Adaptation, Speciation, & Diversity

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.
is the basic unit in the classification of life. For a species to be successful (that is, able to survive and reproduce), it must be well adapted to its environment. Adaptation occurs over time by natural selection favoring variationsVariation:
The differences among individuals in a population.
that improve the species' fitnessFitness:
The relative probability of survival and reproduction for a genotype.
. These adaptations can be physical, chemical, or behavioral.

Camouflage and mimicry are two forms of physical adaptation. CamouflageCamouflage:
Structural adaptation that enables an individual to blend with its surroundings, and that allows an individual to avoid detection by predators.
occurs when an individual "blends" into the environment in the eyes of a potential predator. MimicryMimicry:
A phenomenon in which an individual gains some sort of survival advantage by looking like an individual of another (often more harmful) species.
is present when one species has evolved to look like another species in a way that will provide some advantage. The best-known examples of mimicry are those in which a species mimics another that is toxic or harmful to a potential predator, such as the nonvenomous Scarlet Kingsnake that closely resembles the venomous Coral Snake in the color bands on its body. Camouflage and mimicry are both adaptations because they increase the likelihood of an individual evading predation long enough to reproduce.

Bivalves also provide good examples of camouflage and mimicry. The spines on the shells of Spiny Oysters (family Spondylidae) provide crevices in which algae, sponges, and other epibiontsEpibiont:
An organism that lives on the surface of another living organism.
can settle and grow to cover and camouflage the shell. With time, this growth helps the Spiny Oyster to “disappear” on the surface of the reef, lowering the threat of predation from top-level predators such as fish. Octopuses and squid (both cephalopods, and close relatives of bivalves) are the “camouflage champions” of the molluskMollusk:
A member of the phylum Mollusca; also spelled mollusc (most especially in the United Kingdom).
world, able to change the color and texture of their soft bodies to blend into their surroundings.

A living Thorny Oyster (Spondylus varians) shows a thick cover of epibionts — mainly sponges and algae — that help it blend into its surroundings to avoid predation. When closed, this oyster is barely noticeable on the rock.

This Thorny Oyster shell (Spondylus regius) probably looked much like the encrusted Spondylus varians before it was cleaned (a laborious process, as many shell collectors know). The spines are an adaptation that provides added surface for the attachment of a fouling communityFouling Community:
Community of organisms found attached to hard substrata, most usually human-made, e.g., on the sides of docks, marinas, harbors, or vessels.

Females of the freshwater pearl mussel called the Orange-Nacre Mucket (Hamiota perovalis) have evolved a marvelous example of mimicry that works to their reproductive advantage. Freshwater pearl mussel larvae, called glochidiaGlochidium:
The specialized larval form of freshwater pearl mussels that usually has hooks that enable it to attach itself to a host (e.g., to the gills of a fish) for a period of time before it detaches and falls to the bottom and takes on the typical form of a juvenile mussel. (pl. glochidia)
, must attach to the gills or fins of a fish to successfully metamorphose. The female Orange-Nacre Mucket produces a discrete mass of larvae, called a superconglutinate, that resembles a small fish in shape and coloration. After release from the female mussel, the mass of larvae looks just like a small fish darting about in the current of the stream (very similar to the complex behavior of “mantle lures” of other freshwater mussels, such as the Cracking Pearl Mussel [see below]). The fish-like superconglutinate attracts a predatory fish that is interested in eating the “small fish.” When the predatory fish attacks, the mass disintegrates, releasing the larvae to attach to the fish.

Shells of the Orange-Nacre Mucket (Hamiota perovalis). Scientists at Missouri State University have produced a video of the Hamiota superconglutinate in action.

The fleshy “lure” of the freshwater Cracking Pearl Mussel (Hemistena lata) flutters in the water to attract the host fish required for its glochidia larvae.

Over a long period of time, natural selection is able to utilize many variations and evolve new species in the process of speciationSpeciation:
The process in which one species evolves over time into a different species (anagenesis) or in which one species diverges to become two or more species (cladogenesis); see also allopatric speciation, parapatric speciation, peripatric speciation, sympatric speciation.
. All species have evolved from earlier forms of life. Earth's great diversityDiversity:
The variety of species in a sample, community, or area.
of life is the result of 3.8 billion years of natural selection favoring advantageous variations, some of which ultimately lead to new species. Genetic driftGenetic Drift:
Change in the gene pool that occurs when the frequency of an allele changes over generations due to random chance.
also plays a part in speciation, but due to its randomness, it is very difficult to identify specific instances.

There are four broad categories of speciation:

  1. Hoover Dam, on the Arizona-Nevada border in the southwestern United States, impounded the Colorado River to create Lake Mead. The average depth of the Colorado River is 20 feet (6 meters), whereas Lake Mead is at its greatest 500 feet deep (> 150 meters). Such a change in water depth and conditions, inflicted on a split population of freshwater mussels, could be a powerful force for adaptation leading to speciation.

    Allopatric speciationAllopatric Speciation:
    Speciation in which the evolution of reproductive isolating mechanisms occurs during physical separation of the populations.
    occurs when a population is split into two geographically isolated populations. Each population is then exposed to different selective pressures that result in genotypic and phenotypic changes. Over time, the populations develop into two different species and if they ever come into contact again, they would be unable to reproduce.

    Allopatric speciation could occur if a population of freshwater mussels is cut in half by the construction of a dam. The mussels upstream would be isolated in the newly formed lake and the mussels downstream would be isolated in the remaining river. Assuming that both populations could survive the new living conditions, the populations would evolve separately and eventually become two distinct species. The mussels in the lake would adapt to the slower moving, deeper water. The mussels in the river would remain adapted to the faster moving, shallow water, more similar to the original conditions of the population.

  2. Peripatric speciationPeripatric Speciation:
    Process by which a new species evolves in a subpopulation that colonized a new habitat or niche within the same geographical area of the ancestral species.
    also occurs when a single population is physically split into two isolated populations (= allopatric speciation), but this term is used when one population is much smaller than the other.

    Peripatric speciation could occur if a few members of a clam population get swept ashore during a hurricane and become isolated in a brackish lake. Assuming that the clams in the lake can survive the brackish-water conditions, the population in the ocean and the new population in the lake would then evolve separately and eventually become two distinct species.

  3. Vesicomyid clams on the Bucky Hydrothermal Vent Field, East Pacific Rise, 2,493 meters (> 14,000 feet) depth, ALVIN submersible dive 3934. Each of these impressive clams is 20-30 centimeters (8-12 inches) in length.

    Parapatric speciationParapatric Speciation:
    Speciation in which the evolution of reproductive isolating mechanisms occurs when a population enters a new niche or habitat within the range of the parent species.
    occurs when a population is in the process of splitting, due to some non-physical factor (such as a change in physiology or ecology), but continue to overlap. Each part of the population has its own ecological niche. The diverging members might occasionally come into contact but their offspring are not well suited for either niche. Over time, the populations develop into two different species, each suited for their own niche, and eventually they will lose the ability to reproduce with each other.

    Vesicomyid clams (family Vesicomyidae) are dominant organisms in chemosynthetic-based deep-sea communities, such as methane or cold seeps and hydrothermal vents. In a recent study involving molecular and morphometric analyses[1], five lineages (including three undescribed species) were identified. All five species are broadcast spawners (adults release eggs and sperm freely into the water where fertilization occurs), but are nevertheless now segregated by depth, suggesting that the species diverged and are maintained by bathymetric (depth) and substratum factors exerted on the larvae rather than on the adults. This might be an example of perapatric speciation — the gametes of the five species can commingle in the water column, but “hybrid” larvae might not be as well adapted to the niches of the “pure” larvae.

  4. The small colorful octopus, Octopus rubescens, from off of Washington State

    Sympatric speciationSympatric Speciation:
    Speciation in which the evolution of reproductive isolating mechanisms occurs within the range and habitat of the parent species.
    occurs when a single population diverges into two while inhabiting the same physical space. This usually occurs as a result of sexual selectionSexual Selection:
    A type of selection in which the forces determined by mate choice act to cause one genotype to mate more frequently than another genotype.

    Sympatric speciation might occur if, within one population of octopuses, some females favor large aggressive males whereas other females favor smaller more colorful males. Over a very long time, the two preferences could result in divergence and two new species — one large and agressive, the other small and colorful.


  1. Goffredi, S. K., L. A. Hurtado, S. Hallam, & R. C. Vrijenhoek. 2003. Evolutionary relationships of deep-sea vent and cold seem clams (Mollusca: Vesicomyidae) of the "pacifica/lepta" species complex. Marine Biology, 142: 311-320.