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Terms

Viable

Able to live and develop.

Adaptation

A characteristic or feature that is maintained by natural selection because it offers a fitness advantage.

Allele Frequency

The frequency of a certain allele within a population or species, usually written as the percent of individuals that possess that allele.

Allopatric

Two species are allopatric if their ranges are completely disconnected.

Anagenesis

The evolutionary changes that occur within a lineage.

Assortative Mating

A form of non-random mating in which individuals prefer to mate with other individuals that are similar to them in some important characteristic. In other words, like mates with like.

Bell Curve

The statistical distribution (also known as a normal distribution) that has a mean of zero and a standard deviation of one.

Cladogenesis

The evolutionary changes that result in the splitting of lineages.

Congruence

In a state of agreement; if two ideas are in agreement with each other, there is congruence between them.

Divergence

The accumulation of differences (genetic, behavioral, ecological, etc.) between two groups of the same lineage, often resulting in the splitting of that lineage into two distinct species.

Fitness

The reproductive success of an individual or genotype.

Fixation

When an allele frequency falls to 0% (meaning the allele is lost from the populations) or raises to 100% (meaning all individuals in the population have the allele).

Gene Flow

The movement of genes through a population caused by interbreeding among individuals. There can also be gene flow between populations if migration occurs and is followed by interbreeding. Disruption of gene flow (via reproductive isolation) is required for speciation.

Hybrid Infertility

Occurs when the hybrid offspring of two parent species does not produce fertile, viable, or compatible gametes and therefore cannot reproduce. Also known as Hybrid Sterility.

Hybrid Inviability

Occurs when the hybrid offspring of two parent species has reduced fitness to the extent that it cannot mature into a healthy, reproductive adult.

Hybrid

The resulting offspring from a successful mating of two different species.

Hybridization

When members of different species mate to produce an offspring. If the resulting offspring is inviable or infertile then the parent species are considered distinct species. However, if the hybrid offspring is healthy and reproduces then it challenges the distinction between the two parent species.

Incipient Species

a group of organisms that are thought to be on the brink of diverging into a distinct species, but are still capable of interbreeding with other members of the species.

Intraspecific Variation

The variation (genetic or phenotypic) that exists among all the individuals of the same species.

Inviable

Unable to live and/or develop, even under favorable conditions.

Microhabitat

A small, specialized habitat within a larger habitat. Most habitats contain many microhabitats. For example, a pine forest habitat might include microhabitats such as underneath moist rocks, in the tree canopy, inside hollow trees, near streams, etc.

MRCA (Most Recent Common Ancestor)

The most recent lineage from which all members of a given set of species descended. For example: 

Natural Selection

The evolutionary process in which individuals that are better adapted to their environment have more offspring than individuals that are not well-adapted to their environment, resulting in greater numbers of well-adapted individuals in the population over several generations.

Parapatric

Two species are parapatric if their ranges are right next to each other but do not significantly overlap.

Phenetic

Relating to observable traits (often morphological) of a species.

Phylogeny

The evolutionary history of a species or group of species, represented graphically by a phylogenetic tree.

Polymorphism

When two or more distinct phenotypes of a given trait exist within the same population or species.

Population

Postzygotic Isolation

A form of reproductive isolation in which individuals of two species mate, but their zygote cannot successfully reproduce. This can be because the zygote is inviable and dies (either before or after birth), or because the resulting offspring is sterile.

Prezygotic Isolation

A form of reproductive isolation in which no hybrid zygote is ever formed, either because the individuals of two species never mate, or because their gametes (sperm and egg) are incompatible and never form a zygote.

Reinforcement

The process in which natural selection acts to increase reproductive isolation among sister species.

Reproductive Isolation

If populations or individuals cannot produce fertile, viable offspring together, then they are said to be reproductively isolated. Causes of reproductive isolation can be differences in geography, behavior, ecology, morphology, etc.

Sexual Selection

The evolutionary process in which individuals that have desirable sexual characteristics are more successful at attracting mates.

Speciation Event

A branching point on a phylogenetic tree denoting one lineage splitting into two. Not really an "event" by our time frame, as speciation occurs on evolutionary timescales.

Speciation

The evolutionary process by which new species arise from a single evolutionary lineage.

Subspecies

A subset of a species whose members are capable of interbreeding with outside member of the species but usually don't, often because of a geographic barrier.

Sympatric

Species are sympatric if they occupy the same range or, at least, have ranges with significant overlap.

Viable

Able to live and develop.

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