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Macroclimate: Large-scale variations in climate; the climate of an entire region.
Macroevolution: Evolutionary change at or above the species level including the appearance of major evolutionary developments, such as flight.
Macromolecule: A giant molecule formed by the joining of smaller molecules, usually by condensation reaction. Polysaccharides, proteins, and nucleic acids are macromolecules.
Macronucleus: The larger of the two nuclei found in ciliate cells. It functions in ways similar to the somatic cells of animals.
Macronutrient: A chemical substance that an organism must obtain in relatively large amounts.
Macrophage: A phagocytic cell present in many tissues that functions in innate immunity by destroying microbes and in acquired immunity as an antigen presenting cell.
Magnetic reversal: A reversal of the polarity of Earth’s magnetic field.
Magnoliids: A flowering plant clade that evolve later than basal angiosperms but before monocots and eudicots. Extant examples are magnolias. laurels, and black pepper plants.
Major depression: Depressive mental illness characterized by experiencing a low mood most of the time.
Major histocompatibility complex (MHC): A set of closely linked genes in vertebrates that play a key role in the immune response. In humans, it is known as the HLA (human leukocyte antigen) complex. These genes encode a large set of cell surface proteins; Class I and II MHC molecules function in antigen presentation to T cells.
Major transition: Identify by Maynard Smith and Szathmary as major changes in the way hereditary information is transmitted.
Malignant tumor: A cancerous tumor that is invasive enough to impair the functions of one or more organs.
Malleus: The first of three middle ear bones.
Malnourished: Referring to an animal whose diet is missing one or more essential nutrients.
Malpighian tubule: A unique excretory organ of insects that empties into the digestive tract, removes nitrogen waste from the hemolymph, and functions in osmoregulation.
Mammal: Member of the class Mammalia, amniotes with mammary glands that produce milk.
Mammary glands: Exocrine glands that secrete milk to nourish the young.
Mandible: One of a pair of jaw-like feeding appendages found in myriapods, hexapods, and crustaceans.
Mantle: A fold of tissue in molluscs that drapes over the visceral mass and may secret a shell.
Mantle cavity: A water-filled chamber that houses the gills, anus, and excretory pores of a molluscs.
Marine benthic zone: The ocean floor.
Marsupian: A mammal, such as a koala, kangaroo, or opossum, whose young complete their embryonic development inside a maternal pouch called the marsupium.
Map unit: A unit of measurement of the distance between genes. One map unit is equivalent  to a 1% recombination frequency.
Mating types: A polymorphism in which individuals can mate only with a different type. See self-incompatibility.
Mass: The quantity of matter as determined from its weight or form Newton’s second law of motion.
Mass number: The sum of the number of protons and neutrons in an atom nucleus.
Mast cell: A vertebrate body cell that produces histamine and other molecules that triggers the inflammatory response.
Maternal effect gene:  a gene that, when mutated in the mother, results in a mutant phenotype in the offspring, regardless of the genotype.
Matter: Anything that takes up space and has mass.
Maximum likelihood: A principle that states that when considering multiple phylogenetic hypothesis, one should take into account the one that reflects the most likely sequence of evolutionary events, given certain rules about how DNA changes over time.
Maximum parsimony: A principle that states that when considering multiple explanations for an observation, one should first investigate the simplest explanation that is consistent with the facts.
Mean: Average, the sum of n values divided by n.
Mechanoreceptor: A sensory receptor that detects physical deformation in the body’s environment associated with pressure, touch, stretch, motion, and sound.
Medulla oblongata: The lowest part of the vertebrate brain, commonly called the medulla, a swelling of the hindbrain dorsal to the anterior spinal cord that controls autonomic homeostatic functions, including breathing, heart and blood vessel activity, swallowing, digestion, and vomiting.
Medusa: The floating, flattened, mouth-down version of the cnidarian body plan. The alternate form is the polyp.
Megapascal (Mpa): A unit of pressure equivalent to 10 atmospheres of pressure.
Megaphyll: A leaf with a highly branched vascular system, characteristic of the vast majority of vascular plants.
Megaspore: A spore from a heterosporous plant  species that develop into a female gametophyte.
Meiosis: A two stage cellular division process (meiosis I and II)  that is involved in sexual reproduction in eukaryotes in which gametes are produced having half the number of copies of each chromosome as the parents.
Meiotic drive: Preferential movement of a chromosome during meiosis toward the pole of the cell that will go on to produce gametes.
Melanocyte-stimulating hormone (MSH): A hormone produced and secreted by the anterior pituitary that regulates the activity of pigment-containing cells in the skin of some vertebrates.
Melatonin: A hormone secreted by the pineal gland that regulates body functions related to seasonal day length.
Membrane attack complex (MAC):  A molecular complex consisting of a set of complement proteins that forms a pore in the membrane of bacterial and transplanted cells, causing the cells to die by lysis.
Membrane potential: The charge difference between the cell’s  cytoplasm and the extracellular fluid, due to the differential distribution of ions. Membrane potential affects the activity of excitable cells (nerve and muscle) and the transmembrane  movement of all charged substances.
Memory cell: One of a clone of long-lived lymphocytes, formed during the primary immune response, that remains in a lymphoid organ until activated by exposure to the same antigen that triggered its formation. Activated memory cells mount the secondary immune response.
Menopause: The cessation of ovulation and menstruation.
Menstrual cycle: A type of reproductive cycle in higher female primates, in which the nonpregnant endometrium is shed as a bloody discharge.
Meristem: Plant tissue that remains embryonic as long as the plant lives, allowing for indeterminate growth.
Meristem identity gene: A plant genet that promotes the switch from vegetative growth to flowering.
Meroblastic cleavage: A type of cleavage in which there is incomplete division of yolk-rich egg; characteristic of avian development.
Mesentery: A membrane that suspends many of the organs of vertebrates inside fluid-filled body cavities.
Mesoderm: One of the three layers found in bilaterian embryos (the other two being ectoderm and endoderm). The mesoderm goes on to form structures such as the notochord, the lining of the coelom, muscles, and internal bones, most of the circulatory system, and portions of organs such as the kidney and reproductive system.
Mesohyl: A gelatinous region between two layers of cells of a sponge.
Mesophile:  An organism that prefers to live at moderate temperatures.
Mesophyll: The ground tissue of a leaf, sandwiched between upper and lower epidermis and specialized for photosynthesis.
Mesophyll cell: A loosely arranged photosynthetic cell located between the bundle-sheath and the leaf surface.
Mesoproterozoic: Division of time from 1600 to 1000 Mya.
Messenger RNA (mRNA): The RNA molecule that is transcribed from the DNA and takes sequence information to the ribosome, where it is translated into protein.
Messenger RNA processing: The collective term for the modifications to eukaryotic RNA That are necessary before the RNA can be transported to the cytoplasm for translation.
Metabolic pathway: A series of chemical reactions that either builds a complex molecule (anabolic pathway) or breaks down a complex molecule into simpler compounds (catabolic pathway).
Metabolic rate: The total amount of energy an animal uses in a unit of time.
Metabolism: The totality of an organism’s chemical reactions including anabolic and catabolic pathways.
Metagenomics: Large-scale sequencing of DNA isolated directly from environmental samples (e.g. soil, water, and air).
Metamorphosis:The resurgence of development in an animal larva that transforms it into a sexually mature adult.
Metanephridium:  In annelid worms, a type f excretory tubule with internal openings called nephrostomes that collect body fluids and external openings called nephridiospores.
Metaphase: The third stage of mitosis, in which the spindle is complete and the chromosomes, attached to microtubules at their kinetochores, are all aligned at the metaphase plate.
Metaphase plate: An imaginary plate during metaphase in which the centromeres of all the duplicated chromosomes are located midway between the poles.
Metapopulation: A collection of discrete demes.  A subdivided population of a single species.
Metastasis: The spread of cancer cells to locations distant form their original site.
Metazoa: The group that includes all animal phyla, including sponges, ctenophores, cnidarias, and the bilaterians.
Methanogen: A microorganism that obtains energy by using carbon dioxide to oxidize hydrogen, producing methane as a waste product.
Microarray: An array of short oligonucleotides, bound to a substrate, that can be used to simultaneously measure the concentration of large numbers of different DNA or RNA sequences.
Microclimate: Very fine scale variation of climate, such as the specific climate conditions underneath a log.
Microevolution: Evolutionary change below the species level; change in the genetic makeup of a population from generation to generation.
Microfilament: Minute fibers composed primarily of actin involved in the structural integrity and movement of eukaryotic cells. Together with microtubules these make up the cytoskeleton.
Micronucleus: The smaller of the two nuclei found in ciliate cells. It functions in ways similar to the germ cells of animals.
Micronutrient: An element that an organism needs in very small amounts and that functions as a component or cofactor of enzymes.
Microphyll: In lycophytes, a small leaf with a single unbranched vein.
Micropile: A pore in the integument(s) of an ovule.
MicroRNA: A family of RNA molecules, approximately 22 nucleotides long, that regulates the expression of some eukaryotic genes; miRNA binds to a complementary sequence in mRNA molecules and direct associated proteins to degrade or prevent translation of the target mRNA.
Microsatellite: A short array of repeated sequences, each a few base pairs long. Microsatellites tend to be highly polymorphic and so widely used as genetic marker.
Microspore: A spore from a heterospoprous plant species that develops into a male gametophyte.
Microsporidia: A group of single-celled eukaryotes that were once considered to be their own phylum but are now considered part of the fungal phylum.
Microtubule: A major component of the cytoskeleton, composed of the protein tubulin. It is used in eukaryotic cells to regulate their shape and control their movements. Present in cilia, flagella, and the cytoskeleton.
Microvillus: One of many fine, fingerlike projections of the epithelial cells in the lumen of the small intestine that increases its surface area.
Midbrain: One of three ancestral and embryonic regions of the vertebrate brain, develops into sensory integrating and relay centers that send sensory information to the cerebrum.
Middle ear: One of the three main regions of the vertebrate ear; a chamber containing three small bones (the hammer, anvil, and stirrup) that convey vibrations from the ear drum to the oval window.
Middle lamella: A thin layer of extracellular material, primarily pectins, found between the primary walls of adjacent young plant cells.
Migration: Movement from place to place. In evolution, it is used synonymously with gene flow.
Mimicry: An adaptive resemblance between an organism and another.
Mineral: In nutrition, a chemical element other than hydrogen, oxygen, or nitrogen that an organism requires for proper body functioning.
Mineral nutrient: An essential chemical element absorbed  from the soil in the form of inorganic ions.
Mineralocorticoid: A steroid hormone secreted by the adrenal cortex that regulates salt and water homeostasis.
Minimum viable population: The smallest population size at which a species is able to sustain its numbers and survive.
Minisatellite: Multiple copies of short sequences, from 9 base pairs up to several hundred base pairs. They are scattered over many sites, with 10-100 repeats, and are highly variable and hence useful as genetic markers.
Mismatch distribution: The distribution of numbers of differences between random pairs of sequences sampled from a population.
Mismatch repair: The process of repairing damage to DNA that has already been replicated.
Missense mutation: A nucleotide substitution within a protein coding region of a gene that leads to the replacement of one amino acid by a different amino acid.
Mitochondria: The eukaryotic organelle responsible for aerobic respiration. Mitochondria are derived form α-protobacteria.
Mitochondrial DNA (mtDNA): The genome contained within mitochondria. Animal mtDNA is widely used as a genetic marker, because it does not recombine.
Mitochondrial matrix: The compartment of the mitochondrion enclosed by the inner membrane and containing enzymes and substrates for the Krebs (citric acid) cycle.
Mitosis: Cellular division process that is involved in asexual reproduction in eukaryotes in which each daughter cell gets a copy of the chromosomes of the parent, thus conserving the chromosome number. Mitosis  is divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.
Mitotic phase: The phase of the cell cycle that includes mitosis and cytokinesis.
Mitotic spindle: An assemblage of microtubules and associated proteins that is involved in the movements of chromosomes during mitosis.
Mixotroph: An organism that is capable of both photosynthesis and heterotrophy.
Model: A representation of a theory or process.
Model organism: An organism chosen to study broad biological principles.
Modern synthesis: A comprehensive theory of evolution emphasizing populations as units of evolution and integrating ideas from genetics, statistics,  paleontology, taxonomy, and biogeography.
Modifier: In the strict sense, an allele that has no direct effect on fitness but that has some other effect on the genetic system that may be indirectly selected. More broadly, it refers to an allele that alters the expression of alleles at other loci.
Modularity: Subdivision into distinct parts or modules that can function independently. In evolutionary psychology, the term refers to independent mental functions.
Molarity: A common measure of solute concentration, referring to the number of moles of solute per liter of solution.
Mold: A rapidly growing fungus that reproduces asexually by producing spores.
Mole (mol): The number of gram of a substance that equals its molecular weight in daltons and contains Avogadro’s number of molecules.
Molecular clock: The constant rate of accumulation of amino acid or DNA sequence differences.
Molecular formula: A type of molecular notation indicating only the quantity of the constituents atoms.
Molecular mass: The sum of the masses of all the atoms in a molecule; sometimes called molecular weight.
Molecular recognition: The binding of two molecules through noncovalent bonds in which the shape of the molecules plays a key role in the strength of the binding.
Molecular recombination: The physical cutting and joining of DNA molecules.
Molecular systematics: The comparison of nucleic acids or other molecules in different species to infer relatedness.
Molecule:Two or more atoms held together by covalent bonds.
Mollusks: A major phylum within the lophotrochozoa. This group of animals include clams, mussels, chitons, octopus, squid, and nudibranchs.
Molt: To cast or shed the feathers, skin, pelage, or the like that will be replaced by a new growth.
Monandrous: Where females mate with a single male.
Monoclonal antibody: Any of a preparation of antibodies that have been produced by a single clone of cultured cells and thus are all specific for the same epitope.
Monocots: A clade consisting of flowering plants that have one embryonic seed leaf or cotyledon.
Monocyte: A type of white blood cell that migrates into tissues and develops into a macrophage.
Monoecious: A term typically used to described an angiosperm species in which carpellate and staminate flowers are on the same plant.
Monogamous: A type of relationship in which one male mates with just one female.
Monohybrid: An organism that is heterozygous with respect to a single gene of interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids. For example, parents of genotype AA and aa produce a monohybrid of genotype Aa.
Monomer: The subunit that serves as the building block of a polymer.
Monomorphic: Having only one form. Mono-one and morphic-form.
Monophyletic: Describes a group of organism or genes that share a common ancestor to the exclusion of all other entities; a clade.
Monosaccharide: The simplest carbohydrate (sugar), active alone or serving as a monomer for disaccharides and polysaccharides.  The molecular formula is generally a multiple of CH₂O.
Monosomy: The presence of only one chromosome instead of a pair in a cell’s nucleus.
Monotreme: An egg-laying mammal, represented by the platyplus and achidna.
Morphogen: A substance that provides positional information in the form of a concentration gradient along an embryonic axis.
Morphogenesis: The development of body shape and organization.
Morphological species concept:  Defining species by measurable anatomical criteria.
Morgan: The unit of distance on a genetic map.
Morph: An individual of one particular form in a species that occurs in two or more forms.
Morula: A solid ball of blastomeres formed by early cleavage.
Moss: A small, herbaceous nonvascular plant that is a member of the phylum Bryophyta.
Motor neuron: A nerve cell that transmits signals from the brain or spinal cord to muscles or glands.
Motor unit: A single motor neuron and all the muscle fibers it controls.
Movement corridor: A series or small clumps or a narrow strip of quality habitat, usable by organisms, that connects otherwise isolated patches of quality habitat.
MPF: Maturation-promoting factor (M-phase-promoting factor); a protein complex required for a cell to progress from late interphase to mitosis. The active form consists of cyclin and a protein kinase.
Mucous membrane: Smooth moist epithelium that lines the digestive tract and air tubes leading to the lungs.
Mullerian mimicry: A distasteful species evolves to resemble another distasteful species. Both gain increased protection because predators learn to avoid the common pattern more quickly.
Muller’s ratchet: The degeneration of an asexual population that arises from the random and irreversible loss of the fittest genotype.
Multifactorial: Referring to a phenotypic character that is influenced by multiple genes and environmental factors.
Multigene family: A collection of genes with similar or identical sequences, presumably of common origin.
Multiple fruit: A fruit derived from an inflorescence, a group of flowers tightly clustered together.
Muscle spindle: A mechanoreceptor stimulated by mechanical distortion.
Muscle tissue: Tissue consisting of long muscle cells that are capable of contracting when stimulated by nerve impulses.
Mutagen: A chemical or physical agent that interacts with DNA and causes a mutation.
Mutation: A heritable change in the genetic material of an organism that does not involve reciprocal recombination. Mutations ultimately create genetic diversity
Mutation rate: The rate at which mutations are generated.
Mutator: An allele that causes an increased mutation rate.
Mutualism: An interaction between species from which all involved gain.
Mycelium: The dense branched network of hyphae in a fungus.
Mycorrhizae: Mutualistic associations of plant root and fungi.
Mycosis: The general term for fungus infection.
Myelin sheath: In a neuron, an insulating coat of cell membrane from Schwann cells that is interrupted by nodes of Ranvier, where saltatory conduction occurs.
Myofibril: A fibril collectively arranged in longitudinal bundles in muscle cell (fibers); composed of thin filaments of actin, a regulatory protein, and thick filaments of myosin.
Myofilaments: The thick and thin filaments that form the myofibers.
Myogenic heart: A type of heart, such as in vertebrate animals, in which the pacemaker is made up of specialized muscle tissues and located within the heart itself.
Myoglobin: An oxygen storing, pigmented protein in muscle cells.
Myosin: A type of protein filament that interacts with actin filaments to cause cell contraction.
Myotonia: Increased muscle tension.
Myriapod: A terrestrial arthropod with many body segments and one or two pairs of legs per segment. Millipedes and centipedes comprise the two classes of living myriapods.