In geology, the idea that older rock layers or strata were deposited earlier and so will generally be found beneath younger strata.

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.

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

(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.

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.

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

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

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 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.

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.

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

(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.

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.

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.

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.

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

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.

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.

(1785-1873)
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 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.

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