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Find definitions for the terminology used throughout the Understanding Evolution pages.

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natural selection
Differential survival or reproduction of different genotypes in a population leading to changes in the gene frequencies of a population. The conditions required for the operation of evolution by natural selection include variation, a system of heredity, differential reproduction, and time. For a more detailed explanation, see our resource on natural selection in Evolution 101.

(nebulae — pl.) A large cloud of gas and dust in space. Star formation takes place when clumps of matter in a nebula collapse to form stars. These new stars may be associated with protoplanetary disks, which go on to form solar systems.

neutral theory
The idea that most of the molecular variation within populations is not being selected for or against — it is just neutral variation "drifting" around. The neutral theory de-emphasizes the role of natural selection in explaining molecular variation and emphasizes the importance of mutation and genetic drift. For a more detailed explanation, see our resource on neutral theory in Evolution 101.

In ecology, the part of the environment occupied by a particular species along with the resources it uses and produces. A species' niche includes such factors as energy consumed, time of consumption, space occupied, temperature required, mode of reproduction, and behavior.

A point on a phylogeny where a single ancestral lineage breaks into two or more descendent lineages.

non-random mating
A mating system in which at least some individuals are more or less likely to mate with individuals of a particular genotype than with individuals of other genotypes.

norm of reaction
The pattern of phenotypic plasticity for a particular genotype. A norm of reaction describes the way in which a genotype is expressed as a trait under different environmental circumstances. For example, for a particular plant genotype that affects height, the norm of reaction in relation to watering level might look like a bell curve: very small and very large amounts of watering result in shorter plants and normal amounts of watering result in taller plants. For plant with a different genotype, the ideal amount of water to grow a tall plant might be different because this genotype might have a different norm of reaction.

A flexible rod running the length of a chordate, providing structural support. The notochord is one of the inherited characteristics shared by all chordates.

The building blocks of DNA. A chain of nucleotides forms DNA. Nucleotides are made of a sugar, a phosphate, and a base. See also base.

obligate trait
A trait in which the phenotypic expression of the genotype is not adaptive (i.e., the norm of reaction has not been shaped by natural selection). Obligate traits are those that are not facultative. Obligate traits rarely change once they have reached their adult form and generally produce similar phenotypes across a range of "normal" environments. For example, height, eye color, and blood type are all obligate traits. For more details, see our resource on genotype vs. phenotype in Evolution 101.

An organism that eats both plants and animals (omni = all, vorare = to swallow up).

See development.

Onychoporans (also known as velvet worms) share certain characters with arthropods, but are lacking a hard exoskeleton or jointed legs. Onychophorans are probably closely related to arthropods and branched off the tree just before a fully hardened exoskeleton and jointed legs evolved.


Onychophoran photo provided by Dr. Lynn Kimsey and the Bohart Museum of Entomology, University of California Davis

Pertaining to compounds containing carbon. Also refers to living things or the materials made by living things.

Any living creature.

Mating between very distantly related individuals.

A lineage in a phylogenetic analysis that falls outside the clade being studied. All members of the clade being studied will be more closely related to each other than to the outgroup, so the outgroup will branch off at the base of that phylogeny.

Owen, Richard
English anatomist and student of Cuvier. Owen reconstructed the skeletons of many extinct animals, even working on some of Darwin's specimens. He was, nonetheless, an early opponent of Darwin, arguing that God created new species by modifying a basic anatomical idea — an "archetype." Later he modified his own views to accept a kind of "divine" evolution. Owen is also known for overstating the differences between the human brain and those of other apes in his struggle to place humans on a kind of pedestal, apart from the rest of the animal kingdom.

Having some features of the ancestral juvenile stage, but being an adult (with a mature reproductive system). This word means "child form," and a paedomorphic change is any evolutionary change in the development of an organism that generates an adult with a "child's form."

A paleontologist that studies fossils of humans and their closest relatives.

A scientist who studies fossils (paleo = ancient, onto = being, ology = study of; study of ancient beings).

The study of fossils.

Term used to describe a group of organisms that includes the most recent common ancestor of all of its members, but does not include all of the descendants of that most recent common ancestor.

Organism that lives on or within another organism, on which it feeds.

A principle stating that the simplest explanation accounting for the observations is the preferred explanation. When reconstructing the evolutionary relationships among lineages, the principle of parsimony implies that we should prefer the phylogeny that requires the fewest evolutionary changes.

The physical features of an organism. Phenotype may refer to any aspect of an organism's morphology, behavior, or physiology. An organism's phenotype is affected by its genotype and by its environment.

phenotypic plasticity
Degree to which an organism's phenotype changes depending upon its current or past environment. Two organisms with the same genotype (e.g., identical twins) may have different phenotypes (e.g., one may be taller or heavier) if raised in different environments; those differences represent phenotypic plasticity. All organisms exhibit some degree of phenotypic plasticity (e.g., an animal that receives more food will generally be heavier than a genetically identical animal that receives less food), but sometimes phenotypic plasticity can be extreme (e.g., some fish become either male or female depending upon the temperatures they were exposed to as an egg). For more details, see our news story on the topic of phenotypic plasticity.

phylogenetic classification
A system of classification that names groups of organisms according to their evolutionary history. Like Linnaean classification, phylogenetic classification produces a nested hierarchy where an organism is assigned a series of names that more and more specifically locate it within the hierarchy. However, unlike Linnaean classification, phylogenetic classification only names clades and does not assign ranks to hierarchical levels.

The evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered. Many of the phylogenies you encounter are the "family trees" of groups of closely related species, but we can also use a phylogeny to depict the relationships between all life forms. For a more detailed explanation, see our resource on phylogenies in Evolution 101.

Substance that absorbs light. Pigments absorb light of particular wavelengths, which gives the pigment a characteristic color.

In placental mammals, the organ that connects a fetus to the wall of its mother's uterus. Nutrients and oxygen pass through the placenta from the mother to the developing embryo and waste products pass back through it into the mother's bloodstream.

placental mammal
A mammal, such as a human, whose young completes its embryonic development in the uterus, joined to the mother by a placenta.

plate tectonics
A broad theory that uses movements of continental plates to explain many geographic, geologic, seismic, and even biological observations. The idea is that the Earth's crust and upper mantle are made up of many differently sized and irregularly shaped plates that "slide around" on the lower mantle. The plates may crash into one another, slide under one another, and change shape as they are broken down and reformed.

A situation in which a single gene influences more than one trait or has more than one phenotypic effect. For example, the gene that causes sickle cell anemia affects the shape of red blood cells, blood flow, tendency towards fatigue, malaria resistance, and many other traits.

The ancestral character state for a particular clade. This character state may change depending on the clade under consideration. For example, "has four legs" is plesiomorphic for the clade of terrestrial vertebrates, but "has two legs and two wings" is plesiomorphic for the clade of owls.

The number of copies of each chromosome an organism carries. For example, humans are diploid (i.e., we have a ploidy of two) because we carry two copies of each chromosome.

Term used to describe to a group of organisms that does not include the most recent common ancestor of those organisms.

A node on a phylogeny where more than two lineages descend from a single ancestral lineage. A polytomy may indicate either that we don't know how the descendent lineages are related or that we think that the descendent lineages speciated simultaneously. For a more detailed explanation, see our resource on polytomies in Evolution 101.

population bottleneck
See bottleneck.

population genetics
The study of how allele and genotype frequencies in a population change over time through the action of genetic drift, natural selection, mutation, migration, recombination, and reproduction. For more details, see our historical essay on the topic.

Generally, a group of organisms living close to one another that interbreed with one another and do not breed with other similar groups; a gene pool. Depending on the organism, populations may occupy greater or smaller geographic regions.

An organism that hunts and eats other organisms. Predators may eat plants or meat.

Organism killed for food by a predator.

Having a chemical composition close to the original composition of the material from which the Solar System formed. Some samples from the solar wind or from comets have compositions that are close to primordial and can tell us about the early history of the Solar System.

Elongated organ associated with the mouth. For example, in elephants, the trunk is the proboscis, while in butterflies, the long, coiled feeding tube is the proboscis.

A molecule made of a string of amino acids, folded into a complex three-dimensional structure. Proteins are coded for by DNA and are essential molecules for life.

protoplanetary disk
A rotating disk of gas and dust surrounding a young star. Our own Sun and Solar System formed from a protoplanetary disk.

quantitative genetics
The study of the genetic basis of traits that vary continuously, not discretely — for example, height or weight, as opposed to blood type.

A way of classifying humans into large groups based on inherited characteristics, physical appearance, and many other factors. While it is true that some groups of humans share large portions of their evolutionary history (and that this shared evolutionary history has biological implications), scientists agree that there is no biological basis for dividing Homo sapiens into discrete "races," as they have been socially defined. For more details, see our resource on genetic variation and ethnicity in Evolution 101 or our news story on why evolutionary history matters.

radial symmetry
A property of an item (e.g., a shape or an animal) that can be divided into two matching halves by many different lines, which all intersect one another at a single point in the center. For example, pies, snowflakes, and starfish are radially symmetric because they have many different lines of symmetry (dividing them into matching halves) and the lines cross one another at the center.

radial symmetry

radiometric dating
A method of determining the date at which an igneous rock solidified based upon the rate of decay of radioactive atoms within the rock. For a more detailed explanation see our resource on radiometric dating.

random mating
A mating system in which an individual is equally likely to mate with any other individual in the population, regardless of the individual's genotype.

Unpredictable in some way. Mutations are "random" in the sense that the sort of mutation that occurs cannot generally be predicted based upon the needs of the organism. However, this does not imply that all mutations are equally likely to occur or that mutations happen without any physical cause. Indeed, some regions of the genome are more likely to sustain mutations than others, and various physical causes (e.g., radiation) are known to cause particular types of mutations.

recessive gene version
Gene version with an effect that is only observed when it is found paired with an identical version in the same individual.

reciprocal altruism
A situation in which an altruistic behavior (one that represents a fitness cost to the actor and a fitness benefit to the receiver) is likely to be reciprocated in the future by another individual in the population, benefitting the original altruist. Reciprocal altruism can help explain how cooperation and altruism in general can evolve via natural selection.

A process in which pairs of chromosomes swap DNA with one another. This happens during gamete formation. A single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes each). In the process of forming these daughter cells, recombination happens so that the chromosomes the daughter cells have are "mosaic," composed of different pieces of the parent cells' chromosomes. Recombination is important for evolution because it brings new combinations of genes together — a source of variation for natural selection to act upon.

Red Queen hypothesis
The idea that, in order for a species to maintain a particular niche in an ecosystem and its fitness relative to other species, that species must be constantly undergoing adaptive evolution because the organisms with which it is coevolving are themselves undergoing adaptive evolution. When species evolve in accordance with the Red Queen hypothesis, it often results in an evolutionary arms race.

regulatory gene
A gene that controls when protein-coding genes are turned on or off.

Ribonucleic acid, a molecule similar to DNA involved in carrying information and producing proteins in cells. Some viruses carry RNA as their genetic material instead of DNA.

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