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

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Sedgewick, Adam
English geologist who studied the fossils in different geologic strata and helped give the strata (and corresponding time periods) the names we use today — Cambrian, Devonian, etc. Although he accepted naturalistic explanations for geologic events and studied them using the biostratigraphic methods of William Smith, Sedgwick rejected Darwin's naturalistic explanation for the origin of species and argued that God created new forms of life at the beginning each geologic period.

The process in which pairs of chromosomes separate and are shuttled off to different gametic daughter cells. When gametes are formed, a single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes each). In the process, the paired chromosomes of the parent cell separate into different daughter cells. This process is segregation.

sexual selection
Selection acting on an organism's ability to obtain or successfully copulate with a mate. This process may produce traits that seem to decrease an organism's chance of survival, while increasing its chances of mating. For a more detailed explanation, see our resource on sexual selection in Evolution 101.

shocked quartz
Crystals with a pattern of fracturing that can be caused by the intense pressure and heat of events such as asteroid impacts.

sickle cell anemia
A genetically caused disease that generally results in the death of the person with it unless medical interventions are available. Sickle cell anemia is a popular topic for biology courses because it is one the few, well-worked out examples of heterozygote advantage that we have. People carrying two copies of the sickle cell allele have the disease, people with no copies of the sickle cell allele are normal, but people carrying just one copy of the sickle cell allele are resistant to malaria (though they may occasionally have symptoms of sickle cell). So, if you live in a region where malaria is common, you are at an advantage if you are a heterozygote (i.e., if you carry one sickle cell allele and one normal allele). For a more detailed explanation, see our resource on sickle cell anemia in Evolution 101.

signal detection
In evolution, the ability of individuals to detect signaling behavior from other individuals — e.g., the ability of females to locate a male on the basis of his calling display, or the ability of a predator to determine that a prey item bears coloration associated with toxic organisms.

single nucleotide polymorphisms
A variation at a single, specific location in the genome. Comparison of SNPs is a common way to quantify genetic diversity either within or between populations.

Refers to an organism consisting of one cell, such as bacteria, protozoa, and some algae, fungi, and yeasts.

sister groups
(sometimes called sister taxa) Clades that are each other's closest relatives. On a phylogeny, sister groups occur anytime a single ancestral lineage gives rise to two daughter lineages: the daughter lineages are sister groups, and since they arose from the same ancestor at the same time, sister groups are always the same age. Sister groups may differ widely in diversity level: one clade may be comprised of a single species, while its sister group may be comprised of 100 species.

In evolution, the tendency to form social groups in which individuals of a species interact with one another outside of opportunities to mate.

solar system
A star, its planets, and other smaller bodies orbiting the star. Our own Solar System consists of the Sun, the eight planets, dwarf planets, asteroids, and comets.

solar wind
The ionized atoms that stream through the Solar System from the surface of the Sun. The Earth's magnetic field protects it from bombardment by these particles, but in interplanetary space, the solar wind may bombard comets, blasting material from the surface and creating long tails.

somatic mutation
Mutations occurring in cells that do not form gametes, mutations that do not end up being carried by eggs or sperm. For example, mutations in your skin, muscle, or liver tissue are somatic mutations.

The process by which species form. This involves the reproductive isolation of different parts of an ancestral species so that they form distinct descendent species. For a more detailed explanation, see our resource on speciation in Evolution 101.

A group of individuals that actually or potentially interbreed in nature. In this sense, a species is the largest gene pool possible under natural conditions. For a more detailed explanation, see our resource on species in Evolution 101.

To cut introns out of an RNA transcript and rejoin the RNA molecule.

(strata — pl.) A layer of sedimentary rock.

A grouping of organisms less inclusive than a species. The term is usually applied to groups within a species that have distinct forms and live in a restricted area.

(supernovae — pl.) An exploding massive star. A supernova may briefly outshine its entire host galaxy and, as it explodes, may fuse light elements to make heavier elements as well as cosmic dust.

An organism that lives in close contact with another organism (usually with an organism of a different species).

A relationship between two different organisms that live in close contact with each other. The relationship may be beneficial to both organisms (mutualism), beneficial to just one (commensalism), or harmful to one (parasitism).

An ancestral character state (i.e., a plesiomorphy) shared by two or more lineages in a particular clade. For example, within the clade of terrestrial vertebrates (in which the ancestral character state is "has four legs"), both elephants and salamanders have four legs — and so having four legs is a symplesiomorphy for those two lineages.

A derived or changed character state (i.e., an apomorphy) shared by two or more lineages in a particular clade. Synapomorphies are indicators of common ancestry. For example, within the clade of terrestrial vertebrates the ancestral, or plesiomorphic, character state is "has four legs." However, both owls and parrots have the synapomorphic character state "has two legs and two wings," indicating that owls and parrots are closely related.

(taxa — pl.) Any named group of organisms (e.g., the reptiles, Felidae, beetles, Homo sapiens), whether or not it forms a clade.

terminal taxon
A clade, species, or lineage that appears at the tip of a phylogenetic tree. Terminal taxa may be extant or extinct.

The animal clade containing vertebrates with sturdy legs (as opposed to fins).

A broad explanation for a wide range of phenomena. Theories are concise, coherent, systematic, predictive, and broadly applicable. They usually integrate many individual hypotheses. A scientific theory must be testable with evidence from the natural world. If a theory can't be tested with experimental results, observation, or some other means, then it is not a scientific theory.

In animals with three body regions, the middle body region, usually between the head and abdomen.

tidal heating
The repeated deformation of a body (for example, a moon) due to tides from another body (for example, a planet), which leads to heating of the former's interior. Tidal forces arise due to variations in the gravitational forces between bodies. For example, tidal forces from the Moon cause tides in the Earth's oceans of course, but they also very slightly deform the Earth itself. Tidal heating may have resulted in a water ocean under the ice covering Jupiter's moon Europa.

An internal tube that carries air into the body of an animal for breathing. For example, in humans, a trachea carries air to the lungs; in insects, a network of tracheae carries air directly to tissues throughout the body. (plural = tracheae)

In evolution, a situation in which undergoing natural selection improving performance in one arena (e.g., attracting a mate with an extra-long tail) means simultaneously decreasing performance in another arena (e.g., avoiding predation). For more details, read about trade-offs in Evolution 101.

The process of building an RNA molecule using DNA as a template. In this process, complimentary RNA bases are matched to their DNA counterparts so that the strand of RNA that is produced carries the "imprint" of one strand of the DNA molecule.

transitional forms
Fossils or organisms that show the transformation from an ancestral form to descendant species' form. For example, there is a well-documented fossil record of transitional forms for the evolution of whales from their amphibious ancestor. For a more detailed explanation, see our resource on transitional forms.

Part of the process of decoding an RNA molecule composed of nucleotide bases into a protein composed of amino acids.

tree thinking
The ability to understand evolutionary tree graphics and use them to organize one's knowledge of biodiversity.

Trilobites are an extinct group of arthropods, distinguished by the following characters:

  • a body built from a cephalon, thorax, and pygidium
  • a body divided into three lobes, running from head to tail
  • one pair of antennae


The last trilobites went extinct about 245 million years ago, but they are well represented by the fossil record.

trilobite trilobite

Differences in genes, traits, or behaviors among members of a population, which may result in differences in reproductive success. When variation is genetic in origin, it may be acted upon by natural selection.

Any member of the animal clade Vertebrata. All vertebrates have a backbone that surrounds and protects the nerve cord, a character that they all inherited from their common ancestor. Vertebrates are a subgroup of the chordates. Modern vertebrates include fish, sharks, mammals, and amphibians.

vestigial structure
A feature that an organism inherited from its ancestor but that is now less elaborate and functional than in the ancestor. Usually, vestigial structures are formed when a lineage experiences a different set of selective pressures than its ancestors, and selection to maintain the elaboration and function of the feature ends or is greatly reduced.

A process in which a species' range is divided even though the species has remained in place. This might happen through tectonic action, geologic activity (like the rise of a mountain range or shift in the course of a river), or other processes. Vicariance is usually contrasted with dispersal as a biogeographic mechanism.

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