paleoanthropologist: A scientist who uses fossil evidence to study early human ancestors.
paleobiology: The biological study of fossils.
paleontologist: A scientist who studies fossils to better understand life in prehistoric times.
paleontology: The scientific study of fossils.
Pangaea: A supercontinent which began to break apart into the modern continents about 260 million years ago, causing the isolation (and separate evolution) of various groups of organisms from each other.
panmixis: Random mating throughout a population.
paradox: A seemingly absurd or contradictory, though often true, statement.
parapatric speciation: Speciation in which the new species forms from a population contiguous with the ancestral species' geographic range.
paraphyletic group: A set of species containing an ancestral species together with some, but not all, of its descendants. The species included in the group are those that have continued to resemble the ancestor; the excluded species have evolved rapidly and no longer resemble their ancestor.
parasite: An organism that lives on or in a plant or animal of a different species, taking nutrients without providing any benefit to the host.
Parish, Amy: A biological anthropologist and primatologist whose research focuses on the social behavior of bonobos ("pygmy chimpanzees," or Panpaniscus). In addition to comparative work with chimpanzees (Pan troglodytes)and endocrinological investigations, Dr. Parish studies reciprocity in chimpanzees, bonobos, and hunter-gatherers.
parsimony: The principle of phylogenetic reconstruction in which the phylogenyof a group of species is inferred to be the branching pattern requiring the smallest number of evolutionary changes.
parthenogenesis: Development from an egg cell that has not been fertilized. The term for a certain form of asexual reproduction that is found in some lizards, insects (notably among aphids), and certain other organisms.
particulate: (as property of theory of inheritance) A synonym of atomistic.
paternity: The identity of the father of an offspring.
pathogen: A microorganism that causes disease.
pathological: Related to or caused by disease.
penicillin: The first antibiotic discovered, penicillin is derived from the mold Penicillium notatum. It is active against a wide variety of bacteria, acting by disrupting synthesis of the bacterial cell wall.
peripatric speciation: A synonym of peripheral isolate speciation.
peripheral isolate speciation: A form of allopatric speciation in which the new species is formed from a small population isolated at the edge of the ancestral population's geographic range. Also called peripatric speciation.
pesticide-resistant insects: Insects with the ability to survive and reproduce in the presence of pesticides. These resistant variants increase in frequency over time if pesticides remain present in their environment.
Petrie, Marion: A behavioral ecologist at the University of Newcastle in the U.K., Dr. Petrie's research interests include the links between sexual selection and speciation, and how males and females assess genetic quality in a mate.
phenetic classification: A method of classification in which species are grouped together with other species that they most closely resemble phenotypically.
phenetic species concept: A concept of species according to which a species is aset of organisms that are phenotypically similar to one another. Compare with biological species concept, cladistic species concept, ecological species concept, and recognition species concept.
phenotype: The physical or functional characteristics of an organism, produced by the interaction of genotype and environment during growth and development.
phenotypic characters: Individual traits that can be observed in an organism(including appearance and behavior) and that result from the interaction between the organism's genetic makeup and its environment.
pheromone: A chemical substance produced by some organisms and emitted into the environment to communicate with others of the same species. Pheromones play an important role in the social behavior of certain animals, especially insects and some mammals. They are used to mark out territories, to attract mates, to lay trails, and to promote social cohesion and coordination in colonies. Examples are the sex attractants secreted by moths to attract mates and the queen substance produced by queen honeybees, which controls the development and behavior of worker bees. Pheromones are usually volatile organic molecules which are effective at very low concentrations, as little as one part per million.
photoreceptor cell: A cell, functionally part of the nervous system, that reacts to the presence of light. It usually contains a pigment that undergoes a chemical change when light is absorbed. This chemical change stimulate selectrical changes in the photoreceptor that, when passed along and processed by other neurons, form the basis of vision.
photosynthesis: The fundamental biological process by which green plants make organic compounds such as carbohydrates from atmospheric carbon dioxide and water using light energy from the Sun. The process has two main phases: the light-dependent light reaction responsible for the initial capture of energy, and the light-independent dark reaction in which this energy is stored in the chemical bonds of organic molecules. Because virtually all other forms of life are directly or indirectly dependent on green plants for food, photosynthesis ist he basis for almost all life on Earth.
phylogeny: The study of ancestral relations among species, often illustrated with a "tree of life" branching diagram, which is also known as a phylogenetic tree.
phylum (plural phyla): One of the highest levels of taxonomic classification. See taxon.
phytoplankton: Microscopic aquatic organisms that, like plants, use photosynthesis to capture and harness solar energy.
Pickford, Martin: A paleontologist at the College de France in Paris. In 2000,Pickford and Brigitte Senut discovered Orrorin tugensis, a proto-hominid dated at 6 million years old.
Pinker, Steven: A psychologist and professor with a special interest in language, linguistic behavior, and cognitive science. Pinker's publications include the popular science books The Language Instinct and How the Mind Works.
placental mammals: A group (specifically, an order) of mammals in which the young develop inside the mother, attached to her and nourished by a specialized structure called the placenta. In placental mammals, the young are born in an advanced stage of development. Compare with marsupial and monotreme.
placoderm: An extinct bottom-dwelling fish that was among the first to develop jaws and paired fins.
plankton: Minute or microscopic animals (zooplankton) and plants (phytoplankton)that float and drift in water, usually near the surface. In the top meter or two of water, both in the sea and in freshwater, small plants can photosynthesize, and abundant microscopic life can be observed. Many organisms that are sessile (attached to a surface) as adults disperse by means of a planktonic larval stage.
plan of nature: The philosophical theory that nature is organized according to a plan. It has been influential in classification, and is a kind of idealism.
plasmid: A genetic element that exists (or can exist) independently of the main DNA in the cell. In bacteria, plasmids can exist as small loops of DNA and be passed between cells independently.
plate tectonics: The theory that the surface of Earth is made of a number of plates, which have moved throughout geological time to create the present-day positions of the continents. Plate tectonics explains the location of mountain building, as well as earthquakes and volcanoes. The rigid plates consist of continental and oceanic crust together with the upper mantle, which "float" on the semi-molten layer of the mantle beneath them, and move relative to each other across the planet. Six major plates (Eurasian, American, African, Pacific, Indian, and Antarctic) are recognized, together with a number of smaller ones. The plate margins coincide with zones of seismic and volcanic activity.
Poisson distribution: A frequency distribution for number of events per unit time, when the number of events is determined randomly and the probability of each event is low.
polyandry: A reproductive system in which one female mates with many males. Seahorses and jacanas are examples of polyandrous species, which are less common than monogamous or polygynous species.
polygyny: Reproductive strategy in which one male mates with several females. Lions, peacocks, and gorillas all have polygynous mating systems. Compare with polyandry and monogamy.
polymorphism: A condition in which a population possesses more than one allele at a locus. Sometimes it is defined as the condition of having more than one allele with a frequency of more than five percent in the population.
polyphyletic group: A set of species descended from more than one common ancestor. The ultimate common ancestor of all species in the group is not a member of the polyphyletic group.
polyploid: An individual containing more than two sets of genes and chromosomes.
population: A group of organisms, usually a group of sexual organisms that interbreed and share a gene pool.
population genetics: The study of processes influencing gene frequencies.
postulate: A basic principle.
postzygotic isolation: A form of reproductive isolation in which a zygote is successfully formed but then either fails to develop or develops into a sterile adult. Donkeys and horses are postzygotically isolated from one another; a male donkey and a female horse can mate to produce a mule, but the mule is sterile.
prezygotic isolation: A form of reproductive isolation in which the two species never reach the stage of successful mating, and thus no zygote is formed. Examples would be species that have different breeding seasons or courtship displays, and which therefore never recognize one another as potential mates.
primate: A mammal belonging to the order Primates (about 195 species), which includes prosimians, monkeys, apes, and humans. Primates probably evolved from insectivorous climbing creatures like tree shrews and have many adaptations for climbing, including five fingers and five toes with opposable first digits (except in the hind feet of humans). They have well-developed hearing and sight, with forward-facing eyes allowing binocular vision, and large brains. The young are usually produced singly and undergo a long period of growth and development to the adult form. Most primates are arboreal, but the great apes and humans arelargely terrestrial.
prokaryote: A cell without a distinct nucleus. Bacteria and some other simple organisms are prokaryotic. Compare with eukaryote. In classificatory terms, the group of all prokaryotes is paraphyletic.
proofreading enzymes: Mistakes during DNA replication can be recognized and repaired by proofreading enzymes. A mismatched nucleotide may occur at the rat eof one per 100,000 base pairs, causing a pause in replication. DNA repair enzymes perform a proofreading function and reduce the error rate to one per billion. Many complex mechanisms have evolved to repair damage and alterations in DNA, which can occur as a result of damage from ultraviolet radiation, X-rays etc. as well as from errors during replication. (Over 50 genes have been found in yeast to be involved in repair mechanisms).
prosimian: One of the group of primates that includes lemurs and lorises; the other two primate groups are tarsoids and anthropoids.
protein: A molecule made up of a sequence of amino acids. Many of the important molecules in a living thing -- for example, all enzymes -- are proteins.
protozoa: A group of unicellular, usually microscopic, organisms now classified in various phyla of the kingdom Protoctista. They were formerly regarded either as a phylum of simple animals or as members of the kingdom Protista. Most feed on decomposing dead organic matter, but some are parasites, including the agents causing malaria (Plasmodium) and sleeping sickness (Trypanosoma), and a few contain chlorophyll and carry out photosynthesis, like plants.
pseudogene: A sequence of nucleotides in the DNA that resembles a gene but is nonfunctional for some reason.
pupa (plural pupae): The third stage of development in the life cycle of some insects, including flies, butterflies (in which it is the chrysalis), ants, bees, and beetles. During the pupal stage locomotion and feeding cease and metamorphosis from the larva to the adult form takes place. The adult emerges by cutting or digesting the pupal case after a few days or several months.
purine: A kind of base in the DNA; adenine (A) and guanine (G) are purines.
pyrimidine: A kind of base. In DNA, cytosine (C) and thymine (T) arepyrimidines. In RNA, cytosine (C) and uracil (U) are pyrimidines.