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

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A collection of stars (and associated gas and dark matter) held together by its own gravity. Galaxies can contain many millions to hundreds of billions of stars. Our own Galaxy is known as the Milky Way.

gene conversion
A mechanism of mutation associated with recombination in which a cell's DNA repair systems "convert" the sequence on a section of one chromosome to a sequence that is found on the homologous chromosome. Gene conversion can be biased, tending to favor some alleles over others.

gene flow
The movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location).

gene frequency
(also called allele frequency) Proportion of genes/alleles in a population that are of a particular type. For example, at a particular locus, pea plants may have either a "yellow pea" allele or a "green pea" allele — so a population of pea plants would have some frequency of yellow pea alleles ranging from zero to one (100%).

gene pool
All of the genes in a population. Any genes that could wind up in the same individual through sexual reproduction are in the same gene pool.

gene-environment interaction
A change in how a particular allele is expressed that is caused by an environmental influence. Gene-environment interactions are an example of phenotypic plasticity.

The unit of heredity. Generally, it means a region of DNA with a particular phenotypic effect. Technically, it may mean a stretch of DNA that includes a transcribed and regulatory region.

genetic drift
Random changes in the gene frequencies of a population from generation to generation. This happens as a result of sampling error — some genotypes just happen to reproduce more than other genotypes, not because they are "better," but just because they got lucky. This process causes gene frequencies in a population to drift around over time. Some genes may even "drift out" of a population (i.e., just by chance, some gene may reach a frequency of zero). In general, genetic drift has the effect of decreasing genetic variation within a population. For a more detailed explanation, see our resource on genetic drift in Evolution 101.

genetic variation
Loosely, a measure of the genetic differences there are within populations or species. For example, a population with many different alleles at a locus may be said to have a lot of genetic variation at that locus. Genetic variation is essential for natural selection to operate since natural selection can only increase or decrease frequency of alleles already in the population.

All the genetic information an organism carries.

The set of genes an organism has. Sometimes, genotype refers to the entire genome of an organism and sometimes it refers to the alleles carried at a particular locus.

(genera — pl.) The rank above species in Linnaean classification. For example, the genus of humans is Homo. Other species in our genus include Homo erectus and Homo neanderthalensis.

germ line mutation
Mutation that occurs in reproductive cells and ends up being carried by gametes (e.g., eggs and sperm).

An organ used for breathing in many water-dwelling animals, including most fish and many arthropods. Gills generally have a large surface area and are filled with blood; gas exchange occurs by diffusion across the surface area of the gill as oxygen passes into the blood and carbon dioxide passes out of the animal.

habitable zone
The region surrounding a star in which planets with liquid water on their surfaces may exist. The width and position of the habitable zone depend on how hot the star is.

Place and conditions in which an organism normally lives.

Hamilton's rule
The hypothesis that a gene version coding for an altruistic behavior will spread via kin selection when rB > C — where r is the relatedness between the altruist and recipient, B is the fitness benefit given to the recipient, and C is the fitness cost to the altruist.

Individual or cell that carries one set of its chromosomes. Human eggs and sperm are haploid. They usually carry one copy of each of our 22 regular chromosomes and either an X or a Y sex chromosome.

An organism that eats almost entirely plants (herb = plant, vorare = to swallow up).

A property of systems in which intrinsic characteristics are passed from parent(s) to offspring. This results in offspring that resemble their parent(s) more than they resemble randomly chosen individuals of the population. In biology, heredity occurs largely through transmission of the genetic materials DNA and RNA. However, epigenetic heredity also occurs.

An evolutionary change in the timing of a developmental event. For example, relative to the lineage's ancestor, the early maturation of sex organs is an example of heterochrony.

An individual carrying two different gene versions for a particular gene (e.g., Aa as opposed to AA).

Here, humans and their extinct relatives (i.e., organisms on the "human side" of the human/chimpanzee lineage split). However, some scientists use the term hominid to refer to a larger group: humans, other great apes (gorillas, chimpanzees, bonobos, and orangutans), and their extinct relatives. However you decide to name the groups, the important thing is how all these species are related to one another and not exactly what we decide to call each lineage.

Clade that includes all humans, human ancestors, and human relatives that are more closely related to us than chimpanzees are.

homology/homologous structure
Inherited from a common ancestor. Human eyes and mouse eyes are homologous structures because we each inherited them from our common ancestor that also had the same sort of eyes. Contrast this with homoplasious and analogous.

Similar but not because of inheritance from a common ancestor. Homoplasious characters may be explained by convergent evolution in two different organisms or character reversals.

An individual carrying two identical gene versions for a particular gene (e.g., AA as opposed to Aa).

horizontal transfer
A process which results in the transfer of genetic material between members of different species. Bacteria, for example, frequently pass copies of particular genes to one another and pick up foreign genetic material from their environment, resulting in horizontal transfer.

Organism that serves as a habitat for another organism. A host may provide nutrition to a parasite or simply a place in which to live.

Hox gene
A gene that regulates the development and organization of the major body units. For a more detailed explanation, see our resource on Hox genes in Evolution 101.

Hutton, James
Scottish farmer and geologist. In his travels around Britain, he made observations which suggested to him that the geologic processes that shaped the ancient Earth could be seen operating all the time, an idea which would later form the basis of Lyell's uniformitarianism. Hutton used his observations and hypothesis to argue that the Earth must be extremely old.

The production of offspring from different parental forms. For example, if two recognizably different species of plant fertilized one another and produced viable, fertile offspring, the process would be called hybridization.

hydrostatic skeleton
A fluid-filled cavity that supports the body of an animal because the fluid cannot be compressed into a smaller volume (hydro = liquid or water, statos = standing, unchanging).

hygiene hypothesis
The idea that while reduced exposure to microorganisms and parasitic worms has reduced morbidity and mortality in humans, it has been associated with a rising prevalence of atopic disorders and autoimmune diseases, particularly in industrialized countries, because of insufficient or inappropriate development of the immune system in the absence of these infectious agents.

A proposed explanation for a narrow set of phenomena. A hypothesis must be testable with evidence from the natural world. If an explanation can't be tested with experimental results, observation, or some other means, then it is not a scientific hypothesis.

inbreeding depression
Decrease in health and fitness often observed in offspring resulting from inbreeding.

Mating between relatives. Technically, this is defined as a pattern of mating in which mates are more closely related than two individuals selected at random from the population.

incipient species
A group of organisms that is about to become a separate species from other, related individuals.

Insects are a group of arthropods distinguished by the following characters:

  • a body divided into head, thorax, and abdomen
  • one pair of antennae, three pairs of mouth appendages, three pairs of legs on thorax, and often one or two pairs of wings

body divided into head, thorax, and abdomen, three pairs of legs on thorax, and often one or two pairs of wings

one pair of antennae, three pairs of mouth appendages

Examples of insects include flies, moths and beetles.

Flies, moths and beetles are all insects

Photos by T. W. Davies © California Academy of Sciences

intelligent design movement
The intelligent design (ID) movement promotes the idea that many aspects of life are too complex to have evolved without the intervention of a supernatural being — the intelligent designer. Because it relies on supernatural explanations, ID is not science. To learn more, read our brief on the intelligent design movement.

intermediate form
A partially assembled adaptation. Complex adaptations evolve in a series of smaller steps and these steps along the history of an adaptation's evolution are called intermediate forms.

An intervening section of a gene that does not code for the main product of the gene. Introns are removed during RNA processing.

A rare element that is found in relatively high concentrations in asteroids.

junk DNA
DNA that doesn't code for proteins. The term "junk DNA" is a bit of a misnomer since some of this non-coding DNA performs important functions like helping to turn genes on and off.

key innovation
An adaptation that allows an organism to exploit a new niche or resource.

kin selection
A form of natural selection that is caused by interactions among related individuals. To learn more, see Hamilton's rule and altruism.

ladderized phylogenetic tree
A phylogenetic tree in which the most taxon-rich clades are consistently positioned to the right of less diverse clades on vertically-oriented trees or above less diverse clades on horizontally-oriented trees.

Late Heavy Bombardment
The period about 600 million years after the Solar System started to form (early in the Solar System's history, but "late" in its formation) when the rate of collisions between bodies was much higher than it is now.

An area where animals congregate to perform courtship displays and select mates.

life history
Traits that make up the life cycle of an organism. An organism's life history includes characteristics related to reproduction, development, and growth (e.g., fecundity, types of larval stages passed through, size at adulthood, and habitats used at different points in the life cycle).

lineage splitting
An event in which a single historical lineage gives rise to two or more descendent lineages. Every node on a phylogeny is a lineage-splitting event.

A continuous line of descent; a series of organisms, populations, cells, or genes connected by ancestor/descendent relationships.

Linnaean classification
The standard system of classification in which every organism is assigned a kingdom, phylum, class, order, family, genus, and species. This system groups organisms into ever smaller and smaller groups (like a series of boxes within boxes, called a nested hierarchy).

The place in the DNA where a gene is located. For example, the pea color locus is the place in a pea plant's DNA that determines what the color of the peas will be. The pea color locus may contain DNA that makes the peas yellow or DNA that makes the peas green — these are called the yellow and green alleles.

Long life; long duration of existence.

The name given to a particular female hominid (of the species Australopithecus afarensis) who lived in what is now Ethiopia about three million years ago. "Lucy" is famous because she left behind a very complete fossilized skeleton found in 1974.

Evolution above the species level. The adaptive radiation of a lineage into many different niches is an example of macroevolution. Since evolutionary change above the species level, means that populations and species must be evolving, macroevolutionary change entails microevolutionary change.

marsupial mammal
A mammal, such as an opossum or kangaroo, whose young are suckled and protected inside a maternal pouch.

mass extinction
Event in which many different lineages go extinct around the same time. Mass extinctions involved higher rates of extinction than the usual rate of background extinction that is going on all the time. For a more detailed explanation, see our resource on mass extinctions in Evolution 101.

Mendelian genetics
Aspects of genetic inheritance documented by Gregor Mendel. Mendelian genetics mainly refers to the ideas that (1) traits are influenced by discrete heritable elements (now known as genes) that come in different varieties (now known as alleles), (2) for a particular gene, each individual carries two alleles, one inherited from each parent, (3) during reproduction, one allele from each pair is randomly selected to be passed to the offspring and united with the other parent's alleles, (4) because of these characteristics, trait ratios among offspring are predictable if the parental genotypes are known. For more details, see our historical essay on the topic.

A piece of meteoroid, asteroid, or comet that survives its journey through the atmosphere and hits the ground.

A small (under about one meter across), rocky or metallic object traveling through space. If a meteoroid enters the Earth's atmosphere it may heat up and glow as a meteor, or "shooting star."

The collection of microbial organisms that live internally and externally upon larger organisms.

Changes in gene frequency in a population from one generation to the next.

The movement of individuals between populations.

In evolution, lack of fit between an organism's traits and its environment. A mismatch is likely to occur if a species that evolved in one environment moves to a different environment or if a species' environment changes rapidly and the species does not experience natural selection and adapt to these changes. Evolutionary mismatches are frequently used to explain aspects of human biology that are problematic in modern life, but that may have been beneficial over the course of our evolutionary history.

An organelle in eukaryotic cells where cellular respiration takes place. Mitochondria contain a short loop of DNA that is distinct from the DNA contained in the cell's nucleus.

In science, the term model can mean several different things (e.g., an idea about how something works or a physical model of a system that can be used for testing or demonstrative purposes). However, as a research method, modeling often means creating a mathematical model — a set of equations that indirectly represents a real system. These equations are based on relevant information about the system and on sets of hypotheses about how the system works. Given a set of parameters, a model can generate expectations about how the system will behave in a particular situation. A model and the hypotheses it is based upon are supported when the model generates expectations that match the behavior of its real-world counterpart. Modeling often involves idealizing the system in some way — leaving some aspects of the real system out of the model in order to isolate particular factors or to make the model easier to work with computationally.

modern synthesis
A conception of the processes and mechanisms of evolution that builds on Darwin's original ideas about evolution by incorporating advances in our understanding of Mendelian genetics, molecular genetics, and population genetics. The modern synthesis was developed in the 1930s and 40s and integrates the work of Charles Darwin, Theodosius Dobzhansky, Ronald Fisher, Sewall Wright, and others. For more details, see our historical essay on the topic.

molecular clock
The idea that some stretches of DNA evolve at a constant enough rate that they can be used to estimate the dates of past events. Learn more about molecular clocks.

molecular genetics
Aspects of genetics associated with the molecules responsible for inheritance: DNA and RNA. The field involves, for example, the sequence of bases on a DNA molecule, the copying of DNA, the transcription of DNA into RNA, and the translation of RNA into amino acids.

In evolutionary biology, having to do with DNA sequences or the amino acid sequences of proteins.

Group of two or more atoms bonded together.

A process in which an animal sheds all or part of its outer covering, which is then regenerated in some way. For example, arthropods molt their exoskeletons in order to grow, and birds molt their feathers in order to replace worn out feathers or to prepare for a different season or for breeding.

The study of the form and structure of organisms. For example, comparing the shape of the femur in different grazing mammals is a morphological study.

most recent common ancestor
(abbr: MRCA) The youngest common ancestor that two taxa share. On a phylogenetic tree, one can locate the most recent common ancestor of two terminal taxa by tracing each lineage back in time until they meet. The node at which the lineages meet represents the most recent common ancestor of the taxa.

A change in a DNA sequence, usually occurring because of errors in replication or repair. Mutation is the ultimate source of genetic variation. Changes in the composition of a genome due to recombination alone are not considered mutations since recombination alone just changes which genes are united in the same genome but does not alter the sequence of those genes. For a more detailed explanation, see our resource on mutation in Evolution 101.

A species interaction in which both of the interacting species profit from the interaction.

A segment of muscle.

Myriapods (myria = ten thousand, pod = foot) are a group of arthropods distinguished by the following characters:
  • a body built from a head and long, repeating trunk
  • one pair of antennae (number of other appendages on head varies), many (but not 10,000!) limbs on trunk

typical myriapod

Examples of myriapods include centipedes and millipedes.

Centipedes and millipedes are myriapods
Centipede Millipede

Centipede photo by James T. Johnson © California Academy of Sciences; Millipede photo © 2003 John White

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