mean and variance tracked across generations.

Mean of selected trait shifts toward selected extreme over generations. Variance may decrease as population becomes more uniform. If selection is relaxed, trait may revert toward original mean.

• Explain why the trait changed over generations using natural selection logic
• Predict what would happen if selection pressure were reversed
• Distinguish artificial from natural selection
• Identify heritable vs. non-heritable variation

Hardy-Weinberg equilibrium describes a non-evolving population. Deviations from predicted genotype frequencies (p²+2pq+q²=1) indicate that one or more evolutionary mechanisms are acting.

Allele frequencies are estimated from phenotype data (recessive phenotype frequency = q²). Genotype frequencies are calculated and compared to H-W predictions across generations.

A population in H-W equilibrium shows stable allele and genotype frequencies across generations. A shift in q² over generations indicates evolution is occurring.

• Calculate q, p, 2pq from a given q² value
• Determine which H-W condition is violated from a described scenario
• Identify whether a population is evolving based on frequency data

DNA sequence similarity reflects evolutionary relatedness. More recently diverged species share more identical base pairs. Molecular phylogenies constructed from sequence data provide independent evidence for evolutionary relationships.

DNA or protein sequences are aligned and compared using BLAST or sequence alignment tools. Percent sequence identity is calculated. A phylogenetic tree is constructed from the similarity matrix.

Closely related species share a higher percentage of identical nucleotides. Phylogenetic trees constructed from molecular data should be consistent with morphological or fossil evidence.

• Explain why more similar sequences indicate closer evolutionary relationship
• Construct or interpret a cladogram from sequence data
• Explain why the universal genetic code supports common ancestry
• Identify a synapomorphy (shared derived character) from sequence data

Water moves by osmosis across a selectively permeable membrane from high water potential (ψ) to low water potential. Solute potential (ψs = −iCRT) and pressure potential (ψp) determine total water potential.

Dialysis tubing or potato/celery cores placed in solutions of varying sucrose concentration. Mass or length change measured after equilibration. The equilibrium concentration (zero mass change) = intracellular solute concentration.

Cells gain mass in hypotonic solutions, lose mass in hypertonic solutions, show zero mass change at the isotonic point. A graph of % mass change vs. sucrose concentration crosses zero at the equilibrium point.

• Calculate ψs from concentration and temperature data
• Determine intracellular solute concentration from the zero-change point
• Predict direction of osmosis from water potential values
• Explain why plant cells become turgid vs. plasmolyzed

Photosynthesis produces O₂ as a byproduct of the light reactions. In the floating leaf disk assay, disks sinking in solution float as O₂ accumulates in intercellular spaces. The rate of flotation = net photosynthesis rate.

Leaf disks vacuum-infiltrated with sodium bicarbonate solution (CO₂ source) are placed under different light intensities. Number of floating disks per unit time is counted. Gross photosynthesis = Net photosynthesis + Respiration rate.

Flotation rate increases with light intensity up to a plateau (light saturation point), after which another factor limits the rate. In darkness, no flotation occurs (no net photosynthesis; respiration consumes O₂).

• Calculate gross photosynthesis rate from net photosynthesis + respiration data
• Explain why the assay measures NET, not gross, photosynthesis
• Identify the limiting factor at the plateau region
• Predict the effect of changing CO₂ concentration, temperature, or wavelength

Aerobic cellular respiration consumes O₂ and produces CO₂. Respirometers measure O₂ consumption by tracking a colored dye or water column displacement. KOH in the chamber absorbs CO₂, isolating the O₂ signal.

Germinating seeds (high metabolic activity) vs. dry seeds (low activity) placed in sealed vials with KOH. Water displacement in a capillary tube is measured over time. Rate = Δvolume / Δtime, corrected for temperature.

Germinating seeds show a higher rate of O₂ consumption than dry seeds. Temperature increases increase the rate up to an optimum. The water/dye column moves toward the organism as O₂ is consumed.

• Calculate respiration rate from volumetric displacement data
• Explain the purpose of the KOH-containing vial (control for CO₂)
• Predict the effect of temperature change on respiration rate
• Identify what the non-germinating seed control establishes

Mitosis produces two genetically identical diploid daughter cells for growth and repair. Meiosis produces four genetically unique haploid gametes. Crossing over during prophase I increases genetic diversity.

Onion root tip slides (mitosis) or prepared slides of meiotic divisions are examined under a microscope. Stage frequency (number of cells in each stage) is counted and used to calculate relative time spent in each stage.

Most cells are in interphase (highest count). Mitotic index = cells in mitosis / total cells. High mitotic index indicates rapid cell division (e.g., cancer tissue, meristematic tissue).

• Identify the stage of mitosis or meiosis from a figure description
• Calculate mitotic index from cell count data
• Explain why cells spend more time in interphase than in mitosis
• Distinguish mitosis from meiosis II based on chromosome configuration

Genetic transformation introduces foreign DNA (a plasmid) into bacteria. Transformed bacteria express the new gene (e.g., GFP or ampicillin resistance). Transformation efficiency quantifies how many cells successfully incorporated the plasmid.

Bacteria treated with CaCl₂ and heat shock to increase membrane permeability. Plated on selective media (LB + ampicillin). Colony count on +pGLO/+amp plates vs. control plates determines transformation efficiency = colonies / μg DNA.

Colonies grow only on plates with both ampicillin and the resistance plasmid. Plates without ampicillin show lawn of growth (non-selective). GFP colonies fluoresce under UV light if arabinose promoter is induced.

• Explain the purpose of the CaCl₂ and heat shock steps
• Calculate transformation efficiency from colony count and DNA mass
• Predict which control plates show growth and why
• Explain why bacteria must be plated on both selective and non-selective media

Restriction enzymes cut DNA at specific recognition sequences, producing fragments of predictable sizes. Gel electrophoresis separates fragments by size — smaller fragments migrate farther. Fragment sizes can be estimated from a standard DNA ladder.

DNA samples are digested with restriction enzymes, then run on an agarose gel. Bands are visualized with ethidium bromide or SYBR dye. Fragment sizes are estimated by comparing band position to the DNA ladder.

A restriction map shows cut sites along the DNA. Expected fragment sizes can be calculated from the map. If two samples produce identical banding patterns, they likely have the same restriction sites (same sequence at those regions).

• Interpret a gel image to determine relative fragment sizes
• Construct a restriction map from fragment size data
• Explain why smaller fragments migrate farther in gel electrophoresis
• Determine whether two DNA samples are identical based on banding patterns

Energy flows through ecosystems via feeding relationships. Only ~10% of energy is transferred from one trophic level to the next (10% rule). The rest is lost as heat via cellular respiration. Primary productivity is the rate at which producers fix energy.

Organisms at different trophic levels are collected and dried for caloric content or biomass measurements. Energy transfer efficiency = (energy at higher level / energy at lower level) × 100%. Net primary productivity (NPP) = GPP − respiration.

Energy pyramids show decreasing energy at higher trophic levels. Transfer efficiency averages ~10%, meaning 90% of energy is lost per level. NPP is always less than GPP.

• Calculate energy transfer efficiency between trophic levels
• Explain why ecosystems can support fewer top predators than primary producers
• Calculate NPP from GPP and respiration data
• Predict the effect of removing a trophic level on ecosystem energy flow

Water moves through plants via the cohesion-tension mechanism: transpiration at leaves creates negative pressure that pulls water up through the xylem. Transpiration rate is affected by environmental factors (light, humidity, wind, temperature).

A potometer measures water uptake rate (proxy for transpiration) as a bubble moves along a capillary tube. Variables tested include light, humidity, fan (wind), and temperature. Rate = bubble distance / time.

Transpiration rate increases with light intensity, low humidity, wind, and high temperature. Stomata open in response to light, increasing transpiration. High humidity reduces the water vapor gradient, reducing transpiration.

• Explain the cohesion-tension mechanism driving water movement
• Predict the effect of environmental changes on transpiration rate
• Calculate transpiration rate from potometer data
• Explain why plants wilt in high temperature or low humidity

Animal behavior has both innate (genetic) and learned components. Courtship behaviors in Drosophila are largely innate and species-specific, serving as pre-zygotic reproductive isolating mechanisms. Mate choice (sexual selection) can drive trait evolution.

Drosophila courtship sequences (tapping, wing vibration, licking, attempted copulation) are observed and timed. Courtship success rate and latency to copulation are quantified. Interspecific mating attempts are compared to intraspecific mating.

Flies preferentially mate with conspecifics (same species). Courtship sequence completion rate is higher between conspecifics than between different Drosophila species. Behavioral isolation is a pre-zygotic isolating mechanism.

• Explain how behavioral isolation contributes to reproductive isolation
• Describe how courtship behavior relates to sexual selection
• Predict the effect of disrupting pheromone signaling on mating success
• Identify the courtship behavior sequence as innate vs. learned

Animals exhibit kinesis (non-directional movement rate change) and taxis (directional movement toward or away from a stimulus). Isopod behavior (preference for moist vs. dry, dark vs. light) is a model for testing stimulus-response relationships.

Isopods placed in a choice chamber with two contrasting conditions (dry/moist, light/dark). Number of isopods on each side recorded at intervals. Chi-square test determines whether distribution differs significantly from 50:50 (null hypothesis).

Isopods show strong preference for moist, dark conditions (consistent with their terrestrial habitat and desiccation avoidance). Chi-square value above critical value (df=1, p=0.05: 3.841) indicates non-random distribution.

• State the null hypothesis for the choice chamber experiment
• Perform chi-square test on observed vs. expected distribution (50:50)
• Determine whether the null hypothesis is rejected
• Explain the adaptive value of the observed behavior