Types of Aquatic Organisms - Freshwater Ecology: Concepts and Environmental Applications - page 114

THE SPECIES CONCEPT

Biologists consider the species as the fundamental unit of taxonomic di-
vision, and the scientific system for naming organisms is based on distin-
guishing species. Species are then classified into ever-broader groups (Table
7.1). A traditional definition of biological species is “a genetically distinctive
group of populations whose members are able to interbreed freely under nat-
ural conditions and are reproductively isolated from all members of other
such groups” (McFadden and Keeton, 1995). However, many aquatic or-
ganisms (especially the microbes, but also many plants and animals) do not
reproduce sexually. Other species may be able to reproduce with organisms
that are considered to belong to different species (e.g., many species of trout
are able to hybridize). Bacteria have no sexual reproduction but can pass
genes within or among different taxonomic groups. Even for those organ-
isms that do reproduce sexually, it is often difficult to test if two individuals
will interbreed successfully or to determine the degree of genetic similarity.
We have limited data on the reproductive biology of most aquatic organisms
in their habitat except for some game fish and emergent wetland plants. Fi-
nally, there is no objective measure of how “genetically distinctive” an or-
ganism must be from another before it qualifies as its own species.
In practice, systematists differentiate among most nonbacterial species
on the basis of morphological characteristics. The operational species def-
inition uses an older, formal definition of a species as “a group of organ-
isms that more closely resemble each other, with respect to their physical
appearance (morphology), physiology, behavior, and reproductive patterns,
than they resemble any other organisms” (McFadden and Keeton, 1995).
However, no hard and fast line delineates the amount of morphological dif-
ferentiation that is necessary for organisms to be considered distinct species.
Difficulty arises because of the natural morphological variation found in
the same species (Fig. 7.2), especially those living in different environments.

TABLE 7.1 Botanical and Zoological Naming Scheme for Organisms, Name Endings Associated
with Each Taxonomic Group, and Examples of Naming of a Cattail and a Beaver

Taxonomic classification Botanical name Zoological name

(sub- or supergroup) endings Example endings Example

Super kingdom or domain -a Eukarya -a Eukarya

Kingdom -ae Plantae -a Animalia

Phylum or division -phyta Anthophyta -a Chordata

(subphylum
Vertebrata)

Class -opsida Liliopsida -a Mammalia

Order -ales Typhales -a (-formes, Rodentia

birds; -oidea,
some mammals)

Family (subfamily, -aceae Typhaceae -idae Castoridae

tribe, subtribe)
Genus Typha Castor

Species (subspecies) latifolia canadensis