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P

paleontology

The study of life in past geologic time.

pallial sinus

An embayment in the posterior part of the pallial line that indicates the attachment of siphonal retractor muscles and demarcates that part of the mantle cavity into which the siphons can retract in bivalves.

parapatric speciation

Speciation in which the evolution of reproductive isolating mechanisms occurs when a population enters a new niche or habitat within the range of the parent species.

parasitism

A form of symbiosis in which one organism (the parasite) benefits at the expense of another organism of different species (the host). The association can also lead to the injury of the host.

peripatric speciation

Process by which a new species evolves in a subpopulation that colonized a new habitat or niche within the same geographical area of the ancestral species.

phenotype

The observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences; what an organism “looks like.” (adj. phenotypic)

phylogenetic analysis

Analytical method used to find a hypothesis of relationships among species, by coding the various states of homologous characters; also called cladistics.

phylogeny

Sequence of ancestors of a particular lineage.

phytoplankton

Freely floating photosynthetic organisms in the oceans.

ploidy

The number of sets of chromosomes in a cell or organism.

polychromism

Polymorphism expressed as existing in several different colors. (adj. polychromic)

polymorphism

Existing in several different forms. (adj polymorphic)

polyploidy

More than two sets of chromosomes in a cell or organism. (adj. polyploid)

population

A group of organisms, all of the same species, which occupies a particular area.

population size

The number of individuals sharing a gene pool; a complete set of alleles; also as N.

punctuated equilibrium

The evolutionary process involving long periods without change (stasis) punctuated by short periods of rapid speciation.