Introduction to Comparative Anatomy

Homecercal Tail

In the homecercal tail, both lobes are equal in size, making the tail symmetrical.

Heterocercal Tail

In the heterocercal tail, found in sharks and a few other groups, the upper lobe is elongated.

Functional Morphology

The discipline that relates a structure to its function.

Evolutionary Morphology

A discipline combining morphology and evolutionary biology.

High Reproductive Potential

The natural increase in number of members of any species if left unchecked.

Competition

The struggle for resources among organisms.

Survival of the Few

The mechanism by which nature weeds out the less fit organisms.

Natural Selection

Nature's way of determining which organisms survive and which do not.

Primitive

Term preferred over "lower" to emphasize early evolutionary appearance.

Derived

Term preferred over "higher" to emphasize later evolutionary appearance.

Archetype

A supposed underlying plan upon which an organism was built.

Morphology

The study of the form and structure of organisms.

Homology

Similarity between features due to shared ancestry.

Analogy

Similarity between features due to similar function.

Homoplasy

Similarity between features that simply look alike.

Serial Homology

Similarity between successively repeated parts in the same organism.

Symmetry

The way in which an animal's body meets the surrounding environment.

Radial Symmetry

Symmetry around a central axis, resulting in mirrored halves.

Bilateral Symmetry

Symmetry where the body is divided into left and right mirrored portions.

Anterior

Refers to the head end of an organism.

Posterior

Refers to the tail end of an organism.

Dorsal

Refers to the back side of an organism.

Ventral

Refers to the belly or front side of an organism.

Medial

Refers to the midline of the body.

Lateral

Refers to the sides of the body.

Distal

Farthest from the body.

Proximal

Closest to the body.

Pectoral Region

Area supporting the forelimbs.

Pelvic Region

Area supporting the hindlimbs.

Frontal Plane (Coronal Plane)

Divides a bilateral body into dorsal and ventral sections.

Sagittal Plane

Divides a bilateral body into left and right portions.

Transverse Plane

Divides a bilateral body into anterior and posterior portions.

Superior

Refers to a position higher in the body.

Inferior

Refers to a position lower in the body.

Segment

Repeated section of an organism's body.

Segmentation

Process of dividing a body into duplicated sections.

Septa

Dividing walls between segments in segmented animals.

Function

The action or property of a part as it works in an organism.

Biological Role

How a part is used in the environment during an organism's life history.

Preadaptation

Possession of necessary form and function before the biological role arises.

Incipient Structures

Small and formative structures when they first appear in evolution.

Phylogeny

The course of evolution depicted in graphic schemes or dendrograms.

Quantum Evolution

Long intervals of unchanged evolution occasionally interrupted by rapid change.

Punctuated Equilibrium

Theory describing long periods of stasis punctuated by rapid change.

Primitive Condition

The earlier state of a character.

Derived Condition

The later state of a character after transformation.

Taxon

A named group of organisms.

Natural Taxon

A group that accurately depicts a naturally occurring group of organisms.

Artificial Taxon

A group that does not correspond to an actual unit of evolution.

Sister Group

Taxon most closely related to the group being studied.

Grade

Expression of the degree of change or level of adaptation reached by an evolving group.

Clade

A lineage including an ancestor and all its descendants.

Traditional Systematics

Placing together organisms with similar characteristics.

Phylogenetic Systematics

Placing together organisms belonging to the same clade.

Cladistics

A method of classification based on evolutionary relationships.

Cladogram

A dendrogram depicting evolutionary relationships among taxa.

Ingroup

The assortment of taxa being examined.

Outgroup

Taxa used as a reference but not part of the ingroup.

Monophyletic

Including an ancestor and all its descendants.

Polyphyletic

Including members with different ancestors.

Paraphyletic

Including a common ancestor and some, but not all, of its descendants.

Crown Group

The smallest clade including all living members and relevant fossils.

Stem Group

Extinct taxa most closely related to the crown group.

Total Group

Crown group plus stem group.

Exposures

Natural areas where rock layers are exposed.

Strata

Layers of rock containing fossils.

Taphonomy

The study of how decay affects fossilization.

Stratigraphy

Placing fossils in a relative sequence based on rock layers.

Time Horizon

Each layer of rock containing fossils from a specific time period.

Uniform Extinctions

Gradual loss of species over long periods.

Catastrophic Extinctions

Abrupt loss of species occurring over short geological time.

Electromyogram

Electric record of muscle activity.

Ecomorphology

The study of morphological systems in relation to ecology.