I

I band: The area near the edge of the sarcomere where there are only thin filaments.
Identical twins: Twins formed from a single zygote and therefore genetically identical.
Identity by descent (IBD): Genes that are inherited from the same gene in an ancestral population are identical by descent.
Immigration: The influx of new individuals from other areas.
Immune memory: The increased response of the immune system to an antigen that had been encountered before. This is the basis of vaccination.
Immunization: The process of generating a state of immunity by artificial means; also called vaccination. In active immunization, a nonphatogenic version of a pathogenic microbe is administered, inducing B and T cell responses and immunological memory.  In passive immunization, antibodies specific for a particular microbe are administered conferring immediate but temporary protection.
Immunoglobulin (Ig): Any of the class of proteins that functions as antibodies. There are five classes of Ig that differ in body distribution and antigen disposal activities.
Immunological distance: A measurement of phylogenetic relationship based on differences in antibody-antigen binding affinity between species. The magnitude of the difference is used as a rough evolutionary clock.
Imprinting: A type of learned behavior with a significant innate component, acquired during a limited critical period.
Imprinting, genomic: A phenomenon in which the phenotype of a heterozygote depends on which allele came from the father and which from the mother.
in vitro fertilization: Fertilization of ova in laboratory containers followed by artificial implantation of the early embryo in the mother’s uterus.
in vitro mutagenesis: A technique to discover the function of a gene by introducing specific changes into the sequence of a cloned gene, inserting the mutated gene into a cell, and studying the phenotype of the mutant.
Inbred: Produced by mating between relatives.
Inbred line: A population produced by continued self-fertilization or mating between close relatives. After several generations of inbreeding, an inbred line becomes genetically homogeneous.
Inbreeding coefficient: The chance that two homologous genes in a diploid individual are identical by descent.
Inbreeding depression: Reduced vigor of inbred individuals.
Inclusive fitness: A measure of individual fitness that includes the effects of that individual on its neighbors’ fitness and discounts the effects of neighbors on the individual’s own fitness.
Incomplete dominance: A condition in which heterozygotes express a trait that is distinguishable from both homozygotes.
Incomplete flower: A flower in which one or more of the four basic floral organs (sepals, petals, stamens, and carpels) are either absent or nonfunctional.
Incomplete metamorphosis: A type of development in certain insects, such as grasshoppers, in which the young (nymphs)  resemble adults but are smaller and have different body proportions; after a series of molts reaches full size.
Incus: The second of the three middle ear bones.
Indel: An insertion or a deletion mutation involving a small number of bases.
Indeterminate cleavage: A type of embryonic development  in deuterostomes, in which each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo.
Indirect selection: Selection on a trait that arises from its association with other traits rather than because of its direct causal effect on fitness. The term may also refer to selection that arises from the effects of a gene in one individual on the fitness of other individuals.
Individualistic hypothesis: A plant community is a chance assemblage of species found in the same area simply because they happen to have similar abiotic requirements.
Induced fit: The change in shape of the active site of an enzyme so it binds more snuggly  tot eh substrate, induced by entry of the substrate.
Inducer: A specific small molecule that inactivates the repressor in an operon.
Induction: (1) Increase in gene expression in response to a regulatory signal. (2) The ability of one group of embryonic cells to influence the development of another.
Inductive reasoning: A type of logic in which generalizations are based on a large number of specific observations.
Infer: to derive by reasoning, conclude or judge by premises or evidence.
infinite-alleles model: A model that assumes that every mutation generates a new allele.
Infinitesimal model: A model that assumes that quantitative trait variation is caused by a very large number of loci, with infinitesimally small additive effects.
Inflammatory response:  A localized innate immune defense triggered by physical injury or infection of tissue in which changes in nearby small blood vessels enhance the infiltration of white blood cells, antimicrobial protein, and clotting element that aid in tissue repair and destruction of invading pathogens.
Inflorescence: A group of flowers tightly clustered together.
Informational genes: Genes involved in core “informational” processes including DNA replication and repair, transcription, and translation. They are thought to be less prone to lateral gene transfer than operational genes.
Ingestion: A heterotrophic mode of nutrition in which other organisms or detritus are eaten whole or in pieces.
Ingroup: In a cladistic study of evolutionary relationships among taxa of organisms, the group of taxa that is actually being analyzed.
Initials: Cells that remain within an apical meristem as source of new cells.
Innate behavior: Behavior that is developmental fixed and under strong genetic control. It is exhibited in virtually the same form by all individuals in a population despite internal or external environmental differences during development and throughout their lifetimes.
Innate immunity: The kind of defense that is mediated by phagocytes, antimicrobial proteins, the inflammatory response, and natural killer cells (NK). It is present before exposure to pathogen and it is effective from the time of birth.
Inner cell mass: A cluster of cells in a mammalian blastocyst that protrudes into one end of the cavity and subsequently develops into the embryo proper and some of the extraembryonic membranes.
Inner ear: One of the three main regions of the vertebrate ear; includes the cochlea, organ of Corti, and the semicircular canals.
Innovation: A change to a preexisting feature.
Inositol triphosphate (IP₃): A second messenger that functions as an intermediate between certain nonsteroid hormones and a third messenger; a rise of cytoplasmic calcium concentration.
In situ hybridization: A labeled DNA or RNA probe is hybridized to a tissue section or whole embryo and view under the microscope to determine when and where a specific mRNA is expressed. Alternatively, the probe can be hybridized to a chromosomal spread and view under the microscope to determine the location of a specific genomic sequence.
Insertion: A mutation involving the addition of one or more nucleotide pairs to a gene.
Insertion sequence (IS): A class of transposable element found in bacterial and archaea. The simplest kind of transposable element, consisting of inverted repeats of DNA flaking a gene for transposase.
Insulin: A hormone secreted by pancreatic beta cells that lowers glucose blood levels. It promotes the uptake of glucose by most body cells, the synthesis and storage of glucogen in the liver, and stimulates protein and fat synthesis.
Insuiln-like growth factor: A hormone produced by the liver whose secretion is stimulated by growth hormone. It stimulates bone and cartilage growth.
Integral protein: A transmembrane protein with hydrophobic regions that spans the hydrophobic interior of the plasma membrane.
Integral hypothesis: A community is an assemblage  of closely linked species, locked into association by mandatory biotic interactions that cause the community to function as a unit.
Integration: The interpretation of sensory systems within neural processing  centers of the central nervous system.
Integrin: A receptor protein built into the plasma membrane that interconnects the extracellular matrix to the cytoskeleton.
Integument: Layer of sporophyte tissue that contributes to the structure of an ovule of a seed plant.
Integumentary system: The outer coverage of a mammal’s body including skin, hair, and nails.
Intercalating agent: A chemical that resembles DNA bases and can insert into DNA backbones during replication, leading to insertion or deletion mistakes in replication.
Intercalating disk: A specialized junction between cardiac muscle cells that provides direct electrical coupling between cells.
Interferon: A protein that has antiviral or immune regulatory functions. There are three types of interferon; alpha and beta interferons secreted by virus-infected cells help nearby cells resist viral infection. Interferon gamma secreted by T cells help activate macrophages.
Intermediate filament: A component of the cytoskeleton that includes all filaments intermediate in size between microtubules and microfilaments.
Interneuron: An association neuron;  a nerve cell within the central nervous system that forms synapses with sensory and motor neurons and integrates sensory input and motor output.
Internode: A segment of a plant stem between points where leaves are attached.
Interoreceptor: A sensory receptor that detects stimuli within the body, such as blood pressure and body position.
Interphase: The period in the cell cycle when the cell is not dividing. During interphase cellular metabolic activity is high, chromosomes and organelles are duplicated, and cell size may increase.
Intersexual selection: Selection where by individuals of one sex are choosy in selecting their mates from individuals of the other sex.
Interspecific competition:  Competition for resources between plants, between animals, and between decomposers when resources are in short supply.
Interspecific interaction: Relationships between species of a community.
Interstitial fluid: The internal environment of vertebrates, consisting of the fluid filling the spaces between cells.
Intertidal zone: The shallow zone of the ocean where land meets water.
Intracellular digestion: The joining of food vacuoles and lysosomes to allow chemical digestion to occur within the cytoplasm of a cell.
Intrasexual selection: Direct competition among individuals of one sex for mates of the opposite sex.
Intricate: having many interrelated parts or facets, complex.
Introduced species: A species moved by humans, either intentionally or accidentally from its native location to a new geographic region; also called an exotic species.
Introgression: Movement of genes from one genetic background to another, as a result of hybridization between individual from distinct populations.
Intron: A noncoding, intervening sequence within an eukaryotic cell.
Invagination: The folding of cells.
Invasive species: A species that takes hold outside its native range; usually introduced by humans.
Invention: A fundamental new feature of an organism.
Inversion: An aberration of chromosome structure resulting from reattachment in a reverse orientation of a chromosomal fragment to the chromosome from which the fragment originated. The process involves excision and reinsertion of the DNA.
Invertebrate: An animal without a backbone. Invertebrates make up 95% of animal species.
Involution: Cells rolling over the edge of the lip of the blastopore into the interior of the embryo during gastrulation.
Ion: An atom that has gained or lost electrons, thus acquiring a charge.
Ion channel: Protein channel in a cell membrane that allows passage to a specific ion down its concentration gradient.
Ionic bond: A chemical bond resulting from the attraction between oppositely charged ions.
Ionic compound: A compound resulting from the formation of an ionic bond, also called a salt.
Irreducible complex: A system that cannot function if any of its components is missing.
Island model: The simplest model of population structure. A fraction of the genes in a deme comes from outside. Immigration may be from the mainland or from other demes.
Islets of Langerhans: Cluster of endocrine cells within the pancreas that produce and secrete the hormones glucagon (alpha cells) and insulin (beta cells).
Isogamous: Producing a single kind of gamete.
Isolation by distance: Divergence between allele frequencies in different places within a spatially continuous population. The term usually refers to divergence caused by random genetic drift.
Isomer: One of several organic compounds with the same molecular formula but different structures and therefore different properties. There are three types of isomers: structural, geometric, and enantiomers.
Isomorphic: Referring to alternative generations in plants and certain algae in which the sporophytes and gametophytes look alike, but differ in chromosome number.
Isopod: A member of one of the largest groups of crustaceans, which includes terrestrial, freshwater, and marine species.
Isotonic: Having the same solute concentration as another solution.
Isotope: Forms of an element that differ in the number of neutrons and thus differ in atomic mass.
Isozymes: Enzymes with different amino acid sequences that catalyze the same reaction. This includes variants coded by different genetic loci as well as allozymes, which are coded by homologous genes at the same locus.
Iteroparity: A life history in which adults produce large numbers of offspring over many years; also called repeated reproduction.