Tuesday, April 21, 2009

Dictionary of Evolutionary Nomenclature - Letter M

macroevolution: A vague term generally used to refer to evolution on a grand scale, or over long periods of time. There is no precise scientific definition for this term, but it is often used to refer to the emergence or modification of taxa at or above the genus level. The origin or adaptive radiation of a higher taxon, such as vertebrates, could be called a macroevolutionary event.

macromutation: Mutation of large phenotypic effect, one that produces a phenotype well outside the range of variation previously existing in the population.

malaria: A sometimes-fatal disease transferred to humans by mosquitoes, infecting the bloodstream.

Malthus, Thomas: A British economist and demographer best known for his treatise on population growth, which states that people will always threaten to outrun the food supply unless reproduction is closely monitored. His theory was in opposition to the utopians of the 18th century.

mammals: The group (specifically, a class) of animals, descended from a common ancestor, that share the derived characters of hair or fur, mammary glands, and several distinctive features of skeletal anatomy, including a particular type of middle ear. Humans, cows, and dolphins are all mammals.

mammary glands: Only found in mammals, these are specialized glands that canp roduce milk for feeding young.

mandible: A part of the bony structure of a jaw. In vertebrates, it is the lower jaw; in birds, the lower bill; in arthropods, one of the paired appendages closest to the mouth.

Margulis, Lynn: A biologist who developed the serial endosymbiosis theory of origin of the eukaryotic cell. Although now accepted as a plausible theory, both she and her theory were ridiculed by mainstream biologists for a number of years.

marsupial mammals: A group (specifically, an order) of mammals whose females give birth to young at a very early stage of development. These newborns complete their development while sucking in a pouch, which is a permanent feature on the female. Examples include kangaroos and opossums.

mastodon: An extinct elephant-like mammal.

Mayr, Ernst: Mayr's work has contributed to the synthesis of Mendelian genetics and Darwinian evolution, and to the development of the biological species concept. Mayr has been universally recognized and acknowledged as one of the leading evolutionary biologists of the 20th century.

McGinnis, William: A professor of biology at the University of California, San Diego. Discover (with Mike Levine) of homeoboxes, the sequences of DNA that are characteristic of homeotic genes, which play a central role in specifying body development. His current research uses both genetics and biochemistry to examine such questions as how molecular variations in the Hox genes that specify the head-tail pattern of an organism can generate variety in animal shapes during evolution, and what the molecular changes were that allowed single-celled animals to become multicellular.

meiosis: A special kind of cell division that occurs during the reproduction of diploid organisms to produce the gametes. The double set of genes and chromosomes of the normal diploid cells is reduced during meiosis to a single haploid set in the gametes. Crossing-over and therefore recombination occur during a phase of meiosis.

meme: The word coined by Richard Dawkins for a unit of culture, such as an idea, skill, story, or custom, passed from one person to another by imitation or teaching. Some theorists argue that memes are the cultural equivalent of genes, and reproduce, mutate, are selected, and evolve in a similar way.

Mendelian inheritance: The mode of inheritance of all diploid species, and therefore of nearly all multi-cellular organisms. Inheritance is controlled by genes, which are passed on to the offspring in the same form as they were inherited from the previous generation. At each locus an individual has two genes -- one inherited from its father and the other from its mother. The two genes are represented in equal proportions in its gametes.

Mendel, Gregor: An Austrian monk whose plant breeding experiments, begun in1856, led to insights into the mechanisms of heredity that are the foundation of genetics today. His work was ignored in his lifetime and only rediscovered in1900. See Mendelian inheritance.

messenger RNA (mRNA): A kind of RNA produced by transcription from the DNA and which acts as the message that is decoded to form proteins.

metabolism: The chemical processes that occur in a living organism in order to maintain life. There are two kinds of metabolism: constructive metabolism, or anabolism, the synthesis of the proteins, carbohydrates, and fats which form tissue and store energy; and destructive metabolism, or catabolism, the breakdown of complex substances, producing energy and waste matter.

metamorphosis: One or more changes in form during the life cycle of an organism, such as an amphibian or insect, in which the juvenile stages differ from the adult. An example is the transition from a tadpole to an adult frog. The term "complete metamorphosis" is applied to insects such as butterflies in which the caterpillar stage is distinct from the adult. "Incomplete metamorphosis" describes the life histories of insects such as locusts in which the young go through a series of larval stages, each of which bears similarities to the adult. Metamorphosis in both insects and amphibians is controlled by hormones, and often involves considerable destruction of larval tissues by enzymes.

metazoans: All animals that are multicellular and whose cells are organized into tissues and organs. In the simplest metazoans only an inner and outer layer can be distinguished.

microbe: A nonscientific and very general term, with no taxonomic significance, sometimes used to refer to microscopic (not visible to the unaided eye) organisms. The term often refers to bacteria or viruses that cause disease orinfection.

microevolution: Evolutionary changes on the small scale, such as changes in gene frequencies within a population.

Miller, Geoffrey: Author of The Mating Mind, Miller is known for his research on evolutionary psychology and sexual selection. He believes that our minds evolved not only as survival machines, but also as courtship machines -- at least in part, to help us attract a mate and pass on genes.

Miller, Ken: A cell biologist and professor of biology at Brown University. Miller's academic research focuses on the structure and function of biological membranes. He is the coauthor of widely used high school and college biology textbooks, and he has also written Finding Darwin's God: A Scientist's Search for Common Ground Between God and Evolution.

Miller, Veronica: A German virologist whose research has focused on HIV-AIDS. Miller was the first researcher to announce that an interruption in drug treatment among AIDS patients may result in reversion of drug-resistant virus to its wild type. This led other researchers and clinicians to explore "structured treatment interruptions" among some patients as an experimental treatmentoption.

mimicry: A case in which one species looks more or less similar to another species. See Batesian mimicry and Müllerian mimicry.

mitochondrial DNA: DNA found in the mitochondrion, a small round body found inmost cells. Because mitochondria are generally carried in egg cells but not in sperm, mitochondrial DNA is passed to offspring from mothers, but not fathers.

mitochondrion: A kind of organelle in eukaryotic cells. Mitochondria produce enzymes to convert food to energy. They contain DNA coding for some mitochondrial proteins.

mitosis: Cell division. All cell division in multicellular organisms occurs by mitosis except for the special division called meiosis that generates the gametes.

Müllerian mimicry: A kind of mimicry in which two poisonous species evolve to look like one another.

modern synthesis: The synthesis of natural selection and Mendelian inheritance. Also called neo-Darwinism.

molecular clock: The theory that molecules evolve at an approximately constantrate. The difference between the form of a molecule in two species is then proportional to the time since the species diverged from a common ancestor, and molecules become of great value in the inference of phylogeny.

molecular geneticists: Scientists who study genes and characters through as a fleshy, muscular body. The phylum Molluscaincludes snails, bivalves, squids, and octopuses.

"monkey trial": In 1925, John Scopes was convicted and fined $100 for teaching evolution in his Dayton, Tenn., classroom in the first highly publicized trial concerning the teaching of evolution. The press reported that although they lost the case, Scopes's team had won the argument. The verdict had a chilling effect on teaching evolution in the classroom, however, and not until the 1960s did it reappear in schoolbooks.

monogamy: A reproductive strategy in which one male and one female mate and reproduce exclusively with each other. Contrast with polygyny and polyandry.

monophyletic group: A set of species containing a common ancestor and all of its descendants, and not containing any organisms that are not the descendants of that common ancestor.

monotremes: A group (specifically, an order) of mammals whose females lay eggs. The young hatch and continue to develop in the mother's pouch, which is present only when needed. Two species of spiny anteater and the duck-billed platypus are the only living monotremes.

Moore, James: The author, with Adrian Desmond, of an authoritative biography of Charles Darwin, Moore has made a 20-year study of Darwin's life. With degrees in science, divinity, and history, he has taught the history of science at Harvard University and at the Open University in the U.K.

morphology: The study of the form, shape, and structure of organisms.

Mueller, Ulrich G.: A zoologist and professor whose research aims at understanding microevolutionary forces and macroevolutionary patterns that govern the evolution of organismal interactions, particularly the evolution of mutualisms and the evolution of social conflict and cooperation. Mueller's current research focuses on the coevolution between fungus-growing ants and their fungi and the evolutionary ecology of halictine bees.

Murray, Charles: An author and policy analyst who has written many controversial and influential books on social policy. He is coauthor with Richard J. Herrnstein of The Bell Curve: Intelligence and Class Structure in American Life(1994). He has also written Losing Ground: American Social Policy 1950-1980(1984), which argues for the abolishment of the welfare system, The Underclass Revisited (1999), and Income, Inequality and IQ (1998).

mutation: A change in genetic material that results from an error in replication of DNA. Mutations can be beneficial, harmful, or neutral.

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