AP Biology Review

Hydrolysis

Chemical reaction involving the addition of water to split a larger molecule into two smaller ones. This is a catabolic process.

Dehydration Synthesis

Chemical reaction involving the removal of a water molecule to combine two smaller molecules into a larger one. This is an anabolic process.

Adhesion

The attraction between water molecules and different substances, such as water adhering to plant cell walls. This property is crucial for processes like capillary action in plants.

Cohesion

The attraction between water molecules. This property contributes to surface tension and the formation of droplets.

Water Potential

(Ψ = Ψs + Ψp) This number always has to be less than zero. Water moves from areas of high water potential to low water potential, such as in a tree water moves upwards and evaporates because the water potential is lower higher up. This is measured in ‘bars.’

Solute Potential (Ψs)

This is determined by the concentration of solutes in a solution. The more solutes, the lower the solute potential (and thus the water potential), as water molecules are drawn to the solutes, making them less likely to move.

Ψs = -iCRT

• i is the ionization constant (number of ions a solute dissociates into)

• C is the molar concentration of the solute

• R is the pressure constant

• T is the temperature in Kelvin

Ionization Constant (i)

The number of ions a solute dissociates into when dissolved in a solution.

Ions like glucose and sucrose do not ionize in water, therefore i = 1. Whereas ions like NaCl dissolve into 2 making i = 2, and CaCl² i = 3.

Molar Concentration (C)

This is the moles/Liter of the solute in the water, or the molarity of the solution.

Pressure Constant (R)

This is always 0.0831 liter bars/mole K

Kelvin (T)

273 + C = T

Pressure Potential (Ψp)

Plants have cell walls, therefore their walls exert pressure on them, which is what this number is.

Osmosis

The movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration. Osmolarity is the total solute concentration in a solution.

Active Transport

The movement of ions or molecules across a cell membrane against their concentration gradient, requiring energy (typically from ATP).

• Endocytosis- outside to inside

  • Phagocytosis- large particles

  • Pinocytosis- extracellular fluid with dissolved substances

  • Receptor-Mediated Endocytosis- receptor proteins on cell used to capture specific target molecules

• Exocytosis- inside to outside

  • Proteins (signaling), Hormones, Waste

Passive Transport

The movement of substances across a cell membrane without the need for energy input, typically down their concentration gradient. NO ATP

• Diffusion: molecules moving from high to low concentrations (small nonpolar)

• Facilitated Diffusions: high to low through transport proteins, hydrophilic molecules and ions

Proteins

Protein function is dependent on its shape. Proteins have 4 levels of structure: primary, secondary, tertiary, and quaternary.

1. The sequence of amino acids in the polypeptide chain

2. The initial folding (alpha helices or beta sheets) of the polypeptide due to H-bonds

3. The interactions between the R groups of the amino acids

4. The overall 3D shape formed by multiple polypeptide chains

Amino Acids

Building blocks of proteins, consisting of a central carbon atom, an amino group, a carboxyl group, and a variable R group that determines their identity.

DNA

The carrier of genetic information in living organisms, composed of two strands that form a double helix structure, with sequences of nucleotides encoding the instructions for protein synthesis. It runs double parallel, with one strand in the 5’ to 3’ and the other in the 3’ to 5’ direction.

RNA

A nucleic acid that plays a crucial role in coding, decoding, regulation, and expression of genes. It is involved in protein synthesis and exists in several forms, including mRNA, tRNA, and rRNA. It does not have Thymine as a base and has Uracil instead, it is also made of a ribose base, not a deoxyribose base like DNA.

Ribosomes

They synthesize proteins according to mRNA, which originate in the genome of a cell, reflecting a common ancestry between prokaryotes and eukaryotes. Ribosomes have two subunits (not membrane enclosed) and are made of rRNA and proteins (tRNA).

Nucleus

Unique to eukaryotic cells, the nucleus is an organelle that houses the cell's genetic material, managing gene expression and coordinating cell activities such as growth and reproduction.

Endoplasmic Reticulum

Network of membrane tubules within cytoplasm of eukaryotes.

1. Rough ER

   • Ribosomes in membrane

   • Packages newly synthesized proteins for possible export from cell

2. Smooth ER

   1. Lacks ribosomes

   2. Functions are detox and lipid synthesis

Golgi Complex

Series of flattened membrane bound sacs in eukaryotes. Involved in proper folding and chemical modification of new proteins and packaging for protein trafficking (through transport vesicles).

Mitochondria

Has a double membrane and through cellular respiration and is known as the powerhouse of the cell as it produces ATP. Its outer membrane is smooth, inner is folded into cristae.

1. The Krebs Cycle (citric acid cycle) occurs in the matrix (inner)

2. Electron transport and ATP synthesis occur in inner mitochondrial membrane

   • Folding of the inner membrane increases surface area which allows for more ATP creation (through cristae folding)

Lysosomes

Contains hydrolytic enzymes to digest damaged cell parts or macromolecules.

• Intracellular digestion

• Apoptosis

Vacuoles

Membrane bound sacs in eukaryotes.

• Used for storage (of water and macromolecules), and for the release of waste

• Helps maintain turgor pressure

Chloroplasts

Found in eukaryotes like algae and plants, they capture energy from the sun and convert it into sugars. Has a double outer membrane.

1. Thylakoids

   • Highly folded membrane components stacked in grana

   • Containing chlorophyll pigments that comprise the photosystems and ETC proteins can be found between photosystems embedded in the thylakoid membrane

   • Light dependent reactions occur here

   • More folding = higher efficiency

2. Stroma

   1. Fluid between inner chloroplast membrane outside thylakoids

   2. Carbon Fixation reactions occur here

Surface Area to Volume Ratio

This is important because it limits the size of cells, as a larger SA:V ratio will yield in greater efficiency and transport/exchange of materials with the environment.

Adaptations for Better Exchange with Environment (SA:V)

Membrane Folding can be used to increase SA through

• Villi and Microvilli on Small Intestine

• Root Hairs on plants

Bigger animals have a harder time exchanging heat with their environment.

• Leaves use stomata to increase exchange with environment.

Cholesterol

A type of steroid used to regulate the bilayer fluidity, wedged between phospholipids in the cell membrane of eukaryotes.

Phospholipids

Are amphipathic and have a hydrophilic, polar phosphate head, and a hydrophobic, nonpolar fatty acid tail to form a bilayer in aqueous environment.

Membrane Proteins

Peripheral Proteins

• Loosely bound to surface

• Hydrophilic with charge and polar side groups

Integral Proteins

• Span the membrane

• Hydrophobic with nonpolar side groups in the hydrophobic interior of bilayer and hydrophilic with charged and polar side groups on the exterior (like transmembrane proteins)

Functions

1. Transport

2. Cell-Cell Recognition

3. Enzymatic Activity

4. Signal Transduction

5. Intercellular joining

6. Attachment to extracellular matrix or cytoskeleton

Fluid Mosaic Model

The structure of the phospholipid bilayer is fluid and held together by hydrophobic interactions weaker than covalent bonds, allowing some lipids and proteins to flow along the surface of the membrane or across the bilayer.

Carbohydrates in the Fluid Mosaic Model

Both function as markers (cell markers)

1. Glycoproteins: one or more carbohydrates attached to membrane proteins

2. Glycolipids: lipid with one or more carbohydrates attached

Selective Permeability

Small Hydrophobic Molecules: can easily pass through membrane (ex: O₂, CO₂)

Large Polar, Hydrophilic Molecules: cannot move freely across the membrane and need the help of transport proteins:

• Channel Proteins: hydrophilic tunnel allows specific target molecules to pass through

• Carrier Proteins: spans the membrane and changes shape to move a target molecule from one side to the other

Cell Walls

Made of complex carbohydrates:

• Plants: Cellulose (polysaccharide)

• Fungi: Chitin (polysaccharide)

• Prokaryotes: Peptidoglycan (polymer of sugar and amino acids)

Facilitated Diffusion

Movement of molecules from high to low concentrations through transport proteins, and example of active transport.

• Aquaporins for water

• Charged ions like Na+ and K+ (Carrier Proteins- Pumps) (can create membrane potential through the pumping of charged ions)

• Large and small polar molecules

Cotransport, secondary active transport using energy from electrochemical gradient to transport two different ions across the membrane through a protein.

• Symport- two different ions in same direction

• Antiport- two different ions in opposite directions

Tonicity

Hypertonic Environment: more solute and less solvent than inside the cell (water will flow out of cell)

Isotonic: equal concentrations of solute and solvent to the cell

Hypotonic: less solute and more solvent than the cell (water will flow in)

• Animal cells lyse (explode)

• Plant cells are turgid

• Paramecium use contractile vacuole to pump water out

Enzymes

Biological catalysts that speed up biochemical reactions (most enzymes are proteins and their shape is necessary for their function).

• Active Site that interacts with substrates with a unique shape and charge (can be changed slightly to fit the substrate)

• Enzyme names indicate chemical reaction or substrate (Ex: Sucrase digests sucrose)

• Reusable, not changed by reactions

Activation Energy

Biochemical reactions need a starting energy called the activation energy

• Some reactions result in a net release of energy, and some result in a net absorption of energy

• Reactions resulting in a net release of energy need less activation energy than those resulting in a net absorption of energy

• Enzymes lower the activation energy, thereby accelerating the rate of reactions

Controls

Negative control: not exposed to experimental treatment or any treatment known to have an effect

Positive control: exposed to a treatment with a known effect, not exposed to the experimental treatment

Denaturation

• Changes in environmental temperature and pH can lead to denaturation (loss of enzyme shape), and therefore enzyme catalytic ability is lost or decreased

  • Higher temperatures result in denaturation, but lower temperatures generally just slow down the reaction rate without denaturation

  • Denaturation can occur at both increased or decreased pH because the change in hydrogen ion concentration can disrupt the H-bonds in enzyme structure

• Most times this is irreversible

pH

pH measures the concentration of hydrogen ions in a solution. It is measured on a logarithmic scale so a pH of 6 has 10x more hydrogen ions in a solution than a pH of 7.

Low pH is acidic, high pH is alkaline or basic.

Inhibitors

Competitive Inhibitor

• Can bind reversibly or irreversibly to active site of an enzyme

• Competes with substrate for enzyme’s active site

Noncompetitive Inhibitor:

• Bind to an enzyme’s allosteric site, not the active site, and thereby change the shape of the enzyme

Second Law of Thermodynamics

• Every energy transfer increases the disorder of the universe

• Living cells are not at equilibrium; there is a constant flow of materials in and out of them

• Cells manage energy resources through energy coupling (energy releasing processes drive energy storing processes)

Photosynthesis: Light Dependent Reactions

Light-dependent Reactions: capture light energy through pigments

• Pigments transform light energy into chemical energy

  • Chlorophyll: capture sun energy into high-energy electrons

  • Energy from these electrons is used to establish a proton gradient and reduce NADP+ to NADPH

• Chemical energy is temporarily stored in the chemical bonds of carrier molecules called NADPH

• Help facilitate ATP synthesis

• ATP and NADPH (products of light-dependent) store chemical energy to power the Calvin Cycle (will be reactants)

• Oxygen is produced as a result of water hydrolysis

Thylakoid Membrane and the ETC:

• Photosystem (PS) II (is actually first) and uses hydrolysis of water and releasing of hydrogen molecules released into the thylakoid space to create a proton gradient

• ATP synthase at the end of the ETC uses the proton gradient to turn ADP into ATP

Photosynthesis: Light-Independent Reactions (The Calvin Cycle)

The Calvin Cycle: uses ATP, NADPH, and CO₂ to make glucose.

1. Carbon Fixation- Stroma: CO₂, enzyme RuBisCO, and RuBP (five atoms of carbon and a phosphate group on each end)

   • RuBisCO catalyzes a reaction between CO₂ and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds.

2. Reduction: ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P. (involves the gain of electrons) The molecules of ADP and NAD⁺, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized.

3. Regeneration:

   • One of the G3P molecules leaves the Calvin cycle to form glucose (C₆H₁₂O₆), because it has 6C, it takes six turns of the Calvin cycle to make one

   • The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP.

Fermentation- Anaerobic Respiration

Glucose is broken down through glycolysis and uses 2 ATP to create pyruvate. If oxygen is not present at this point, fermentation occurs to create either lactic acid or ethanol.

Cellular Respiration

Glucose is broken down through glycolysis (in cytoplasm) and uses 2 ATP to create pyruvate. If oxygen is present at this point, cellular respiration occurs.

1. Glycolysis— in the cytoplasm

   • Results in pyruvate, NADH, and ATP

   • 2 pyruvate per one molecule of glucose is then transported actively through the mitochondrial membranes into the matrix

2. Pyruvate Oxidation— in mitochondria

   • Products are NADH+ which becomes NADH and H+, CO₂, and Acethyl CoA

   • Acethyl CoA then enters the Krebs Cycle

3. Krebs Cycle (Citric Acid Cycle)— in mitochondria

   • High energy electrons are transferred to NADH and FADH₂

   • ADP is phosphorylated forming ATP

4. ETC— in inner mitochondrial membrane and internal membrane of chloroplasts (in prokaryotes— ETC is in plasma membrane)

   • Oxidative Phosphorylation

   • Electrons delivered by electron carriers (NADH and FADH₂ to the ETC)

   • Use of Active Transport

   • ATP synthase uses proton gradient to synthesize ATP

   • Decoupling oxidative phosphorylation (proton gradient not used by ATP synthase) is energy released as heat used by endothermic organisms to regulate body temperature

5. Creation of up to 36 ATP

Short Distance Cell Communication

Cell sounds out local regulators (signals), and the target cell is within a short distance of the signal (local signaling). This is often used to communicate with cells of the same type.

Long Distance Cell Communication

The target cell is not in the same area as the cell emitting the signal, and so the signal travels a long distance to reach the target cell, which is usually a cell of a different type.

Immune Cell Communcation

They communicate through cell-to-cell contact. The B-cell is triggered by the binding of specific antigens to the B-cell receptor, which sets off further signals.

Neurotransmitter Communication

Exocytotic vesicles release signals and those signals travel short distances across small gaps between cells, then attach to the target cell.

Signal Transduction Pathway (RTR)

1. Reception of the signal from outside the cell

2. Transduction converts the signal to form that can obtain a cellular response (signal transduction pathway)

   • Phosphorylation cascade

3. Response

   • Regulation of protein synthesis by turning genes in the nucleus on/off

   • Regulation of protein activity in cytoplasm

4. Chemicals can either inhibit or activate pathways (such as blocking neurotransmitters, or activation of a receptor protein)

Transduction In Depth

1. Ligand (chemical messenger- hormone) binds to the receptor on the outside of the cell membrane (ex: G protein-coupled receptors in eukaryotes)

   • GDP —> GTP (G protein coupled receptor is activated by ligand

   • GTP then activates Adenylate cyclase and ATP is released

   • Which is then converted to cAMP (second messenger) which activates protein kinases which amplify the signal

2. Sidenote- steroid transduction is different because the steroid simply passes through the membrane then enters the nucleus immediately to activate the production of a protein

Quorum Sensing

Signal mechanism used by bacteria where they release signals to their neighbors. This type allows bacteria to sense changes in population density and they can then act as a group to respond to environmental change.

Positive Feedback

Amplify responses and processes, one variable initiates a response and this disrupts homeostasis and moves further and further from it. (creates more change) Amplifies the change.

• Example: Ethylene release and ripening of apples, birth

Negative Feedback

If a system is disrupted, negative feedback mechanisms works to reset it back to its normal state, maintaining homeostasis.

The Cell Cycle

Interphase: growth and preparation

1. G1: cell growth

2. S: DNA replicated

3. G2: cytoplasmic components doubled

M-Phase: division of nucleus and cytoplasm, results in TWO GENETICALLY IDENTICAL DAUGHTER CELLS

1. Mitosis: growth, repair, asexual reproduction

   1. Prophase:

      1. Nuclear envelope begins to disappear

      2. DNA coils into chromosomes

      3. Fibers move double chromosomes toward center of cell

   2. Metaphase:

      1. Fibers align double chromosomes across cell

   3. Anaphase

      1. Fibers separate double chromosomes into single ones (chromatids) at the centromere

      2. Migration of chromatids to opposite sides of cell

   4. Telophase:

      1. Nuclear envelope reappears

      2. Each nucleus contains a complete genome and chromosomes begin to uncoil

2. Cytokinesis: division of cytoplasm

G0 Phase: no more cell division

Cell Cycle Checkpoints

G1 Checkpoint: at end of G1 Phase

• Cell size check

• Nutrient check

• Growth Factor check

• DNA damage check

G2 Checkpoint: at end of G2

• DNA replication check

• DNA damage check

M-spindle Checkpoint: at end of metaphase

• Fiber attachment to chromosome check (for proper division)

Cyclins and Kinases

Cyclins: group of related proteins associated with specific cell cycle phases

• different cyclins at different stages

• concentrations fluctuate on cell activity

• used to promote cell cycle progression—> degraded to inhibit cycle progression

• used to activate cyclin-dependent kinases (CDKs) (cyclins are specific to their CDK)

CDKs: group of enzymes in cell cycle regulation

• requires cyclin binding for activation

• phosphorylate substrates and promotes certain cell cycle activities

Meiosis

• Haploid n: gametes for sexual reproduction (one set of chromosomes)

• Diploid 2n: body cells (2 full sets of chromosomes)

• Produces 4 haploid daughter cells

Meiosis I:

1. Prophase I:

   • Nuclear envelope disappears, fibers form, DNA coils into sister chromatids

   • Double chromosomes pair up and exchange genetic information (crossing over- forms recombinant chromatids)

2. Metaphase I

   • Sister chromatids remain in pairs, fibers align pairs across center of cell

   • Random assortment occurs = affects which chromosomes end up in each gamete = increases variation

3. Anaphase I

   • Fibers separate chromosome pairs

   • Each sister chromatid from the pair migrates to opposite sides of the cell

4. Telophase I

   • Nuclear envelope reappears and establishes separate nuclei with only ONE double chromosome from each pair = half of total info that the parent contained but still double chromosomes (sister chromatids)

   • Chromosomes uncoil

   • Cytokinesis occurs and forms two daughter cells which are haploid and genetically different

Meiosis II:

1. Prophase II:

   • Nuclear envelope disappears, fibers form

2. Metaphase II:

   • Fibers align sister chromatids across center of cell

3. Anaphase II:

   • Fibers separate sister chromatids

   • Chromatids move to different sides of the cell

4. Telophase II:

   • Nuclear envelope reappears and establishes separate nuclei with only a single chromosome

   • Cytokinesis occurs and forms four genetically different daughter cells

Phenotype Plasiticty

Environmental factors can change the expression of genes, and phenotypic plasticity is the ability of one genotype to produce multiple phenotypes.

• Ex: Hydrangea Plant color is determined by the pH of the soil

• Ex: Stomata density dependent on CO₂ levels in atmosphere

Independent Assortment

Genes will be sorted into gametes independently and they are not linked. Genes are linked if they have a map unit lower than 50.

Nondisjunction

The failure of chromosomes to fully separate during the formation of gametes = too few or too many chromosomes in the sex cells. Ex: Down Syndrome, or Trisomy 21.

• Ex: Triple X syndrome

Pyrimidines

Uracil, Cytosine, Thymine = one ring structure

Purines

Adenine, Guanine = double ring structure

DNA Replication

Semiconservative replication: resulting DNA contains one original (template) strand and one newly synthesized complimentary strand.

Directionality of DNA strands: antiparallel

• Phosphate terminus is 5’

• Hydroxyl (OH) terminus is 3’

• New nucleotides can only be added in the 5’—3’ direction so one strand is synthesized continuously (leading strand) and the other (lagging strand) is synthesized in fragments

Helicase: unwinds DNA strands

Topoisomerase: relaxes supercoil at replicatio fork- where two strands are separated

DNA Polymerase: synthesizes new strands

• Needs RNA primers to start synthesis

• Attaches to the 3’ end of the template strand and builds in 5’ to 3’ direction

Ligase: joins DNA fragments on lagging strand

Transcription

DNA strands are separated and the template strand (noncoding strand) is transcribed by RNA polymerase in the 5’ to 3’ direction by reading in the 3’ to 5’ direction and creates mRNA of one particular gene.

RNA Processing/Modification

• Addition of Poly-A tail (3’) and GTP head (5’)

• Introns are excised

• Exons are retained

Alternative Splicing:

• Produce different mRNA transcripts from one primary transcript through keeping of different exons

Retrovirus Translation

Introduction of viral RNA (not DNA) into host cells through the use of the enzyme reverse transcriptase that copies viral RNA into viral DNA, once it converts viral RNA to viral DNA, it is integrated into the host genome and will then be transcribed and translated, this results in the assembly of new viral progeny.

Regulatory Sequences

Stretches of DNA used to promote or inhibit protein synthesis by coding for regulatory proteins = controls transcription.

Epigenetics

Epigenetic changes are reversible modification of DNA or histones (proteins used to wrap DNA around) with slight chemical changes that cause histones to tightly or loosely pack DNA = gene expression regulation with access to DNA.

Transcription Factors

Proteins that promote or inhibit transcription of a gene = determines how a cell differentiates during development.

Operons

Closely linked genes producing a single mRNA molecule during transcription under the control of the same regulatory sequence (includes genes to be transcribed, regulatory sequence, and operator).

• Operator either inhibits or promotes

• Ex: Lac Operon is inducible (usually off, with regulatory protein bound to operator and RNA polymerase cannot bind to DNA)

• Inducers are molecules that bind to regulatory protein and cause it to change shape and unbind or be unable to bind to the operator (Ex: allolactose in Lac Operon)

Small RNA fragments (sRNA)

• can break down mRNA in cytoplasm as a form of gene expression

• can block transcription as well

Mutations in Chromosome Number

Triploidy: 3 copies of a particular chromosome = sterility in some animals

Polyploidy: multiple sets of homologous chromosomes = can result in increased vigor in plants

Trisomy 21: extra chromosome at number 21

Turner Syndrome: missing X chromosome

Horizontal Genetic Transfer

Exchange of genetic info between different genomes or unrelated organisms. Is mainly seen in prokaryotes and contributes to genetic variation in prokaryotes through

• Transformation: DNA comes from external environmental source (naked)

• Transduction: transmission of foreign DNA into a cell when viral genome integrates with host genome

• Conjugation: cell to cell transfer of DNA

• Transposition: movement of DNA segments within or between DNA molecules

Gel Electrophoresis

Separates molecules according to size and charge. DNA is negatively charged and DNA samples with fragments are loading into wells with gel.

• Current is then applied to gel creating positive and negative ends, as DNA moves to the positive end of gel it is separated by size and smaller particles move faster and further throughout the gel.

• The resulting bands indicate molecules that migrated to that location. These band patterns can be used to identify people.

Polymerase Chain Reaction (PCR)

PCR allows the creation of large samples of DNA through DNA denaturing, then primers are added and DNA is replicated until an ample amount of DNA is obtained.

Convergent Evolution

Similar environmental conditions select for similar traits in different populations/species over time = analogous structures

Genetic drift

Random change in the frequency of a particular allele within a population. Like natural catastrophes, etc.

1. Bottleneck Events: sudden population decrease

2. Founder Effect: separation/isolation of population

   • Migration, geologic events

   • Migration/gene flow = movement of individuals between populations leading to an exchange of alleles between populations

Hardy Weinburg Equilibrium

Allele frequencies must stay constant (no genetic drift or gene flow)

• p²+2pq+q²=1

• p+q=1

Prezygotic Barriers

1. Habitat Isolation: species occupy different habitats and rarely come into contact

2. Temporal Isolation: species breed during different times of day, seasons, or years

3. Behavioral Isolation: species have different courtship rituals or mate preferences

4. Mechanical Isolation: reproductive structural differences prevent successful mating and reproduction

5. Gamete Isolation: sperm of one species may not be able to fertilize the eggs of another species

Postzygotic Barriers

1. Hybrid Inviability: Mating results in a zygote but incompatibility may stop its development

2. Hybrid Sterility: A hybrid offspring is produced that is vigorous but may be sterile

3. Hybrid Breakdown: First-generation hybrids are viable and fertile but resulting generations are feeble or sterile

Allopatric Speciation

Evolution of new species due to individuals from the same population being geographically isolated over a long period of time with no gene flow and different selective pressures.

Sympatric Speciation

Evolution of a new species due to individuals being reproductively isolated from a surviving ancestral population with no geographic barrier but instead genetic mutations, habitat differentiation or sexual selection.

Divergent Evolution

Adaptation to new habitats result in phenotypic diversification. Adaptive radiation refers to the evolution of new species that allows empty ecological roles or niches to be filled (such as after an extinction event).