Defining "Evolution"

The word evolution means change. In biological terms, evolutionEvolution:
Inheritable change within a lineage, or the change that occurs between generations within one population of a species. This refers of course to change expressed from one individual to another, but we are usually most interested in the changes so great that the later generation is considered a different species. is inheritedInherited:
Derived from a preformed genetic code present in an ancestor. change within a lineageLineage:
Line of descent from an ancestor.. In other words, evolution occurs when inherited changes from generationGeneration:
A single step or stage in the succession of natural descent. to generation within one populationPopulation:
A group of organisms, all of the same species, which occupies a particular area. of a speciesSpecies:
A group of organisms formally recognized as distinct from other groups; the taxon rank in the hierarchy of biological classification below genus; the basic unit of biological classification, defined by the reproductive isolation of the group from all other groups of organisms. give rise to a new appearance, a new genetic code, and ultimately, a new species.

Scallops — today mainly members of the bivalve family Pectinidae — have been alive on Earth since the early Triassic Period, approximately 240 million years ago. They are characterized by “ears” (called auriclesAuricles:
Ear-shaped structures on the shells of scallops, one of their defining characteristics.) on the shell and a notch below one of them through which a set of elastic threads (called a byssusByssus:
A tuft of long, tough filaments which are formed in a groove of the foot, and issue from between the valves of certain bivalve mollusks, by which they attach themselves to rocks, etc.) emerges to help hold the scallop, at least during its juvenile life, to the sea bottom. At the edge of the byssal notch in most scallops is a comb-like set of spines (called the ctenoliumCtenolium:
A comb-like structure along the ventral edge of the byssal notch in scallops in which the byssal threads rest.) that separate and support the byssal threads. The direct ancestors of scallops were scallop-like bivalves of the family Entoliidae, which lived during the Paleozoic and Mesozoic Eras (400-65 million years ago). Entoliids had auricles and a byssal notch only as juveniles, but they did not have a ctenolium. The ctenolium — a defining feature of the modern family Pectinidae — is a characteristic that evolved within the scallop lineage.

A living Sentis Scallop (Caribachlamys sentis) from the Florida Keys. The notch between the auricle (at left) and the main body of the shell is the byssal notch, through which the foot and byssus extend.

The ctenoliumCtenolium: A comb-like structure along the ventral edge of the byssal notch in scallops in which the byssal threads rest. (the comb like teeth in the V-shaped indentation between the auricle and body of the shell) of Caribachlamys sentis from the Florida Keys helps separate and support its byssal threads. Many adult scallops do not use a byssus beyond the juvenile stage, and so lose the ctenolium as they grow larger.

The family Entoliidae is considered ancestral to modern scallops. This is Entolium aviculatum from the Carboniferous Period of Henry County, Missouri. The auriclesAuricles: Ear-shaped structures on the shells of scallops, one of their defining characteristics. are to the right and left of the umboUmbo: Rounded or pointed extremity of a bivalve shell, often projecting above the hinge line, that reflects the early growth stage (= oldest part of the shell) and includes the prodissoconch (larval shell) and adjacent convex area; also called the “beak.” at top center.

An unidentified fossil scallop (Pectinidae, 3.7 cm diameter), from the Jurassic Period of Bradford-on-Avon, England, is preserved in a similar way to the older Entoliidae at left.

Not all biological change is evolution. The metamorphosis, growth, and physical changes of an individual organism are biological changes but are not evolution.

A baby clam spends the first part of its life as a free-swimming, two-shelled larva called a veligerVeliger:
Planktonic larval type characteristic of most mollusks (including bivalves), characterized by a ciliated locomotory organ (velum) which is either discarded or resorbed at metamorphosis. — its locomotory apparatus is a pair of ciliated lobes called a velumVelum:
The locomotory organ of a veliger larva. that propels it through the water. Eventually, the larva grows a foot, settles onto the ocean bottom, and metamorphosesMetamorphosis:
A change in body form and often habits of an animal following the embryonic stage during normal development. (pl. metamorphoses) into a benthicBenthic:
Occurring at the bottom of a body of water, for example, a seabed, riverbed, or lake bottom. organism, resorbing its velum and growing in size. At this stage, its foot secretes strong but elastic byssal threadsByssus:
A tuft of long, tough filaments which are formed in a groove of the foot, and issue from between the valves of certain bivalve mollusks, by which they attach themselves to rocks, etc. to stabilize it in the sediment. The clam's appearance changes quite dramatically from juvenile to adult; notice that the adult (below right) no longer uses a byssus, and has relatively shorter siphons than the attached juvenile.

Veliger larva	Attached juvenile	Adult
(Original artwork by Marla Coppolino)

Evolutionary change does not necessarily mean improvement or progress. Evolution does not follow a linear progression from simple to complex or primitive to advanced. Evolution is occasionally depicted, inaccurately, as a ladder leading progressively from “lower” to “higher” organisms (usually with humans at the top). In reality, however, evolution is more accurately depicted as a tree with many branches. Some species become extinct, some mutations and variationsVariation:
The differences among individuals in a population. fail, and simple organisms can be highly adapted to their environments. The phrase “more evolved” does not necessarily mean “better.”

There is often confusion between the words evolutionEvolution:
Inheritable change within a lineage, or the change that occurs between generations within one population of a species. This refers of course to change expressed from one individual to another, but we are usually most interested in the changes so great that the later generation is considered a different species. and “Darwinism.” Long before Charles Darwin, many scientists understood that evolution occurred but they could not explain how. Darwin's publication, On the Origin of Species (1859), provided the first mechanism of evolution, natural selectionNatural Selection:
The process by which living forms with traits that better enable them to adapt to specific environmental pressures, e.g., predators, changes in climate, or competition for food or mates, will tend to survive and reproduce in greater numbers than others of their kind, thus ensuring the perpetuation of those favorable traits in succeeding generations.. The terms Darwinism and natural selection can be used interchangeably. Other mechanisms of evolution have also been accepted. Therefore, evolution is species change over time and Darwinism (natural selection) is just one of the mechanisms of how these changes occur.

Why study evolution? Evolution is the unifying theory that explains all biological science. All life is connected and none of it can be fully understood without the theory of evolution. Imagine trying to study chemistry without understanding that molecules are made of atoms (atomic theory). Evolution allows us to understand the past, predict the future, and map the organization of our world. Evolutionary scientists from all fields of biology (paleontology, molecular biology, genetics, etc.) are using the most sophisticated research techniques and technologies to explain life. Understanding the evolution of bivalves can help prevent their extinction, prevent the spread of invasive species around the world, and can even help scientists date geological events. More information about the importance of evolution can be found in the Evolution in Everyday Life section.